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Last Updated :2024/11/07

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Master

Affiliation (Master)

  • Institute of Low Temperature Science　Pan-Okhotsk Research Center

Affiliation (Master)

  • Institute of Low Temperature Science　Pan-Okhotsk Research Center

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Profile and Settings

Profile and Settings

  • Name (Japanese)

    Nishioka

  • Name (Kana)

    Jun

  • Name

    200901061177354071

Alternate Names

Achievement

Research Interests

  • オホーツク海   北太平洋   化学海洋学   Chemical Oceanography   

Research Areas

  • Environmental science/Agricultural science / Environmental dynamics

Research Experience

  • 2022/04 - Today 北海道大学 低温科学研究所 附属環オホーツク観測研究センター センター長
  • 2021/12 - Today Hokkaido University Institute of Low Temperature Science Pan-Okhotsk Research Center Professor
  • 2005/06 - 2021/11 Hokkaido University Institute of Low Temperature Science Pan-Okhotsk Research Center
  • 1995/04 - 2005/05 財）電力中央研究所

Education

  • 1993/04 - 1995/03  北海道大学大学院水産学研究科水産化学専攻博士前期課程

Committee Memberships

  • 2024/01 - Today   GEOTRACES SSC co-chair   Co-chair
  • 2021/10 - Today   ー
  • 2021/04 -2023/03   -

Awards

  • 2024/09 2024年度日本海洋学会 日本海洋学会賞
     北西太平洋および縁辺海における鉄の輸送と生物生産との関わりの解明 
    受賞者: 西岡 純
  • 2023/04 文部科学省 令和5年度 科学技術分野の文部科学大臣表彰 科学技術賞（研究部門）
     北太平洋の生物生産を支える鉄分の供給過程に関する研究 
    受賞者: 西岡 純
  • 2016 北海道大学 北海道大学研究総長賞・奨励賞
     
    受賞者: 西岡 純
  • 2015 北海道大学 北海道大学研究総長賞・奨励賞
     
    受賞者: 西岡 純
  • 2014 北海道大学 北海道大学研究総長賞・奨励賞
     
    受賞者: 西岡 純
  • 2005/04 日本海洋学会 岡田賞
     
    受賞者: 西岡 純

Published Papers

  • Minako Kurisu, Kohei Sakata, Jun Nishioka, Hajime Obata, Tim M. Conway, Hannah R. Hunt, Matthias Sieber, Katsuhiko Suzuki, Teruhiko Kashiwabara, Sayuri Kubo, Makoto Takada, Yoshio Takahashi
    Geochimica et Cosmochimica Acta 378 168 - 185 0016-7037 2024/08/01 
    The availability of dissolved iron (Fe) limits primary production over some regions of the surface ocean, especially in regions such as the subarctic North Pacific. In this work, we use Fe stable isotope ratios (δ56Fe) in bulk and size-fractionated marine aerosol particles and dissolved Fe of surface seawater in the subarctic North Pacific on Japanese GEOTRACES cruise GP02 (Summer 2017) as a tracer to clarify the relative contribution of combustion and natural Fe in both marine aerosol particles and surface seawater. The bulk aerosols collected in the coastal regions of both East Asia and western North Pacific have total δ56Fe values that are as low as −0.5 ‰ when compared to crustal (+0.1 ‰), with both the water-soluble phase and the fine particles even more fractionated (as low as −1.9 and −2.8 ‰, respectively). The negative correlation between the aerosol δ56Fe signatures and the enrichment factors of Fe and other elements dominated by anthropogenic sources (e.g., lead and cadmium) in these coastal regions indicates the presence of Fe emitted from high-temperature combustion sources, such as coal combustion and metal smelting. In these regions, combustion Fe accounts for 4–13 and 13–45 % of the total and water-soluble aerosol Fe, respectively. The results demonstrate that soluble aerosol Fe sourced from combustion Fe can be equivalent to that sourced from natural dust Fe in these coastal regions. By contrast, the aerosol particles in pelagic regions were near crustal δ56Fe in all particle size fractions, indicating the dominance of natural Fe and little to no combustion Fe. The relationships among the fractional Fe solubility, major ion concentration, Fe species, and δ56Fe indicate that the presence of combustion Fe is the dominant reason for the solubility increase in the coastal regions and that the atmospheric processing of mineral dust during transport is more important in the pelagic regions. The dissolved Fe of the surface seawater at 10 m depth had a consistently higher δ56Fe by up to +1.5 ‰ than that of the simultaneously collected water-soluble aerosol Fe. The pattern of the elevated δ56Fe in the surface seawater corresponds to decreasing Fe concentrations and can be approximated by Rayleigh fractionation; we attribute these elevated surface δ56Fe values to the effect of biological uptake. New Fe fluxes from both the atmosphere and deeper depths are limited at least in summer compared with the biological uptake in the open ocean of the subarctic North Pacific.
  • Itsuka Yabe, Shin-ichi Ito, Shigeho Kakehi, Takeyoshi Nagai, Jun Nishioka
    Scientific Reports 14 (1) 2024/07/30 
    Abstract The quasi-stationary jet, a branch of the Kuroshio Extension, transports warm saline water in the mixed water region of the western North Pacific. Around the subarctic front between the quasi-stationary jet and Oyashio and its downstream area is a biologically productive area including small pelagic fishes. However, how nutrient is supplied to the euphotic zone in this region remains elusive, especially into the quasi-stationary jet. Using high-resolution hydrography sections across the jet, we showed that Oyashio water isopycnally intrudes under the jet around 26.5–26.8 σθ and forms nutrient-rich intermediate water. Upwelling associated with ageostrophic secondary circulation across the front, caused by confluence, uplifts the intermediate water. A local nitrate maximum was also identified inside the jet by the hydrographic observation. Upwelling has been suggested as a precondition for nutrient supply from nutrient-rich intermediate water to the jet through water mixing which potentially sustains high biological production in the downstream.
  • Naoto Ebuchi, Yasushi Fukamachi, Kay I. Ohshima, Humio Mitsudera, Jun Nishioka, Toru Takatsuka, Kazuya Ono, Masao Ishikawa, Takaharu Daibo, Kunio Shirasawa, Masaaki Wakatsuchi
    Journal of Oceanography 0916-8370 2024/05/23
  • Mariko Honda, Kay I. Ohshima, Vigan Mensah, Jun Nishioka, Masatoshi Sato, Stephen C. Riser
    Journal of Oceanography 80 (4) 273 - 290 0916-8370 2024/05/11 
    Abstract This study provides the first estimation of sea ice-melt amount in the Sea of Okhotsk based on spring hydrographic data accumulated for nearly a hundred years. Just after sea ice melts completely, a low-salinity layer appears on the ocean surface, overlying the layer of Winter Water at the freezing point. The integration of the salinity decrease from Winter Water should correspond to the total ice-melt amount. We developed an algorithm to extract the profiles that clearly show the salinity deficit and converted the salinity deficit to the ice-melt amount from all available data. The climatological map shows that ice-melt amount decreases toward the ice edge and exhibits large values around the northern Sakhalin Island, reflecting the ice thickness distribution. In the southern area (south of 48°N), where sea ice is transported from the north, the average ice-melt amount is estimated to be ~ 71 cm in thickness. It is clearly shown that the ice-melt amount has decreased by ~ 30% in the southern area since the 1990s. These changes possibly affect the regional climate through the decreased latent heat of sea ice and potentially affect biological production through weakened stratification caused by decreased ice melt. We also suggested that ice-melt amount did not show a significant trend during the 1930s–1970s, implying that our methodology could extract information on sea ice before the era of satellite observations.
  • Joji Oida, Toru Hirawake, Youhei Yamashita, Hiroto Abe, Jun Nishioka, Hisatomo Waga, Daiki Nomura, Shigeho Kakehi
    Deep Sea Research Part I: Oceanographic Research Papers 104313 - 104313 0967-0637 2024/04
  • Kuo Hong Wong, Hajime Obata, Taejin Kim, Hirofumi Tazoe, Asami S. Mashio, Hiroshi Hasegawa, Jun Nishioka
    Marine Chemistry 260 0304-4203 2024/03 
    Dissolved zinc (dZn) is an important micronutrient for phytoplankton in the ocean. In high nutrient low chlorophyll (HNLC) regions like the western Bering Sea basin, Zn deficiency might induce shifts in phytoplankton community species. In this study, we presented the first dZn distribution data in the western Bering Sea and near Kamchatka Strait and investigated the biogeochemical cycles of Zn in these regions. Although dZn and silicate (Si) show similar distribution patterns in the global ocean, there is an excess of dZn relative to Si in the intermediate waters of our study region, similar to that previously observed in the subarctic Pacific. The uptake and subsequent regeneration patterns of dZn, Si, and phosphate (PO4) in the study region reveal that excess dZn in the subarctic Pacific is not derived from biogenic processes, because the uptake and regeneration ratios of dZn to phosphate (PO4) cannot account for the additional dZn supplied. Instead, our analyses indicate that a significant proportion of dZn in the region originates from the non-reductive release from shelf sediments. dZn concentrations were elevated in coastal regions at the same isopycnal where excess dZn was observed in the subarctic Pacific, suggesting that some of these sediment-derived dZn were transported to the subarctic Pacific.
  • Shigeyoshi Otosaka, Yunxuan Hou, Masahide Wakita, Youhei Yamashita, Jun Nishioka, Hajime Obata, Yosuke Miyairi, Yusuke Yokoyama, Hiroshi Ogawa
    Journal of Coastal Research 116 (sp1) 0749-0208 2024/01/04
  • Hajime Obata, Akira Mase, Naoya Kanna, Shigenobu Takeda, Jun Nishioka, Kenshi Kuma
    GEOCHEMICAL JOURNAL 58 (2) 71 - 79 0016-7002 2024
  • Mizuki Kuga, Kay I. Ohshima, Sachiko Kishi, Noriaki Kimura, Takenobu Toyota, Jun Nishioka
    Journal of Oceanography 0916-8370 2023/10/17
  • Kana Nagashima, Hajime Kawakami, Koji Sugie, Tetsuichi Fujiki, Jun Nishioka, Yoko Iwamoto, Toshihiko Takemura, Takuma Miyakawa, Fumikazu Taketani, Maki Noguchi Aita
    Scientific Reports 13 (1) 2023/09/29 [Refereed][Not invited]
     
    Abstract Iron availability limits marine ecosystem activities in large areas of the ocean. However, the sources and seasonal supply of iron, critically important for controlling surface ocean biogeochemistry and carbon cycling, are poorly understood. The western subarctic Pacific is a high-nutrient and low-chlorophyll region, and despite high concentrations of macronutrients, iron limits phytoplankton production in summer. Here, we determine the seasonal deposition flux of Asian dust using scanning electron microscope–cathodoluminescence analysis of single quartz particles derived from the western subarctic Pacific during 2003–2022 to trace provenance. We found a high (up to 6.9 mg m−2 day−1) deposition flux of Asian dust in May, June, and early July, with an annual average of 1.0 ± 0.2 mg m−2 day−1. The supply of dissolved-iron flux calculated from Asian dust was 0.9 ± 0.3 µg m−2 day−1 during the high productivity season (April–July), which is approximately half that from the deeper part of the ocean, calculated from vertical profiles of dissolved iron. Our study provides a reliable approach for estimating iron supply from dust to the surface ocean that may be critical for sustaining biological productivity under future ocean stratification, which suppresses nutrient supply from the subsurface ocean.
  • N. Ebuchi, Y. Fukamachi, K.I. Ohshima, T. Takatsuka, H. Mitsudera, J. Nishioka, K. Ono, M. Ishikawa, T. Daibo, K. Shiwasawa, M. Wakatsuchi
    IGARSS 2023 - 2023 IEEE International Geoscience and Remote Sensing Symposium 2023/07/16
  • Kazuhiro Yoshida, Jun Nishioka, Ichiro Yasuda, Koji Suzuki
    Journal of Oceanography 0916-8370 2023/06/15
  • Youhei Yamashita, Jun Nishioka
    Journal of Geophysical Research: Biogeosciences 2023/03 [Refereed][Not invited]
  • Liu, Kailin, Jun Nishioka, Bingzhang Chen, Koji Suzuki, Shunyan Cheung, Lu, Yanhong, Huijun Wu, Hongbin Liu
    Limnology and Oceanography 0024-3590 2023/01/16 [Refereed]
  • Takenobu Toyota, Noriaki Kimura, Jun Nishioka, Masato Ito, Daiki Nomura, Humio Mitsudera
    Journal of Geophysical Research: Oceans 127 (12) 2169-9275 2022/12 [Refereed][Not invited]
  • Naoya Kanna, Shin Sugiyama, Takuto Ando, Yefan Wang, Yuta Sakuragi, Toya Hazumi, Kohei Matsuno, Atsushi Yamaguchi, Jun Nishioka, Youhei Yamashita
    Global Biogeochemical Cycles 36 (11) 0886-6236 2022/11
  • Kaori Kawana, Yuzo Miyazaki, Yuko Omori, Hiroshi Tanimoto, Sara Kagami, Koji Suzuki, Youhei Yamashita, Jun Nishioka, Yange Deng, Hikari Yai, Michihiro Mochida
    Journal of Geophysical Research: Atmospheres 127 (19) 2169-897X 2022/10/16
  • Shunyan Cheung, Kailin Liu, Kendra A. Turk-Kubo, Jun Nishioka, Koji Suzuki, Michael R. Landry, Jonathan, P. Zehr, Szeki Leung, Lixia Deng, Hongbin Liu
    Limnology and Oceanography Letters 2022/07/05 [Refereed]
  • Dong Yan, Jun Nishioka, Takenobu Toyota, Koji Suzuki
    Progress in Oceanography 204 102806 - 102806 0079-6611 2022/06
  • Hirofumi Tazoe, Hajime Obata, Takuya Hara, Mutsuo Inoue, Takahiro Tanaka, Jun Nishioka
    Frontiers in Marine Science 9 2022/05/31 
    The vertical activity concentration distributions of the radium isotopes, 226Ra and 228Ra, which are useful as radiotracers, were obtained for the first time in the western Subarctic Pacific Gyre (WSAG). It was possible to detect short-lived 228Ra present from the surface to 400 m depth by analyzing large seawater samples. Low 228Ra and high 226Ra activity concentrations in the surface layer in the WSAG were strongly influenced by upwelled deeper water with nutrients. The activity concentration distribution of 226Ra especially was in good agreement with the silicate concentration distribution, which was consistent with previous reported findings. These distributions were uniform from the surface to 100 m of the dichothermal layer due to vertical mixing in winter. 228Ra activity concentration decreased with water depth below the pycnocline and reached the undetectable level at 600 m which was within the oxygen minimum layer. Estimations of vertical fluxes of 228Ra and nitrate according to 228Ra decay indicated that the vertical transport by eddy diffusion was a minor process for the 228Ra and nitrate fluxes, and lateral transport mainly affected 228Ra in the intermediate warm water, that is, the mesothermal layer below the main pycnocline. Vertical mixing or submarine underground water and lateral transport to the WSAG could yield this 228Ra in the intermediate depth.
  • Kuo Hong Wong, Hajime Obata, Jun Nishioka, Youhei Yamashita, Yoshiko Kondo, Taejin Kim, Asami Mashio, Hiroshi Hasegawa
    Limnology and Oceanography Bulletin 31 (2) 31 - 36 2022/05
  • Jun Nishioka, Ichiro Yasuda, Toru Hirawake, Tomohiro Nakamura, Yoshiko Kondo, Yuri N. Volkov
    Progress in Oceanography 203 0079-6611 2022/04
  • Kuo Hong Wong, Jun Nishioka, Taejin Kim, Hajime Obata
    Journal of Geophysical Research: Oceans 127 (2) 2169-9275 2022/02
  • Youhei Yamashita, Motohiro Nakane, Yutaro Mori, Jun Nishioka, Hiroshi Ogawa
    Nature Communications 2022/01/13 [Refereed][Not invited]
  • Yoko Iwamoto, Hidenori Aiki, Osamu Isoguchi, Yumiko Obayashi, Fumiyoshi Kondo, Yoshiko Kondo, and Jun Nishioka
    Oceanography in Japan 30 (5) 199 - 225 0916-8362 2021/11/15 [Refereed][Invited]
  • Michiyo Yamamoto-Kawai, Takeshi Tamura, Eiji Watanabe, Jun Nishioka, Daiki Nomura, Ryusuke Makabe, Kohei Mizobata, Yasunaka Sayaka
    Oceanography in Japan 30 (5) 159 - 178 0916-8362 2021/11/15
  • Diatoms contributing to new production in surface waters of the northern Bering and Chukchi Seas during summer with reference to water column stratification
    Koji Suzuki, Yuta Yoshino, Yuichi Nosaka, Jun Nishioka, Stanford B. Hooker, Toru Hirawake
    199 102692  2021/10
  • Yutaka W. Watanabe, Yuto Nakano, Jun Nishioka, Masanori Ito
    Progress in Oceanography 102693 - 102693 0079-6611 2021/10
  • Toru Hirawake, Joji Oida, Youhei Yamashita, Hisatomo Waga, Hiroto Abe, Jun Nishioka, Daiki Nomura, Hiromichi Ueno, Atsushi Ooki
    Progress in Oceanography 197 102641 - 102641 0079-6611 2021/09 [Refereed]
     
    The northern Bering Sea (NBS) and southern Chukchi Sea (SCS) contain several water masses with different characteristics that have been conventionally classified using temperature and salinity data. However, recent warming and sea ice decline can change these water properties, which suggests that classifying water masses using temperature–salinity diagrams could be problematic for this region. We used the light absorption coefficient of chromophoric dissolved organic matter (CDOM), aCDOM, as an alternative way to classify water masses. The aCDOM spectra of several water depths were measured in the NBS, the Gulf of Anadyr included, and SCS in July 2017 and 2018, and August 2018. Optical parameters that indicate CDOM quantity and quality, aCDOM(350) and spectral slopes (S275–295 and S350–400), were calculated for each sample; water masses were classified using cluster analysis. When surface waters in the NBS and SCS are classified on the basis of the conventional method using a temperature–salinity diagram, they are mistakenly identified as Alaskan Coastal Water because of warmer temperatures. However, our cluster analysis using CDOM parameters evenly classified seven water masses with reasonable distributions. A water mass with the highest aCDOM(350) and lowest S275–295 was found along the coast of the Gulf of Anadyr and Alaska mainland, which suggests that freshwater originates from the Anadyr and Yukon rivers and is transported by the Anadyr Current and Alaskan Coastal Current, respectively. A CDOM-based water mass with high S275–295, indicating CDOM degradation by ultraviolet radiation, was present at the sea surface. A water mass with low S275–295 was found at deeper water depths and river mouths. These results suggest that classification with CDOM parameters is consistent with geographical features. In addition, we recognized a water mass with high nitrate concentrations, which is likely Anadyr Water that originates in the region from the Bering Slope to the Gulf of Anadyr. Overall, this study reveals that water mass classification using CDOM parameters is useful in coastal sea areas in which water mass mixing is complex.
  • Takuya Nakanowatari, Tomohiro Nakamura, Humio Mitsudera, Jun Nishioka, Hiroshi Kuroda, Keisuke Uchimoto
    Progress in Oceanography 197 102615 - 102615 0079-6611 2021/09
  • Jun Nishioka, Toru Hirawake, Daiki Nomura, Youhei Yamashita, Kazuya Ono, Aiko Murayama, Alexey Shcherbinin, Yuri N. Volkov, Humio Mitsudera, Naoto Ebuchi, Masaaki Wakatsuchi, Ichiro Yasuda
    Progress in Oceanography 198 102662 - 102662 0079-6611 2021/08
  • Daiki Nomura, Hiroto Abe, Toru Hirawake, Atsushi Ooki, Youhei Yamashita, Aiko Murayama, Kazuya Ono, Jun Nishioka
    Progress in Oceanography 196 102595 - 102595 0079-6611 2021/08 
    We analyzed temperature and salinity data and oxygen isotope ratios (δ18O) in summer 2018 in order to quantify the formation of dense shelf water (DSW) associated with sea ice freezing in the Gulf of Anadyr in the northwestern Bering Sea. Salinity and δ18O displayed a strong linear relationship in samples upstream (southwest) of the Gulf of Anadyr. However, in bottom water (>26.5 σθ) in the northern Gulf of Anadyr, where a large polynya is the site of high sea ice production in winter, salinity was clearly higher than in the upstream water, whereas δ18O was in the same range. This deviation is consistent with the formation of sea ice through brine rejection and subsequent DSW formation, causing an increase of salinity without changing δ18O. A mass balance calculation based on these results yielded an estimated production of sea ice equivalent to a thickness 0.7 m, a result that matched an independent estimate of 2018 annual ice production based on satellite data. Even though the winter of 2018 had a historically low sea ice extent in the Bering Sea, including the Gulf of Anadyr, our results indicate continued DSW formation in the Gulf of Anadyr.
  • Jun Nishioka, Hajime Obata, Toru Hirawake, Yoshiko Kondo, Youhei Yamashita, Kazuhiro Misumi, Ichiro Yasuda
    Journal of Oceanography 77 (4) 561 - 587 0916-8370 2021/05/13 [Refereed]
     
    AbstractOne of the most important breakthroughs in oceanography in the last 30 years was the discovery that iron (Fe) controls biological production as a micronutrient, and our understanding of Fe and nutrient biogeochemical dynamics in the ocean has significantly advanced. In this review, we looked back both previous and updated knowledge of the natural Fe supply processes and nutrient dynamics in the subarctic Pacific and its impact on biological production. Although atmospheric dust has been considered to be the most important source of Fe affecting biological production in the subarctic Pacific, other oceanic sources of Fe have been discovered. We propose a coherent explanation for the biological response in subarctic Pacific high nutrient low chlorophyll (HNLC) waters that incorporates knowledge of both the atmospheric Fe supplies and the oceanic Fe supplies. Finally, we extract future directions for Fe oceanographic research in the subarctic Pacific and summarize the uncertain issues identified thus far.
  • Hirofumi Tazoe, Hiroshi Amakawa, Katsuhiko Suzuki, Jun Nishioka, Takuya Hara, Hajime Obata
    Talanta 232 122435 - 122435 0039-9140 2021/04 [Refereed]
     
    Determination of neodymium (Nd) isotopic composition in seawater is useful for tracing water masses and geochemical cycles for lithogenic elements in the ocean. A new separation procedure for determination of the Nd isotopic composition of in seawater samples was developed that offers enhanced sample throughput and improved measurement reliability. The procedure consists of conventional Fe hydroxide coprecipitation, solid phase extraction using DGA chelating resin column chromatography, and Ln Resin column chromatography to preconcentrate samples. High selectivity in HNO3 medium and elution by low concentration HCl medium for Nd are characteristics of extraction using DGA Resin®, and they allowed an evaporation step to be omitted between the chromatographic steps. These chromatographic steps, using DGA Resin to separate REEs and Ln Resin® to remove Sm, were refined from a previous study. The procedural blank value of Nd was obtained as 2 pg (n = 6) from 3 L of water samples. Chemical yield of Nd from 3 L of seawater ranged within 90-95%. The developed procedure was combined with multiple collector-ICP-MS and applied to analysis of vertical seawater samples obtained from the western subarctic gyre of the North Pacific Ocean, where εNd ranged from -1.29 ± 0.42 at the surface to -3.80 ± 0.41 at 4000 m depth. These results were validated by comparing them with results obtained by the conventional method verified in the GEOTRACES inter-calibration program.
  • Mitsuhide Sato, Jun Nishioka, Kazuyuki Maki, Shigenobu Takeda
    Marine Chemistry 233 103966 - 103966 0304-4203 2021/04 [Refereed]
  • K. Misumi, J. Nishioka, H. Obata, D. Tsumune, T. Tsubono, M. C. Long, K. Lindsay, J. K. Moore
    Global Biogeochemical Cycles 35 (4) 0886-6236 2021/04 [Refereed]
  • Yutaro Mori, Jun Nishioka, Shinzou Fujio, Youhei Yamashita
    Progress in Oceanography 193 102552 - 102552 0079-6611 2021/04 [Refereed]
     
    Pyrogenic carbon, also called black carbon (BC), is produced by biomass burning as well as by fossil fuel combustion and is an important slow-cycling component in the global carbon cycle. BC occurs in a fraction of marine dissolved organic carbon, and this is called dissolved BC (DBC). Marine DBC has been considered the ultimate repository for fluvial DBC; however, the environmental dynamics of marine DBC have not been well documented. In this study, vertical DBC profiles from the surface to the deep layers of the western subarctic Pacific and its marginal seas were analyzed using the benzenepolycarboxylic acid (BPCA) method. DBC concentrations tended to be highest in the surface layer and decreased with increasing depth, except for those in the Bussol? Strait located in the Kuril Islands between the Sea of Okhotsk and the North Pacific. Vertical distributions of the condensation degree index of DBC (i.e., the ratio of B5CA and B6CA to all BPCAs) did not show a general trend with depth. Atmospheric deposition of BC is likely a major source of DBC in the surface layer. In the intermediate layer, the DBC concentration and the condensation degree index in the Bussol? Strait were higher than those in the western subarctic Pacific. The occurrence of highly condensed DBC observed in the Bussol? Strait was accompanied by low salinity and a high DO concentration, indicating that DBC is transported from the shelf sediments of the Sea of Okhotsk to the Bussol? Strait by dense shelf water (DSW) and Okhotsk Sea Intermediate Water (OSIW). The DBC concentration and the condensation degree index in the intermediate layer of the Bering Sea and the Kamchatka Strait (located in the Aleutian Islands between the Bering Sea and the North Pacific) were higher than those in the western subarctic Pacific but lower than those in the Bussol? Strait; this suggests that DBC from the slope sediments in the Bering Sea is transported to the Kamchatka Strait by the East Kamchatka Current and that the DBC flux from the slope sediments of the Bering Sea is lower than that involving the DSW in the Sea of Okhotsk, which accompanies the resuspension of shelf sediments. The results of this study imply that sedimentary BC is an important source of water column DBC in marginal seas and the adjacent open ocean.
  • Ryo Dobashi, Hiromichi Ueno, Yuki Okada, Takahiro Tanaka, Jun Nishioka, Toru Hirawake, Atushi Ooki, Sachihiko Itoh, Daisuke Hasegawa, Yoshikazu Sasai, Hideharu Sasaki, Ichiro Yasuda
    Journal of Oceanography 77 (2) 229 - 242 0916-8370 2021/04 [Refereed]
     
    We conducted a ship observation of two anticyclonic mesoscale eddies (Eddies A and B) between June and July 2016 in the offshore region of the western subarctic North Pacific to clarify the impact of anticyclonic eddies on biological production. Analysis of satellite data reveals that Eddy A formed south of the Alaskan Peninsula on May 2011 while Eddy B formed in the observation area on November 2015. Iron concentration was low both inside and outside Eddies A and B, and there were no large differences between chlorophyll a (chl-a) concentration inside and outside eddies. Satellite-derived chl-a concentration inside Eddy A had been high compared with that in the surrounding area when the eddy was located along the Aleutian Islands and just after it detached from the islands, suggesting that iron originating from the coastal region was consumed during the two-year journey from the coastal region to the observation area. Field measurements also reveal that vertical diffusivity was low both inside and outside Eddies A and B, which could be another factor underlying low iron both inside and outside Eddies A and B and small differences between chl-a concentration inside and outside the eddies.
  • Kuo Hong Wong, Hajime Obata, Taejin Kim, Yoshiko Kondo, Jun Nishioka
    Limnology and Oceanography 66 (4) 1424 - 1439 0024-3590 2021/04 [Refereed]
  • S. Kishi, K. I. Ohshima, J. Nishioka, N. Isshiki, S. Nihashi, S. C. Riser
    Geophysical Research Letters 48 (6) 0094-8276 2021/03/28 [Refereed]
     
    Seven profiling floats equipped with oxygen sensors deployed in the Sea of Okhotsk provide time series data for 33 cases of spring phytoplankton bloom period, including 9 cases in which sea ice existed just before the bloom (prior-ice case). As an index of biological productivity, we calculated net community production (NCP) based on the increasing oxygen rate using the Redfield ratio. The total NCP in the euphotic layer averaged for prior-ice cases is 31.3 mmolC m(-2) day(-1)), similar to 3 times higher than that of non-ice cases. In addition to intensification of surface stratification, other factors of sea-ice melt likely enhance the bloom. The influence of sea-ice melt is particularly large in the southwestern region, where the iron availability likely limits phytoplankton growth. A suggested scenario is that when the sea ice containing sediment/iron is transported from the northern shelves, a prominent bloom is induced via the iron supply by sea-ice melt.
  • Wiwit, Kuo Hong Wong, Hideki Fukuda, Hiroshi Ogawa, Asami S. Mashio, Yoshiko Kondo, Jun Nishioka, Hajime Obata
    Marine Chemistry 230 103927 - 103927 0304-4203 2021/03 [Refereed]
  • Youhei Yamashita, Tetsu Tosaka, Rise Bamba, Ryuichi Kamezaki, Shuji Goto, Jun Nishioka, Ichiro Yasuda, Toru Hirawake, Joji Oida, Hajime Obata, Hiroshi Ogawa
    Progress in Oceanography 191 102510 - 102510 0079-6611 2021/02 [Refereed]
  • T. Tanaka, D. Hasegawa, I. Yasuda, D. Yanagimoto, S. Fujio, H. Nakamura, R. Inoue, J. Nishioka
    Journal of Oceanography 77 (1) 45 - 53 0916-8370 2021/02 [Refereed]
  • Ichiro Yasuda, Shuhei Masuda, Jun Nishioka, Xinyu Guo, Naomi Harada, Shin-ichi Ito, Toshiyuki Hibiya, Hiroyasu Hasumi
    Journal of Oceanography 77 (1) 1 - 1 0916-8370 2021/02
  • Miyazaki, Y., Suzuki, K., Tachibana, E., Yamashita, Y., M{\"u}ller, A., Kawana, K., Nishioka, J.
    Scientific Reports 11 (1) 2045-2322 2021
  • Kanna Naoya, Sugiyama Shin, Ando Takuto, Asaji Izumi, Yefan Wang, Sakuragi Yuta, Nishioka Jun, Yamashita Yohei
    Abstracts of Annual Meeting of the Geochemical Society of Japan 一般社団法人日本地球化学会 68 41  2021 
    海洋における腐植様蛍光物質(FDOMH)は、海洋の微量栄養物質である鉄と有機錯体を形成し、海水中に鉄を溶解させる役割を担う。FDOMHは、陸由来のものと海洋自生性のものに区別されるが、北大西洋では、北極海に注ぐ河川からのFDOMHの影響が大きいため、塩分とFDOMHの蛍光強度に負の相関がみられる。北大西洋に面するグリーンランド沿岸では、河川水に加えて陸氷(氷河、氷山)の融解水が流入する。この融解水が、海洋におけるFDOMHや鉄の動態にどのような影響を与えるかは不明である。本研究では、グリーンランド北西部のフィヨルドにおいて、氷河の融解水が海水中のFDOMHと鉄の動態に与える影響を調べた。
  • Yoshiko Kondo, Rise Bamba, Hajime Obata, Jun Nishioka, Shigenobu Takeda
    Scientific Reports 11 (1) 2021/01 [Refereed]
     
    AbstractIron (Fe) is well known as a limiting factor to control primary productivity especially in high-nutrient and low chlorophyll area such as the subarctic Pacific. The solubility of Fe is believed to be controlled by its complexation with natural organic ligands, while the distribution of organic ligands is poorly understood. Here, we report that dissolved (< 0.2 µm) organic ligands were unevenly distributed between the western and eastern stations in the subarctic Pacific. The concentration of dissolved organic ligands around the lower part of subarctic Pacific intermediate water was higher in the western station, suggesting that Fe complexation with these organic ligands supports a lateral transport within the water mass. However, a more detailed size-fractionated treatment indicated no significant difference in the soluble (< 1000 kDa) ligands’ distribution between the western and eastern stations. These results suggest that organic and inorganic colloid formations are potentially essential for Fe transport mechanisms in the subarctic Pacific.
  • Yuzo Miyazaki, Koji Suzuki, Eri Tachibana, Youhei Yamashita, Astrid Müller, Kaori Kawana, Jun Nishioka
    Scientific Reports 10 (1) 2045-2322 2020/12 [Refereed][Not invited]
     
    Abstract Linking the amount of organic matter (OM) in sea spray aerosols (SSAs) to biological processes in ocean surface is essential for understanding marine aerosol formation and their potential to affect cloud formation. To date, chlorophyll (Chl) a concentration has been widely used as a surrogate for surface phytoplankton biomass or productivity to predict the relative abundance of OM in SSAs (OMSSA). Here we show a new index to present OMSSA using concentrations of Chl a and chlorophyllide (Chllide) a, which is a breakdown product of Chl a and has been used as a biomarker of senescent algal cells. The index was compared with submicrometer OMSSA, based on surface seawater and aerosol samples obtained during the pre-bloom in the western subarctic Pacific. Our results showed that the OMSSA was highly correlated with this unique index, suggesting that the OMSSA was closely linked with senescent algal cells and/or cell lysis. Furthermore, the hygroscopicity parameters κ derived from water-extracted SSA samples implied a reduction in the SSA hygroscopicity with increasing senescent status of phytoplankton. The index can represent OMSSA on a timescale of a day during the pre-bloom period, which should be further examined over different oceanic regions.
  • Naoya Kanna, Shin Sugiyama, Yasushi Fukamachi, Daiki Nomura, Jun Nishioka
    Global Biogeochemical Cycles 34 (10) 0886-6236 2020/10 [Refereed][Not invited]
  • Yusuke Kawaguchi, Jun Nishioka, Shigeto Nishino, Shinzou Fujio, Keunjong Lee, Amane Fujiwara, Daigo Yanagimoto, Humio Mitsudera, Ichiro Yasuda
    Journal of Geophysical Research: Oceans 125 (9) 2169-9275 2020/09 [Refereed][Not invited]
  • Mutsuo Inoue, Ryosei Takehara, Shotaro Hanaki, Hiroaki Kameyama, Jun Nishioka, Seiya Nagao
    Marine Chemistry 225 103843 - 103843 0304-4203 2020/09 [Refereed][Not invited]
  • Dong Yan, Kazuhiro Yoshida, Jun Nishioka, Masato Ito, Takenobu Toyota, Koji Suzuki
    Frontiers in Marine Science 7 2020/07/24 [Refereed][Not invited]
     
    Phytoplankton communities in seasonally ice-covered areas are largely affected by ice algae. The Okhotsk Sea is the southernmost sea ice zone in the northern hemisphere with a sizeable seasonal ice cover, thus ice algae of the Okhotsk sea ice have a large potential to seed the early spring bloom. Little is known about the Okhotsk ice algal communities and their seeding effects. We investigated the dynamics of the composition and the photophysiological performances of an Okhotsk ice algal community in a 6-day laboratory incubation experiment that simulated the natural ice melt conditions. Centric diatoms, especially Thalassiosira spp., overwhelmingly dominated the ice algal community throughout incubation, whereas pennate diatoms, mostly Navicula and Nitzschia, showed little growth with much higher mortality. The maximum photochemical efficiency of Photosystem II (F-v/F-m) was the lowest at the beginning of the ice melt, suggesting a suppressed photosynthetic functioning by changes in salinity. The cellular pigment contents decreased by 30% due to osmotic stress, evidenced by deformed plastids under a microscope. The transcript level of the rbcL gene that encodes the large subunit of RubisCO was significantly higher during ice melt and decreased in the no-ice period, suggesting an urgent need for carbon fixation under the melt condition. Full recovery of photosynthesis and growth was also made after complete ice melt. Our results indicated high seeding potential of Thalassiosira to spring blooms owing to their photophysiological plasticity to dynamic salinity changes.
  • Jun Nishioka, Hajime Obata, Hiroshi Ogawa, Kazuya Ono, Youhei Yamashita, Keunjong Lee, Shigenobu Takeda, Ichiro Yasuda
    Proceedings of the National Academy of Sciences U.S.A 117 (23) 202000658 - 12673 0027-8424 2020/05/27 [Refereed][Not invited]
     
    The mechanism by which nutrients in the deep ocean are uplifted to maintain nutrient-rich surface waters in the subarctic Pacific has not been properly described. The iron (Fe) supply processes that control biological production in the nutrient-rich waters are also still under debate. Here, we report the processes that determine the chemical properties of intermediate water and the uplift of Fe and nutrients to the main thermocline, which eventually maintains surface biological productivity. Extremely nutrient-rich water is pooled in intermediate water (26.8 to 27.6 σθ) in the western subarctic area, especially in the Bering Sea basin. Increases of two to four orders in the upward turbulent fluxes of nutrients were observed around the marginal sea island chains, indicating that nutrients are uplifted to the surface and are returned to the subarctic intermediate nutrient pool as sinking particles through the biological production and microbial degradation of organic substances. This nutrient circulation coupled with the dissolved Fe in upper-intermediate water (26.6 to 27.0 σθ) derived from the Okhotsk Sea evidently constructs an area that has one of the largest biological CO2 drawdowns in the world ocean. These results highlight the pivotal roles of the marginal seas and the formation of intermediate water at the end of the ocean conveyor belt.
  • Toru Hirawake, Masaki Uchida, Hiroto Abe, Irene D. Alabia, Tamotsu Hoshino, Shota Masumoto, Akira S. Mori, Jun Nishioka, Bungo Nishizawa, Atsushi Ooki, Akinori Takahashi, Yukiko Tanabe, Motoaki Tojo, Masaharu Tsuji, Hiromichi Ueno, Hisatomo Waga, Yuuki Y. Watanabe, Atsushi Yamaguchi, Youhei Yamashita
    Polar Science in press 100533 - 100533 1873-9652 2020/05 [Refereed][Not invited]
  • Naoya Kanna, Delphine Lannuzel, Pier van der Merwe, Jun Nishioka
    Marine Chemistry 221 103774 - 103774 0304-4203 2020/04 [Refereed][Not invited]
  • Kazuhiro Yoshida, Suzu Nakamura, Jun Nishioka, Stanford B. Hooker, Koji Suzuki
    Journal of Geophysical Research: Biogeosciences 125 (3) 2020/03/11 [Refereed][Not invited]
  • Youhei Yamashita, Jun Nishioka, Hajime Obata, Hiroshi Ogawa
    Scientific Reports 10 (1) 2045-2322 2020/03 
    Iron is one of the key elements controlling phytoplankton growth in large areas of the global ocean. Aeolian dust has traditionally been considered the major external source of iron in the North Pacific. Recent studies have indicated that sedimentary iron from the shelf region of the Sea of Okhotsk has a strong impact on the iron distribution in the North Pacific, while the mechanism supporting its long-distance transport remains poorly understood. Here, we report that refractory shelf humic substances, which complex and carry dissolved iron, are transported conservatively at least 4000km from the shallow sediments of the Sea of Okhotsk to the subtropical North Pacific with the circulation of intermediate water. This result indicates that shelf humic substances are probably one of the key factors shaping the distribution of dissolved iron in the ocean interior.
  • Influence of Coastal Oyashio water on massive spring diatom blooms in the Oyashio area of the North Pacific Ocean
    Kuroda, H, Y. Toya, T. Watanabe, J. Nishioka, D. Hasegawa, Y. Taniuchi, A. Kuwata
    Progress in Oceanography 175 328 - 344 2019/05 [Refereed][Not invited]
  • Enhanced vertical turbulent nitrate flux in the Kuroshio across the Izu Ridge
    Tanaka, T, D Hasegawa, I Yasuda, H Tsuji, S Fujio, Y Goto, J Nishioka
    Journal of Oceanography 75 (2) 195 - 203 2019/01 [Refereed][Not invited]
  • Accumulation processes of trace metals into Arctic sea ice: distribution of Fe, Mn and Cd associated with ice structure
    Evans,L.K, J. Nishioka
    Marine Chemistry 209 36 - 47 2019/01 [Refereed][Not invited]
  • Winter iron supply processes fueling spring phytoplankton growth in a sub-polar marginal sea, the Sea of Okhotsk
    Kanna,Naoya, Yusuke Sibano, Tkenobu Toyota, Jun Nishioka
    Marine Chemistry 2018/08 [Refereed][Not invited]
  • Quantitative analysis of Fe, Mn and Cd from sea ice and seawater in the Chukchi Sea, Arctic Ocean
    La Kenya Evansa, Jun Nishioka
    Polar Science 17 50 - 58 2018/08 [Refereed][Not invited]
  • Chemical transfer of dissolved organic matter from surface seawater to sea spray water-soluble organic aerosol in the marine atmosphere
    Yuzo Miyazaki, Youhei Yamashita, Kaori Kawana, Eri Tachibana, Sara Kagami, Michihiro Mochida, Koji Suzuki, Jun Nishioka
    Scientific reports 8 (1) 14861  2018 [Refereed][Not invited]
  • Responses of phytoplankton assemblages to iron availability and mixing water masses during the spring bloom in the Oyashio region, NW Pacific
    T Isada, A Hattori‐Saito, H Saito, Y Kondo, J Nishioka, K Kuma, H Hattori, RML McKay, K Suzuki
    Limnology and Oceanography 2018 [Refereed][Not invited]
  • Reiner Schlitzer, Robert F. Anderson, Elena Masferrer Dodas, Maeve Lohan, Walter Geibert, Alessandro Tagliabue, Andrew Bowie, Catherine Jeandel, Maria T. Maldonado, William M. Landing, Donna Cockwell, Cyril Abadie, Wafa Abouchami, Eric P. Achterberg, Alison Agather, Ana Aguliar-Islas, Hendrik M. van Aken, Morten Andersen, Corey Archer, Maureen Auro, Hein J. de Baar, Oliver Baars, Alex R. Baker, Karel Bakker, Chandranath Basak, Mark Baskaran, Nicholas R. Bates, Dorothea Bauch, Pieter van Beek, Melanie K. Behrens, Erin Black, Katrin Bluhm, Laurent Bopp, Heather Bouman, Katlin Bowman, Johann Bown, Philip Boyd, Marie Boye, Edward A. Boyle, Pierre Branellec, Luke Bridgestock, Guillaume Brissebrat, Thomas Browning, Kenneth W. Bruland, Hans-Jürgen Brumsack, Mark Brzezinski, Clifton S. Buck, Kristen N. Buck, Ken Buesseler, Abby Bull, Edward Butler, Pinghe Cai, Patricia Cámara Mor, Damien Cardinal, Craig Carlson, Gonzalo Carrasco, Núria Casacuberta, Karen L. Casciotti, Maxi Castrillejo, Elena Chamizo, Rosie Chance, Matthew A. Charette, Joaquin E. Chaves, Hai Cheng, Fanny Chever, Marcus Christl, Thomas M. Church, Ivia Closset, Albert Colman, Tim M. Conway, Daniel Cossa, Peter Croot, Jay T. Cullen, Gregory A. Cutter, Chris Daniels, Frank Dehairs, Feifei Deng, Huong Thi Dieu, Brian Duggan, Gabriel Dulaquais, Cynthia Dumousseaud, Yolanda Echegoyen-Sanz, R. Lawrence Edwards, Michael Ellwood, Eberhard Fahrbach, Jessica N. Fitzsimmons, A. Russell Flegal, Martin Q. Fleisher, Tina van de Flierdt, Martin Frank, Jana Friedrich, Francois Fripiat, Henning Fröllje, Stephen J.G. Galer, Toshitaka Gamo, Raja S. Ganeshram, Jordi Garcia-Orellana, Ester Garcia-Solsona, Melanie Gault-Ringold, Ejin George, Loes J.A. Gerringa, Melissa Gilbert, Jose M. Godoy, Steven L. Goldstein, Santiago R. Gonzalez, Karen Grissom, Chad Hammerschmidt, Alison Hartman, Christel S. Hassler, Ed C. Hathorne, Mariko Hatta, Nicholas Hawco, Christopher T. Hayes, Lars-Eric Heimbürger, Josh Helgoe, Maija Heller, Gideon M. Henderson, Paul B. Henderson, Steven van Heuven, Peng Ho, Tristan J. Horner, Yu-Te Hsieh, Kuo-Fang Huang, Matthew P. Humphreys, Kenji Isshiki, Jeremy E. Jacquot, David J. Janssen, William J. Jenkins, Seth John, Elizabeth M. Jones, Janice L. Jones, David C. Kadko, Rick Kayser, Timothy C. Kenna, Roulin Khondoker, Taejin Kim, Lauren Kipp, Jessica K. Klar, Maarten Klunder, Sven Kretschmer, Yuichiro Kumamoto, Patrick Laan, Marie Labatut, Francois Lacan, Phoebe J. Lam, Myriam Lambelet, Carl H. Lamborg, Frédéric A.C. Le Moigne, Emilie Le Roy, Oliver J. Lechtenfeld, Jong-Mi Lee, Pascale Lherminier, Susan Little, Mercedes López-Lora, Yanbin Lu, Pere Masque, Edward Mawji, Charles R. Mcclain, Christopher Measures, Sanjin Mehic, Jan-Lukas Menzel Barraqueta, Pier van der Merwe, Rob Middag, Sebastian Mieruch, Angela Milne, Tomoharu Minami, James W. Moffett, Gwenaelle Moncoiffe, Willard S. Moore, Paul J. Morris, Peter L. Morton, Yuzuru Nakaguchi, Noriko Nakayama, John Niedermiller, Jun Nishioka, Akira Nishiuchi, Abigail Noble, Hajime Obata, Sven Ober, Daniel C. Ohnemus, Jan van Ooijen, Jeanette O'Sullivan, Stephanie Owens, Katharina Pahnke, Maxence Paul, Frank Pavia, Leopoldo D. Pena, Brian Peters, Frederic Planchon, Helene Planquette, Catherine Pradoux, Viena Puigcorbé, Paul Quay, Fabien Queroue, Amandine Radic, S. Rauschenberg, Mark Rehkämper, Robert Rember, Tomas Remenyi, Joseph A. Resing, Joerg Rickli, Sylvain Rigaud, Micha J.A. Rijkenberg, Stephen Rintoul, Laura F. Robinson, Montserrat Roca-Martí, Valenti Rodellas, Tobias Roeske, John M. Rolison, Mark Rosenberg, Saeed Roshan, Michiel M. Rutgers van der Loeff, Evgenia Ryabenko, Mak A. Saito, Lesley A. Salt, Virginie Sanial, Geraldine Sarthou, Christina Schallenberg, Ursula Schauer, Howie Scher, Christian Schlosser, Bernhard Schnetger, Peter Scott, Peter N. Sedwick, Igor Semiletov, Rachel Shelley, Robert M. Sherrell, Alan M. Shiller, Daniel M. Sigman, Sunil Kumar Singh, Hans A. Slagter, Emma Slater, William M. Smethie, Helen Snaith, Yoshiki Sohrin, Bettina Sohst, Jeroen E. Sonke, Sabrina Speich, Reiner Steinfeldt, Gillian Stewart, Torben Stichel, Claudine H. Stirling, Johnny Stutsman, Gretchen J. Swarr, James H. Swift, Alexander Thomas, Kay Thorne, Claire P. Till, Ralph Till, Ashley T. Townsend, Emily Townsend, Robyn Tuerena, Benjamin S. Twining, Derek Vance, Sue Velazquez, Celia Venchiarutti, Maria Villa-Alfageme, Sebastian M. Vivancos, Antje H.L. Voelker, Bronwyn Wake, Mark J. Warner, Ros Watson, Evaline van Weerlee, M. Alexandra Weigand, Yishai Weinstein, Dominik Weiss, Andreas Wisotzki, E. Malcolm S. Woodward, Jingfeng Wu, Yingzhe Wu, Kathrin Wuttig, Neil Wyatt, Yang Xiang, Ruifang C. Xie, Zichen Xue, Hisayuki Yoshikawa, Jing Zhang, Pu Zhang, Ye Zhao, Linjie Zheng, Xin-Yuan Zheng, Moritz Zieringer, Louise A. Zimmer, Patrizia Ziveri, Patricia Zunino, Cheryl Zurbrick
    Chemical Geology 493 210 - 223 0009-2541 2018 [Refereed][Not invited]
     
    The GEOTRACES Intermediate Data Product 2017 (IDP2017) is the second publicly available data product of the international GEOTRACES programme, and contains data measured and quality controlled before the end of 2016. The IDP2017 includes data from the Atlantic, Pacific, Arctic, Southern and Indian oceans, with about twice the data volume of the previous IDP2014. For the first time, the IDP2017 contains data for a large suite of biogeochemical parameters as well as aerosol and rain data characterising atmospheric trace element and isotope (TEI) sources. The TEI data in the IDP2017 are quality controlled by careful assessment of intercalibration results and multi-laboratory data comparisons at crossover stations. The IDP2017 consists of two parts: (1) a compilation of digital data for more than 450 TEIs as well as standard hydrographic parameters, and (2) the eGEOTRACES Electronic Atlas providing an on-line atlas that includes more than 590 section plots and 130 animated 3D scenes. The digital data are provided in several formats, including ASCII, Excel spreadsheet, netCDF, and Ocean Data View collection. Users can download the full data packages or make their own custom selections with a new on-line data extraction service. In addition to the actual data values, the IDP2017 also contains data quality flags and 1-σ data error values where available. Quality flags and error values are useful for data filtering and for statistical analysis. Metadata about data originators, analytical methods and original publications related to the data are linked in an easily accessible way. The eGEOTRACES Electronic Atlas is the visual representation of the IDP2017 as section plots and rotating 3D scenes. The basin-wide 3D scenes combine data from many cruises and provide quick overviews of large-scale tracer distributions. These 3D scenes provide geographical and bathymetric context that is crucial for the interpretation and assessment of tracer plumes near ocean margins or along ridges. The IDP2017 is the result of a truly international effort involving 326 researchers from 25 countries. This publication provides the critical reference for unpublished data, as well as for studies that make use of a large cross-section of data from the IDP2017. This article is part of a special issue entitled: Conway GEOTRACES - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.
  • Takeshi Yoshimura, Jun Nishioka, Hiroshi Ogawa, Atsushi Tsuda
    JOURNAL OF MARINE SYSTEMS 177 1 - 7 0924-7963 2018/01 [Refereed][Not invited]
     
    Phosphorus (P) is an essential element for all organisms and thus the P cycle plays a key role in determining the dynamics of lower trophic levels in marine ecosystems. P in seawater occurs conceptually in particulate and dissolved organic and inorganic (POP, PIP, DOP, and DIP, respectively) pools and clarification of the dynamics in these P pools is the basis to assess the biogeochemical cycle of P. Despite its importance, behaviors of each P pool with phytoplankton dynamics have not been fully examined. We measured the four operationally defined P pools (POPop, PIPop, DOPop, and SRP) during an iron-induced phytoplankton bloom (as part of the subarctic ecosystem response to iron enrichment study (SERIES)) in the eastern subarctic Pacific in summer 2002. During our observations of the iron-enriched patch from day 15 to day 26 after the iron infusion, chlorophyll-alpha concentration in the surface layer decreased from 6.3 to 1.2 mu g L-1, indicating the peak through decline phase of the phytoplankton bloom. At the bloom peak, P was partitioned into POPop, PIPop, and DOPop in proportions of 60, 27, and 13%, respectively. While chlorophyll-alpha and POPop showed similar temporal variations during the declining phase, PIPop showed a different peak timing with a 2 day delay compared to POPop, resulting in a rapid change in the relative proportion of PIPop to total particulate P (TPP = POPop + PIPop) at the peak (25%) and during the declining phase of the bloom (50%). A part of POPop was replaced by PIPop just after slowing down of phytoplankton growth. This process may have a significant role in the subsequent regeneration of P. We conclude that measurement of TPP alone is insufficient to show the interaction between P and phytoplankton dynamics and fractionation of TPP into POPop and PIPop provides useful insights to clarify the biogeochemical cycle of P.
  • Hisashi Endo, Hiroshi Hattori, Tsubasa Mishima, Gen Hashida, Hiroshi Sasaki, Jun Nishioka, Koji Suzuki
    Polar Biology 41 (3) 397  0722-4060 2017/11/01 [Refereed][Not invited]
     
    The original article shows TDFe in Table 1 as µmol L−1. The correct TDFe in Table 1 should be nmol L−1.
  • Hisashi Endo, Hiroshi Hattori, Tsubasa Mishima, Gen Hashida, Hiroshi Sasaki, Jun Nishioka, Koji Suzuki
    POLAR BIOLOGY 40 (11) 2143 - 2159 0722-4060 2017/11 [Refereed][Not invited]
     
    The ongoing rise in atmospheric CO2 concentration is causing rapid increases in seawater pCO(2) levels. However, little is known about the potential impacts of elevated CO2 availability on the phytoplankton assemblages in the Southern Ocean's oceanic regions. Therefore, we conducted four incubation experiments using surface seawater collected from the subantarctic zone (SAZ) and the subpolar zone (SPZ) in the Australian sector of the Southern Ocean during the austral summer of 2011-2012. For incubations, FeCl3 solutions were added to reduce iron (Fe) limitation for phytoplankton growth. Ambient and high (similar to 750 mu atm) CO2 treatments were then prepared with and without addition of CO2-saturated seawater, respectively. Non-Fe-added (control) treatments were also prepared to assess the effects of Fe enrichment (overall, control, Fe-added, and Fe-and-CO2-added treatments). In the initial samples, the dominant phytoplankton taxa shifted with latitude from haptophytes to diatoms, likely reflecting silicate availability in the water. Under Fe-enriched conditions, increased CO2 level significantly reduced the accumulation of biomarker pigments in haptophytes in the SAZ and AZ, whereas a significant decrease in diatom markers was only detected in the SAZ. The CO2-related changes in phytoplankton community composition were greater in the SAZ, most likely due to the decrease in coccolithophore biomass. Our results suggest that an increase in CO2, if it coincides with Fe enrichment, could differentially affect the phytoplankton community composition in different geographical regions of the Southern Ocean, depending on the locally dominant taxa and environmental conditions.
  • Jun Nishioka, Hajime Obata
    Limnology and Oceanography 62 (5) 2004 - 2022 1939-5590 2017/09/01 [Refereed][Not invited]
     
    It is well known that phytoplankton growth is broadly limited by iron (Fe) availability in the subarctic Pacific. To investigate which Fe sources control the amplitude of seasonal variation in biogeochemical parameters in the subarctic Pacific, we examined the spatial variation in the west-to-east distribution of dissolved Fe (DFe) across the western and central subarctic Pacific through the Japanese GEOTRACES program. The vertical section profile of the western subarctic Pacific gyre showed high dissolved Fe concentrations from the bottom of the surface mixed layer to as deep as approximately 3000 m, suggesting that Fe-rich intermediate water is transported laterally and distributed across the western subarctic gyre, over 2000 km. The section data also indicate that the influence of the western Fe-rich intermediate water does not reach the Alaskan gyre. Fe* index (= [DFe] (observed) – [PO4] (observed) × RFe : P) distribution clearly indicates that the western Fe-rich intermediate water is well explained by external sedimentary Fe sources and water transport systems from a subpolar marginal sea, the Sea of Okhotsk, and the continental margin. The spatial pattern of Fe to nutrient stoichiometry supplied from the intermediate water to the surface, in comparison with reported Fe and nutrient demand in surface phytoplankton, quantitatively explains the differences in surface macronutrient consumption between the western and eastern gyre as well as the formation of the high nutrient and low chlorophyll region in the whole subarctic Pacific.
  • Jun Nishioka, Hajime Obata
    LIMNOLOGY AND OCEANOGRAPHY 62 (5) 2004 - 2022 0024-3590 2017/09 [Refereed][Not invited]
     
    It is well known that phytoplankton growth is broadly limited by iron (Fe) availability in the subarctic Pacific. To investigate which Fe sources control the amplitude of seasonal variation in biogeochemical parameters in the subarctic Pacific, we examined the spatial variation in the west-to-east distribution of dissolved Fe (DFe) across the western and central subarctic Pacific through the Japanese GEOTRACES program. The vertical section profile of the western subarctic Pacific gyre showed high dissolved Fe concentrations from the bottom of the surface mixed layer to as deep as approximately 3000 m, suggesting that Fe-rich intermediate water is transported laterally and distributed across the western subarctic gyre, over 2000 km. The section data also indicate that the influence of the western Fe-rich intermediate water does not reach the Alaskan gyre. Fe-star index (5 [DFe] (observed) - [PO4] (observed) x RFe : P) distribution clearly indicates that the western Fe-rich intermediate water is well explained by external sedimentary Fe sources and water transport systems from a subpolar marginal sea, the Sea of Okhotsk, and the continental margin. The spatial pattern of Fe to nutrient stoichiometry supplied from the intermediate water to the surface, in comparison with reported Fe and nutrient demand in surface phytoplankton, quantitatively explains the differences in surface macro-nutrient consumption between the western and eastern gyre as well as the formation of the high nutrient and low chlorophyll region in the whole subarctic Pacific.
  • Taejin Kim, Hajime Obata, Jun Nishioka, Toshitaka Gamo
    GLOBAL BIOGEOCHEMICAL CYCLES 31 (9) 1454 - 1468 0886-6236 2017/09 [Refereed][Not invited]
     
    We investigated the biogeochemical cycling of dissolved zinc (Zn) in the western and central subarctic North Pacific during the GEOTRACES GP 02 cruise. The relationship between dissolved Zn and silicate in the subarctic North Pacific plotted as a concave curve. Values of Zn* were strongly positive in the intermediate waters (26.6-27.5 sigma(theta)) of both the western and the central subarctic North Pacific. There was a distinct kink in the relationship between dissolved Zn and soluble reactive phosphorus (SRP) at the transition from shallow to intermediate water, which is similar to what has been reported for other open oceans. The high Zn: SRP ratio and high Zn* in the intermediate water suggest that intermediate water masses play an important role in the decoupling of dissolved Zn and silicate in the subarctic North Pacific, which implies that the biogeochemical processes that control dissolved Zn and silicate in the intermediate water are different from those in other oceanic regions.
  • T. Tanaka, I. Yasuda, K. Kuma, J. Nishioka
    CONTINENTAL SHELF RESEARCH 143 130 - 138 0278-4343 2017/07 [Refereed][Not invited]
     
    The Green Belt (GB) in the southeastern Bering Sea lying along the continental slope is a biological hotspot where summertime high primary production is sustained by continuous input of nutrients and iron. To understand the mechanisms to sustain the GB, we need to know how dissolved iron (D-Fe), which regulates the GB production, is drawn from the abundant source in the adjacent shelf should be clarified, but no quantification has ever been done yet. In the present paper, using hydrographic and D-Fe data taken by a cruise and hydrographic database, we estimate horizontal D-Fe flux from the outer-shelf along 25.4 sigma(theta) and 26.2 sigma(theta) density surfaces, which are proposed as possible pathways by previous studies. The hydrographic data shows that the cold outer-shelf water is distributed in the slope region, and we estimate that 10% (65%) of the water mass in the slope is originated from the outer-shelf at 25.4 (26.2) sigma(theta). Assuming that this portion of the along slope geostrophic transport is derived from the outer-shelf through horizontal isopycnal mixing, and using the observed D-Fe concentration, we estimate the D-Fe flux of O(10(3)) molFe/day at 25.4 sigma(theta) and O(10(4)) molFe/day at 26.2 sigma(theta). The large flux at 26.2 sigma(theta) is consistent with the vertical maximum of D-Fe concentration previously observed off the shelf break at this density range, and the flux provides sufficient iron into the euphotic zone via the subsequent enhanced vertical mixing off the shelf break, which is estimated to be O(10(3)) molFe/day based on our prior studies. Since our estimated D-Fe flux through horizontal mixing at 25.4 sigma(theta) and the vertical mixing off the shelf break altogether are comparable to the minimum D-Fe requirement by phytoplankton in the GB, which is estimated as O(10(3)-10(4)) molFe/day, we suggest that both processes could play important roles in providing D-Fe to the euphotic zone in the GB.
  • Hongmei Jing, Shunyan Cheung, Xiaomin Xia, Koji Suzuki, Jun Nishioka, Hongbin Liu
    FRONTIERS IN MICROBIOLOGY 8 1664-302X 2017/06 [Refereed][Not invited]
     
    Community composition and abundance of ammonia-oxidizing archaea (AOA) in the ocean were affected by different physicochemical conditions, but their responses to physical barriers (such as a chain of islands) were largely unknown. In our study, geographic distribution of the AOA from the surface photic zone to the deep bathypelagic waters in the western subarctic Pacific adjacent to the Kuril Islands was investigated using pyrosequencing based on the ammonia monooxygenase subunit A (amoA) gene. Genotypes of clusters A and B dominated in the upper euphotic zone and the deep waters, respectively. Quantitative PCR assays revealed that the occurrence and ammonia-oxidizing activity of ammonia-oxidizing archaea (AOA) reached their maxima at the depth of 200 m, where a higher diversity and abundance of actively transcribed AOA was observed at the station located in the marginal sea exposed to more terrestrial input. Similar community composition of AOA observed at the two stations adjacent to the Kuril Islands maybe due to water exchange across the Bussol Strait. They distinct from the station located in the western subarctic gyre, where sub-cluster WCAII had a specific distribution in the surface water, and this sub-cluster seemed having a confined distribution in the western Pacific. Habitat-specific groupings of different WCB sub-clusters were observed reflecting the isolated microevolution existed in cluster WCB. The effect of the Kuril Islands on the phylogenetic composition of AOA between the Sea of Okhotsk and the western subarctic Pacific is not obvious, possibly because our sampling stations are near to the Bussol Strait, the main gateway through which water is exchanged between the Sea of Okhotsk and the Pacific. The vertical and horizontal distribution patterns of AOA communities among stations along the Kuril Islands were essentially determined by the in situ prevailing physicochemical gradients along the two dimensions.
  • Hajime Obata, Jun Nishioka, Taejin Kim, Kazuhiro Norisuye, Shigenobu Takeda, Yohei Wakuta, Toshitaka Gamo
    JOURNAL OF OCEANOGRAPHY 73 (3) 333 - 344 0916-8370 2017/06 [Refereed][Not invited]
     
    Using a clean seawater sampling system for trace metals onboard the R. V. Shinsei-Maru, newly launched in 2013, we investigated the vertical distributions of dissolved iron and zinc in Sagami Bay and the Izu-Ogasawara Trench. We applied appropriate clean sampling and filtering processes for trace metals, so that uncontaminated seawater samples were successfully collected. The distribution of zinc in the trench area was similar to that of silicate and the same as that previously reported in the subtropical North Pacific. There were spatial variations in the iron (Fe) distribution in the trench areas. We used previously reported information about biogeochemical cycling in the trench area, and found that Fe has a residence time of 29 years in the water column. The short residence time of Fe (29 years) corresponds to the vertical variations of dissolved Fe in the water column.
  • Toru Hirawake, Shintaro Takao, Koji Suzuki, Jun Nishioka, Yutaka W. Watanabe, Tomonori Isada
    Oceanography in Japan 26 (3) 65 - 77 0916-8362 2017/05/15
  • Takuya Nakanowatari, Tomohiro Nakamura, Keisuke Uchimoto, Jun Nishioka, Humio Mitsudera, Masaaki Wakatsuchi
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 122 (5) 4364 - 4391 2169-9275 2017/05 [Refereed][Not invited]
     
    Iron (Fe) is an essential nutrient for marine phytoplankton and it constitutes an important element in the marine carbon cycle in the ocean. This study examined the mechanisms controlling seasonal variation of dissolved Fe (dFe) in the western subarctic North Pacific (WSNP), using an ocean general circulation model coupled with a simple biogeochemical model incorporating a dFe cycle fed by two major sources (atmospheric dust and continental shelf sediment). The model reproduced the seasonal cycle of observed concentrations of dFe and macronutrients at the surface in the Oyashio region with maxima in winter (February-March) and minima in summer (July-September), although the simulated seasonal amplitudes are a half of the observed values. Analysis of the mixed-layer dFe budget indicated that both local vertical entrainment and lateral advection are primary contributors to the wintertime increase in dFe concentration. In early winter, strengthened northwesterly winds excite southward Ekman transport and Ekman upwelling over the western subarctic gyre, transporting dFe-rich water southward. In mid to late winter, the southward western boundary current of the subarctic gyre and the outflow from the Sea of Okhotsk also bring dFe-rich water to the Oyashio region. The contribution of atmospheric dust to the dFe budget is several times smaller than these ocean transport processes in winter. These results suggest that the westerly wind-induced Ekman transport and gyre circulation systematically influence the seasonal cycle of WSNP surface dFe concentration.
  • Masato Ito, Kay I. Ohshima, Yasushi Fukamachi, Genta Mizuta, Yoshimu Kusumoto, Jun Nishioka
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 122 (2) 788 - 802 2169-9275 2017/02 [Refereed][Not invited]
     
    In the Sea of Okhotsk, sediment incorporation, transport and release by sea ice potentially plays important roles in the bio-related material (such as iron) cycle and ecosystem. The backscatter strength data of bottom-mounted Acoustic Doppler Current Profilers have suggested signals of frazil ice down to 30 m depth, and signals of upward sediment transport throughout the water column simultaneously in the region northeast of Sakhalin, with a water depth of similar to 100 m. Such events occurred under turbulent conditions with strong winds of 10-20 m s(-1). During such events, newly formed ice was present near the observational sites, shown by satellite microwave imagery. Sediment dispersion from the bottom occurred in association with strong currents of 1.0-1.5 m s(-1). During these events, the mixed layer reaches near the bottom due to wind-induced stirring, inferred from the high frequency component of vertical velocity. Thus the winter time turbulent mixing brings re-suspended sediment up to near the ocean surface. This study provides the first observational evidence of a series of processes on the incorporation of sedimentary materials into sea ice: sedimentary particles are dispersed by the strong bottom current, subsequently brought up to near the surface by winter time mixing, and finally incorporated into sea ice through underwater interaction with frazil ice and/ or flooding of sea ice floes. This wintertime incorporation of bottom sediment into sea ice is a possible mechanism of iron supply to sea ice which melts in spring, and releases bio-reactive iron into the ocean.
  • 衛星による海洋基礎生産力の推定
    平譯 享, 高尾信太郎, 鈴木光次, 西岡 純, 渡邉 豊, 伊佐田 智規
    海の研究 26 (3) - - - 2017 [Refereed][Not invited]
  • 北太平洋亜寒帯における鉄の供給過程
    小畑 元, 金 泰辰, 西岡 純
    海の研究 26 (3) 79 - 93 2017 [Refereed][Not invited]
  • Hirofumi Tazoe, Hajime Obata, Masatoshi Tomita, Shinya Namura, Jun Nishioka, Takeyasu Yamagata, Zin'ichi Karube, Masatoshi Yamada
    GEOCHEMICAL JOURNAL 51 (2) 193 - 197 0016-7002 2017 [Refereed][Not invited]
     
    Sr-90 released into seawater from nuclear bomb testing and nuclear facility accident sites requires long-term monitoring. However, measuring Sr-90 in seawater is still difficult and time-consuming. This study is aimed at improving the preconcentration method for successive purification using DGA Resin chromatography, which is applied to Sr-90 analysis in seawater. At natural pH, the oxalate coprecipitation technique effectively collected Sr (84% in Sr yield) from seawater without Mg or Na. For Sr-90 determination, Y-90 in secular equilibrium with Sr-90 was directly extracted using Fe coprecipitation and DGA Resin chromatography. The proposed method simplifies the analytical processes associated with the conventional method. Analytical results for surface seawater in the western North Pacific (0.81 +/- 0.06 Bq m(-3), n = 5) were consistent with those obtained using the conventional method (0.81 +/- 0.07 Bq m(-3), n = 5).
  • Naoya Kanna, Jun Nishioka
    MARINE CHEMISTRY 186 189 - 197 0304-4203 2016/11 [Refereed][Not invited]
     
    Sea ice contains high concentrations of iron (Fe), and melting of sea ice is one of the possible processes that supplies Fe to surface waters (e.g., Lannuzel et al., 2007). To assess the biological availability (bio-availability) of Fe in sea ice, a shipboard bottle incubation experiment (Exp. I) was conducted with surface water from the western subarctic North Pacific and Fe derived from sea ice collected from the Sea of Okhotsk, as compared to Fe derived from inorganic Fe (FeCl3). Addition of FeCl3 solution stimulated the growth of phytoplankton relative to a control treatment. Addition of sea ice meltwater enhanced the particulate labile Fe (>0.22 mu m) by 112 nM and 0.26 nM of dissolved Fe (<0.22 mu m) in the incubated seawater and allowed the phytoplankton to grow, especially large (>10 mu m) phytoplankton, at their maximum growth rate in the incubated seawater. In contrast, addition of desferrioxamine B (DFB) plus sea ice meltwater, in which the strong ligand DFB reduced the bio-availability of Fe, significantly depressed the growth of the large and small (0.8-10 mu m) phytoplankton relative to the control. These results clearly demonstrated that Fe stored in the sea ice stimulates growth of the phytoplankton. We carried out another shipboard bottle incubation experiment (Exp. II) to determine the relationship between concentrations of Fe and phytoplankton growth rates by adding FeCl3 solution at a series of concentrations to the seawater. The results indicated that the amount of dissolved Fe that the incubation bottles received from addition of the sea ice meltwater in Exp. I was insufficient to saturate the growth rate of the large phytoplankton. Therefore, the maximum growth rate achievedby the large phytoplankton can be accounted for by the use of the particulate labile Fe released from the sea ice meltwater. These results indicate that Fe stored in sea, ice (mainly in particulate form) is bio-available and can contribute to phytoplankton growth when it is released into surface waters in the spring. (C) 2016 Elsevier B.V. All rights reserved.
  • D. Lannuzel, M. Vancoppenolle, P. Van Der Merwe, J. De Jong, K. M. Meiners, M. Grotti, J. Nishioka, V. Schoemann
    Elementa 4 2325-1026 2016/10/27 [Refereed][Not invited]
     
    The discovery that melting sea ice can fertilize iron (Fe)-depleted polar waters has recently fostered trace metal research efforts in sea ice. The aim of this review is to summarize and synthesize the current understanding of Fe biogeochemistry in sea ice. To do so, we compiled available data on particulate, dissolved, and total dissolvable Fe (PFe, DFe and TDFe, respectively) from sea-ice studies from both polar regions and from sub-Arctic and northern Hemisphere temperate areas. Data analysis focused on a circum-Antarctic Fe dataset derived from 61 ice cores collected during 10 field expeditions carried out between 1997 and 2012 in the Southern Ocean. Our key findings are that 1) concentrations of all forms of Fe (PFe, DFe, TDFe) are at least a magnitude larger in fast ice and pack ice than in typical Antarctic surface waters 2) DFe, PFe and TDFe behave differently when plotted against sea-ice salinity, suggesting that their distributions in sea ice are driven by distinct, spatially and temporally decoupled processes 3) DFe is actively extracted from seawater into growing sea ice 4) fast ice generally has more Fe-bearing particles, a finding supported by the significant negative correlation observed between both PFe and TDFe concentrations in sea ice and water depth 5) the Fe pool in sea ice is coupled to biota, as indicated by the positive correlations of PFe and TDFe with chlorophyll a and particulate organic carbon and 6) the vast majority of DFe appears to be adsorbed onto something in sea ice. This review also addresses the role of sea ice as a reservoir of Fe and its role in seeding seasonally ice-covered waters. We discuss the pivotal role of organic ligands in controlling DFe concentrations in sea ice and highlight the uncertainties that remain regarding the mechanisms of Fe incorporation in sea ice.
  • M. Inoue, Y. Shirotani, S. Nagao, H. Kofuji, Y. N. Volkov, J. Nishioka
    JOURNAL OF ENVIRONMENTAL RADIOACTIVITY 162 33 - 38 0265-931X 2016/10 [Refereed][Not invited]
     
    We examined lateral distributions of Cs-134, Cs-137, Ra-226, and Ra-228 in the surface seawaters around the Kuril Islands and the Kamchatka Peninsula in the northwestern North Pacific "Ocean during June 2014. The sampling area included three water current areas, the Oyashio Current, the current from the Okhotsk Sea, and the coastal current along the east Kamchatka Peninsula. Ra-226 and Ra-228 distributions differed along the three currents. Low levels of Cs-134 were detected in the surface waters of the Oyashio Current (0.09-035 mBq/L), but it was <similar to 0.1 mika at the surface along the other two currents. This indicates that the distribution of Fukushima Dai-ichi Nuclear Power Plant (FDNPP)-derived radiocesium in surface waters off the Kamchatka and along the Kuril Islands is predominantly governed by the Oyashio current system. (C) 2016 Elsevier Ltd. All rights reserved.
  • Kazuki Tanaka, Nobuyuki Takesue, Jun Nishioka, Yoshiko Kondo, Atsushi Ooki, Kenshi Kuma, Toru Hirawake, Youhei Yamashita
    SCIENTIFIC REPORTS 6 2045-2322 2016/09 [Refereed][Not invited]
     
    The spatial distribution of dissolved organic carbon (DOC) concentrations and the optical properties of dissolved organic matter (DOM) determined by ultraviolet-visible absorbance and fluorescence spectroscopy were measured in surface waters of the southern Chukchi Sea, western Arctic Ocean, during the early summer of 2013. Neither the DOC concentration nor the optical parameters of the DOM correlated with salinity. Principal component analysis using the DOM optical parameters clearly separated the DOM sources. A significant linear relationship was evident between the DOC and the principal component score for specific water masses, indicating that a high DOC level was related to a terrigenous source, whereas a low DOC level was related to a marine source. Relationships between the DOC and the principal component scores of the surface waters of the southern Chukchi Sea implied that the major factor controlling the distribution of DOC concentrations was the mixing of plural water masses rather than local production and degradation.
  • Observations of frazil ice formation and sediment upward transport off the northeast coast of Sakhalin
    Ito, M, K. I. Ohshima, J. Nishioka, Y. Kusumoto, Y. Fukamachi
    Proceedings of the 31th International Symposium on Okhotsk Sea & Sea Ice 124 - 127 2016/02 [Not refereed][Not invited]
  • べーリング海南東部陸棚縁辺域での潮汐混合と溶存鉄輸送
    田中雄大, 安田一郎, 久万健志, 西岡純, 田中祐希, 大西広二, 上野洋路, 増島雅親
    月刊海洋 号外 58 47 - 55 2016 [Not refereed][Not invited]
  • Y. Yamashita, C. -J. Lu, H. Ogawa, J. Nishioka, H. Obata, H. Saito
    MARINE CHEMISTRY 177 298 - 305 0304-4203 2015/12 [Refereed][Not invited]
     
    The application of in situ fluorometers to determine the high resolution vertical profiles of oceanic fluorescent organic matter (FOM) enhances the study of FOM distributions-in the open ocean and will lead to better understandings of the environmental dynamics of FOM, including the distribution/fate of terrestrial FOM in offshore waters. Here, we applied a commercially available in situ FOM fluorometer that was connected to a conductivity, temperature and depth (CTD) sensors with a carousel multi-sampling system to determine full vertical profiles of the FOM in oceanic environments. It was shown that temperature standardization was necessary to produce robust depth profiles and was performed preceding calibration with a benchtop fluorometer. The calibrated output of the in situ fluorometer was linearly related to apparent oxygen utilization when corrected for differences in fluorometer response over the length of the cruise. The results of this study indicated that (1) in situ FOM fluorometers that are mounted on CTD systems can be used to study the distribution of FOM in the open ocean, and (2) temperature standardization and appropriate calibration are crucial to evaluate the distribution of FOM in the open ocean. Although we used a window filter (weighted average of the output) to overcome the low S/N ratio of the output of the in situ fluorometer, better S/N ratios for in situ fluorometers are expected for open ocean studies. (C) 2015 Elsevier B.V. All rights reserved.
  • Edward Mawji, Reiner Schlitzer, Elena Masferrer Dodas, Cyril Abadie, Wafa Abouchami, Robert F. Anderson, Oliver Baars, Karel Bakker, Mark Baskaran, Nicholas R. Bates, Katrin Bluhm, Andrew Bowie, Johann Bown, Marie Boye, Edward A. Boyle, Pierre Branellec, Kenneth W. Bruland, Mark A. Brzezinski, Eva Bucciarelli, Ken Buesseler, Edward Butler, Pinghe Cai, Damien Cardinal, Karen Casciotti, Joaquin Chaves, Hai Cheng, Fanny Chever, Thomas M. Church, Albert S. Colman, Tim M. Conway, Peter L. Croot, Gregory A. Cutter, Hein J. W. de Baar, Gregory F. de Souza, Frank Dehairs, Feifei Deng, Huong Thi Dieu, Gabriel Dulaquais, Yolanda Echegoyen-Sanz, R. Lawrence Edwards, Eberhard Fahrbach, Jessica Fitzsimmons, Martin Fleisher, Martin Frank, Jana Friedrich, Francois Fripiat, Stephen J. G. Galer, Toshitaka Gamo, Ester Garcia Solsona, Loes J. A. Gerringa, Jose Marcus Godoy, Santiago Gonzalez, Emilie Grossteffan, Mariko Hatta, Christopher T. Hayes, Maija Iris Heller, Gideon Henderson, Kuo-Fang Huang, Catherine Jeandel, William J. Jenkins, Seth John, Timothy C. Kenna, Maarten Klunder, Sven Kretschmer, Yuichiro Kumamoto, Patrick Laan, Marie Labatut, Francois Lacan, Phoebe J. Lam, Delphine Lannuzel, Frederique le Moigne, Oliver J. Lechtenfeld, Maeve C. Lohan, Yanbin Lu, Pere Masque, Charles R. McClain, Christopher Measures, Rob Middag, James Moffett, Alicia Navidad, Jun Nishioka, Abigail Noble, Hajime Obata, Daniel C. Ohnemus, Stephanie Owens, Frederic Planchon, Catherine Pradoux, Viena Puigcorbe, Paul Quay, Amandine Radic, Mark Rehkaemper, Tomas Remenyi, Micha J. A. Rijkenberg, Stephen Rintoul, Laura F. Robinson, Tobias Roeske, Mark Rosenberg, Michiel Rutgers van der Loeff, Evgenia Ryabenko, Mak A. Saito, Saeed Roshan, Lesley Salt, Geraldine Sarthou, Ursula Schauer, Peter Scott, Peter N. Sedwick, Lijuan Sha, Alan M. Shiller, Daniel M. Sigman, William Smethie, Geoffrey J. Smith, Yoshiki Sohrin, Sabrina Speich, Torben Stichel, Johnny Stutsman, James H. Swift, Alessandro Tagliabue, Alexander Thomas, Urumu Tsunogai, Benjamin S. Twining, Hendrik M. van Aken, Steven van Heuven, Jan van Ooijen, Evaline van Weerlee, Celia Venchiarutti, Antje H. L. Voelker, Bronwyn Wake, Mark J. Warner, E. Malcolm S. Woodward, Jingfeng Wu, Neil Wyatt, Hisayuki Yoshikawa, Xin-Yuan Zheng, Zichen Xue, Moritz Zieringer, Louise A. Zimmer
    MARINE CHEMISTRY 177 1 - 8 0304-4203 2015/12 [Refereed][Not invited]
     
    The GEOTRACES Intermediate Data Product 2014 (IDP2014) is the first publicly available data product of the international GEOTRACES programme, and contains data measured and quality controlled before the end of 2013. It consists of two parts: (1) a compilation of digital data for more than 200 trace elements and isotopes (TEls) as well as classical hydrographic parameters, and (2) the eGEOTRACES Electronic Atlas providing a strongly inter-linked on-line atlas including more than 300 section plots and 90 animated 3D scenes. The IDP2014 covers the Atlantic, Arctic, and Indian oceans, exhibiting highest data density in the Atlantic. The TEI data in the IDP2014 are quality controlled by careful assessment of intercalibration results and multi-laboratory data comparisons at cross-over stations. The digital data are provided in several formats, including ASCII spreadsheet, Excel spreadsheet, netCDF, and Ocean Data View collection. In addition to the actual data values the IDP2014 also contains data quality flags and 1-sigma data error values where available. Quality flags and error values are useful for data filtering. Metadata about data originators, analytical methods and original publications related to the data are linked to the data in an easily accessible way. The eGEOTRACES Electronic Atlas is the visual representation of the IDP2014 data providing section plots and a new kind of animated 3D scenes. The basin-wide 3D scenes allow for viewing of data from many cruises at the same time, thereby providing quick overviews of large-scale tracer distributions. In addition, the 3D scenes provide geographical and bathymetric context that is crucial for the interpretation and assessment of observed tracer plumes, as well as for making inferences about controlling processes. (C) 2015 The Authors. Published by Elsevier B.V.
  • Atsushi Tsuda, Hiroaki Saito, Hiromi Kasai, Jun Nishioka, Takeshi Nakatsuka
    JOURNAL OF OCEANOGRAPHY 71 (3) 271 - 285 0916-8370 2015/06 [Refereed][Not invited]
     
    The Sea of Okhotsk is a semi-enclosed marginal sea of the subarctic North Pacific. One of the prominent characteristics of this area is the fast ice coverage during winter and early spring. We investigated the life cycles and vertical distributions of four species of large suspension feeding copepods, Neocalanus plumchrus, N. flemingeri, N. cristatus, and Eucalanus bungii in the southern part of the Sea of Okhotsk during the ice-free season. Neocalanus plumchrus and N. cristatus showed annual life cycles while a major part of the population of N. flemingeri and E. bungii showed biennial life cycles. Their occurrences in the surface layer for grazing and development were 1-2 months later than in the adjacent area of the Pacific Ocean, and they remained longer in the surface layer for development. Consequently, the seasonal overlap among the copepods in the surface productive layer was intensified. In contrast, the vertical distribution of the four species of copepods during the growing period extended to deeper layers and became more finely segregated than those of Oyashio region and open ocean of the North Pacific. Thus, the seasonal segregations in the Oyashio region are replaced by vertical segregation in the Sea of Okhotsk. A major factor modifying the seasonal occurrences in the surface layer was considered to be the presence of fast ice during winter and early spring which greatly reduces the photosynthesis in the water column.
  • Lisa A. Miller, Francois Fripiat, Brent G.T. Else, Jeff S. Bowman, Kristina A. Brown, R. Eric Collins, Marcela Ewert, Agneta Fransson, Michel Gosselin, Delphine Lannuzel, Klaus M. Meiners, Christine Michel, Jun Nishioka, Daiki Nomura, Stathys Papadimitriou, Lynn M. Russell, Lise Lotte Sørensen, David N. Thomas, Jean-Louis Tison, Maria A. van Leeuwe, Martin Vancoppenolle, Eric W. Wolff, Jiayun Zhou
    Elementa: Science of the Anthropocene 3 000038 - 000038 2015/01/23 [Refereed][Not invited]
  • Taejin Kim, Hajime Obata, Toshitaka Gamo, Jun Nishioka
    LIMNOLOGY AND OCEANOGRAPHY-METHODS 13 (1) 30 - 39 1541-5856 2015/01 [Refereed][Not invited]
     
    Zinc (Zn) is an essential micronutrient for bacteria and phytoplankton in the ocean. However, seawater samples for Zn measurements are highly prone to contamination due to the ubiquity of Zn in laboratories and research vessels. To identify a potential Zn contamination problem during a research cruise to the sub-arctic North Pacific, we compared three operational sampling approaches for trace Zn in seawater. Acid-cleaned Niskin-X samplers were: (1) deployed on a conductivity-temperature-depth carousel multisampling system (CTD-CMS) launched at the vessel's starboard side, (2) attached to a Kevlar wire lowered at the vessel's stern, and (3) attached to a Ti wire lowered from a starboard position. Zn contamination in the case of Kevlar wire hydrocasts was problematic for samples from <= 400 m because Zn is used as a sacrificial anode for the ship's propeller shaft and for the pipes of the seawater cooling system. In deep waters, consistent Zn concentrations were found using the three different sampling methods. Based on the clean seawater sampling with CTD-CMS and the onboard analytical method, vertical profiles of dissolved Zn were then obtained. These profiles were compared with those of Si in the subarctic North Pacific. Both Zn and Si concentrations showed a clear west-east gradient in shallow waters.
  • Takeshi Yoshimura, Koji Sugie, Hisashi Endo, Koji Suzuki, Jun Nishioka, Tsuneo Ono
    DEEP-SEA RESEARCH PART I-OCEANOGRAPHIC RESEARCH PAPERS 94 1 - 14 0967-0637 2014/12 [Refereed][Not invited]
     
    Increase in seawater pCO(2) and the corresponding decrease in pH caused by the increasing atmospheric CO2 concentration (i.e., ocean acidification) may affect organic matter production by phytoplankton communities. Organic matter production forms the basis of marine food webs and plays a crucial role in oceanic CO2 uptake through the biological carbon pump, and hence will potentially affect future marine ecosystem dynamics. However, responses of organic matter production in open ocean plankton ecosystems to CO2 increase have not been fully examined. We conducted on-deck microcosm experiments using high nutrient, low chlorophyll (HNLC) waters in the western subarctic Pacific and oceanic Bering Sea basin in summer 2008 and 2009, respectively, to examine the impacts of elevated CO2 on particulate and dissolved organic matter (i.e., POM and DOM, respectively) production. Iron deficient natural plankton communities were incubated for 7-14 days under multiple CO2 levels with and without iron enrichments (hereafter +Fe and -Fe treatments, respectively). By combining with our previous experiments at two sites, we created a comprehensive dataset on responses of organic matter production to CO2 increase during macronutrient replete conditions in HNLC waters. Significant differences in net particulate organic carbon production among CO2 treatments were observed only in the -Fe treatments, whereas that in net dissolved organic carbon production were mainly observed in the +Fe treatments, suggesting that CO2 may affect different processes depending on the Fe nutritional status. However, impacts of CO2 were not consistent among experiments and were much smaller than the consistent positive effects of Fe enrichment. In contrast, no significant differences among the CO2 treatments were observed for organic carbon partitioning into POM and DOM, and carbon to nitrogen ratio of net produced POM. We conclude that CO2 does not play a primary role, but could have secondary effects on controlling the organic matter production under macronutrient replete conditions in HNLC waters. On the other hand, in a nutrient-depleted, declining phase of the phytoplankton bloom induced by Fe enrichment, carbon overconsumption was found in an experiment with elevated CO2 conditions suggesting that CO2 impacts might become more significant in such environments. (C) 2014 Elsevier Ltd. All rights reserved.
  • Nanako Hioki, Kenshi Kuma, Yuichirou Morita, Ryouhei Sasayama, Atsushi Ooki, Yoshiko Kondo, Hajime Obata, Jun Nishioka, Youhei Yamashita, Shigeto Nishino, Takashi Kikuchi, Michio Aoyama
    SCIENTIFIC REPORTS 4 2045-2322 2014/10 [Refereed][Not invited]
     
    The location and magnitude of oceanic iron sources remain uncertain owing to a scarcity of data, particularly in the Arctic Ocean. The formation of cold, dense water in the subsurface layer of the western Arctic Ocean is a key process in the lateral transport of iron, macronutrients, and other chemical constituents. Here, we present iron, humic-like fluorescent dissolved organic matter, and nutrient concentration data in waters above the continental slope and shelf and along two transects across the shelf-basin interface in the western Arctic Ocean. We detected high concentrations in shelf bottom waters and in a plume that extended in the subsurface cold dense water of the halocline layer in slope and basin regions. At sigma(theta) = 526.5, dissolved Fe, humic-like fluorescence intensity, and nutrient maxima coincided with N* minima (large negative values of N* indicate significant denitrification within shelf sediments). These results suggest that these constituents are supplied from the shelf sediments and then transported laterally to basin regions. Humic dissolved organic matter probably plays the most important role in the subsurface maxima and lateral transport of dissolved Fe in the halocline layer as natural Fe-binding organic ligand.
  • Takeshi Yoshimura, Jun Nishioka, Hiroshi Ogawa, Kenshi Kuma, Hiroaki Saito, Atsushi Tsuda
    MARINE CHEMISTRY 165 46 - 54 0304-4203 2014/10 [Refereed][Not invited]
     
    A significant part of phosphorus (P) in seawater is found in the dissolved organic matter (DOM) fraction as DOP, which plays a key role in the marine biogeochemical cycle of P. The DOM pool size changes with biological activity, but DOP production and decomposition processes, unlike carbon (C) and nitrogen, have been only infrequently studied during phytoplankton blooms when rapid production and accumulation of organic matter occurs. We observed the DOP dynamics during two phytoplankton blooms dominated by centric diatoms, the first induced by an in situ mesoscale iron enrichment experiment in the western subarctic Pacific in summer 2001 (SEEDS) and the second that occurred naturally in spring 2003 in the Oyashio region. DOP concentration increased with the buildup of phytoplankton biomass with DOP/chlorophyll-a production ratios (mol/g) of 0.0027 +/- 0.0004 and 0.0044 +/- 0.0010 for the SEEDS and Oyashio blooms, respectively. During the SEEDS and Oyashio blooms the amount of net DOP production corresponded to (4.9 +/- 0.7) % and (4.5 +/- 0.6) % of the consumed soluble reactive P, and (5.5 +/- 0.8) % and (13 +/- 3) % of the newly accumulated organic P was partitioned into DOP, respectively. Seawater culture bottle experiments showed that newly produced DOP during the bloom development was decomposed by free living bacteria over a time scale of a month even under soluble reactive P available conditions. C:P for the decomposed DOM (molar ratio of 78-88) showed a similar value to in situ net produced DOM (66) and POM (83) but much lower than that for the bulk DOM (395-706), suggesting that the composition of the freshly produced DOM with high lability differs significantly from the bulk DOM. (C) 2014 Elsevier B.V. All rights reserved.
  • Masanori Ito, Yutaka W. Watanabe, Masahito Shigemitsu, Shinichi S. Tanaka, Jun Nishioka
    JOURNAL OF OCEANOGRAPHY 70 (5) 415 - 424 0916-8370 2014/10 [Refereed][Not invited]
     
    To estimate benthic denitrification in a marginal sea, we assessed the usefulness of , a new tracer to measure the excess nitrogen gas (N-2) using dissolved N-2 and argon (Ar) with N* in the intermediate layer (26.6-27.4 sigma (theta) ) of the Okhotsk Sea. The examined parameters capable of affecting are denitrification, air injection and rapid cooling. We investigated the relative proportions of these effects on using multiple linear regression analysis. The best model included two examined parameters of denitrification and air injection based on the Akaike information criterion as a measure of the model fit to data. More than 80 % of was derived from the denitrification, followed by air injection. Denitrification over the Okhotsk Sea shelf region was estimated to be 5.6 +/- A 2.4 mu mol kg(-1). The distribution of was correlated with potential temperature (theta) between 26.6 and 27.4 sigma (theta) (r = -0.55). Therefore, we concluded that and N* can act complementarily as a quasi-conservative tracer of benthic denitrification in the Okhotsk Sea. Our findings suggest that in combination with N* is a useful chemical tracer to estimate benthic denitrification in a marginal sea.
  • Hiroshi Itoh, Jun Nishioka, Atsushi Tsuda
    PROGRESS IN OCEANOGRAPHY 126 224 - 232 0079-6611 2014/08 [Refereed][Not invited]
     
    We investigated the community structure of mesozooplankton in the western part of the Sea of Okhotsk in late summer, 2006. We recognized four communities belonging to two assemblages. A coastal assemblage dominated by the arctic planktonic snail Limacina helicina consisted of a gulf community characterized by brackish copepods and a continental shelf community characterized by the hydrozoan medusa Aglantha digitale and the arctic copepod Calanus glacialis. The other assemblage, characterized by the oceanic copepod Neocalanus plumchrus, consisted of a continental slope community characterized by a diverse species composition and a basin community characterized by the oceanic copepod N. cristatus. The continental slope community contained species from the coastal waters and was distributed along the course of the East Sakhalin current. This community may have been assembled by the incorporation of coastal water into the oceanic waters by the strong current. Small coastal copepods such as Oithona similis and Pseudocalanus spp. were the main components in all communities in terms of numbers, but larger copepods such as Neocalanus spp. and Metridia okhotensis were important in terms of weight, especially in the continental slope and basin communities. The population structures of the dominant species suggest that overall biological production is maintained by continuous reproduction or growth (or both) of L. helicina and small coastal copepods after the onset of seasonal dormancy of the large oceanic copepods in late summer. (C) 2014 Elsevier Ltd. All rights reserved.
  • Yuki Tanaka, Ichiro Yasuda, Satoshi Osafune, Takahiro Tanaka, Jun Nishioka, Yuri N. Volkov
    PROGRESS IN OCEANOGRAPHY 126 98 - 108 0079-6611 2014/08 [Refereed][Not invited]
     
    Repeated observations with a period of about 24 h of hydrography, current velocity, and microstructures were performed at three stations surrounding a seamount in the middle of the Bussol Strait, the deepest and widest one of the Kuril Straits, to reveal spatial and temporal variability of internal tides and associated turbulent mixing. It is found that isopycnal displacements are dominated by diurnal tidal components, which show phase differences (namely, time lags) between the three stations that can be explained by a first mode topographically trapped wave (TTW) propagating clockwise around the seamount. Furthermore, at the station located near the center of the strait where energy dissipation rates are largest, diurnal variations of isopycnals and velocities are amplified toward the ocean bottom, consistent with the vertical structure of the first mode TTW. At that station, vigorous turbulent mixing with the energy dissipation rate exceeding 10(-6) m(2) s(-3) and diapycnal diffusivity exceeding 10(-1) m(2) s(-1) was observed in deep layers when the diurnal tidal current consisting of the first mode TTW flows from the Okhotsk Sea to the North Pacific, enhancing the mean current. These spatial and temporal variation patterns are confirmed to be reproduced by a previous numerical model successfully for the isopycnals and velocities, and partially for the turbulent mixing. The total energy dissipation rate is, however, by up to a factor of 3-10 smaller than predicted by the numerical model although the observations were performed during spring tides, suggesting that the actual diapycnal mixing is overall weaker than the previous model estimate and/or that extremely strong mixing occurs within highly localized areas. (C) 2014 Elsevier Ltd. All rights reserved.
  • Keisuke Uchimoto, Tomohiro Nakamura, Jun Nishioka, Humio Mitsudera, Kazuhiro Misumi, Daisuke Tsumune, Masaaki Wakatsuchi
    PROGRESS IN OCEANOGRAPHY 126 194 - 210 0079-6611 2014/08 [Refereed][Not invited]
     
    An ocean general circulation model coupled with a simple biogeochemical model was developed to simulate iron circulation in and around the Sea of Okhotsk. The model has two external sources of iron: dust iron at the sea surface and sedimentary iron at the seabed shallower than 300 m. The model represented characteristic features reasonably well, such as high iron concentration in the dense shelf water (DSW) and its mixing, which extends southward in the intermediate layer from the northwestern shelf along Sakhalin Island and finally flows into the Pacific. Sensitivity experiments for the solubility of dust iron in seawater suggest that a solubility of 1% is appropriate in our simulation. Higher solubilities (5% and 10%) result in too low phosphate in the northwestern North Pacific in summer as well as too high iron concentrations at the sea surface, compared with observations. Besides, these experiments show that dust iron hardly contributes to the high iron concentration in the intermediate layer. To investigate locations from which the iron in the intermediate layer originates, the fate of sedimentary iron input from four regions in the Okhotsk Sea was examined. Results suggest that the western and central parts of the northern shelf are important. (C) 2014 Elsevier Ltd. All rights reserved.
  • Jun Nishioka, Takeshi Nakatsuka, Kazuya Ono, Yu. N. Volkov, Alexey Scherbinin, Takayuki Shiraiwa
    PROGRESS IN OCEANOGRAPHY 126 180 - 193 0079-6611 2014/08 [Refereed][Not invited]
     
    Comprehensive observations of the iron (Fe) distribution in the western Sea of Okhotsk were conducted and revealed the existence of two Fe transport processes in the sub-polar marginal sea. One transport process is Fe loading from the Amur River and transport by the East Sakhalin Current (ESC), and the other is Fe transport by the intermediate water (part of which was reported by Nishioka et al., 2007). Here, we report on quantitative evaluations of these two Fe transport processes. The surface dissolved Fe (Diss-Fe) and low salinity water distribution clearly indicate the influence of Fe discharge from the Amur River and the Fe that is transported by the East Sakhalin Current. The amounts of total dissolvable Fe (TD-Fe) and Diss-Fe that cross the surface of the northeast Sakhalin coastal area are estimated at 9.0 x 10(8) - similar to 1.3 x 10(9) g yr(-1) and 1.0 x 10(8) similar to 1.5 x 10(8) g yr(-1), respectively. Although the ESC surface transport system along the Sakhalin coast is effective, the length-scale estimation of TD-Fe transport indicated that only 1.5% of the Fe at the mouth of the Amur River reached 52 degrees N, which may be due to scavenging by biological particulates. High Fe anomalies were observed at the bottom of the continental shelf and the shelf break along the Sakhalin coast. The extremely low temperature and low N* water indicate that Fe resuspension due to the reducing properties of sediment occurred on the shelf and that the Fe was introduced to Dense Shelf Water (DSW) by tidal mixing. We estimate that the amounts of TD-Fe and Diss-Fe involved in the DSW on the continental shelf are 8.8 x 10(10) similar to 2.5 x 10(11) g yr(-1) and 2.3 x 10(9) - similar to 6.6 x 10(9) g yr(-1), respectively. Length-scale estimates of TD-Fe transport indicate that 20% of the TD-Fe on the continental shelf remained in the Kuril Basin; thus, the TD-Fe in the intermediate water was efficiently transported to the Kuril Basin. These results indicate that two orders of magnitude more Fe is derived from the continental shelf by the intermediate water than by surface water and that Fe is transported a greater distance by intermediate water than by the surface layer. Additionally, the Fe that reached the Kuril Straits was mixed by intensive tidal mixing and influenced the vertical profiles of the water columns on both sides of the Kuril Straits in the Kuril Basin and the Oyashio region. Our quantitative evaluation also indicates other Fe inputs around the Kuril Strait. (C) 2014 Elsevier Ltd. All rights reserved.
  • Jun Nishioka, Humio Mitsudera, Ichiro Yasuda, Hongbin Liu, Takeshi Nakatsuka, Yuri N. Volkov
    PROGRESS IN OCEANOGRAPHY 126 1 - 7 0079-6611 2014/08 [Refereed][Not invited]
  • Kenshi Kuma, Ryohei Sasayama, Nanako Hioki, Yuichiroh Morita, Yutaka Isoda, Tohru Hirawake, Keiri Imai, Takafumi Aramaki, Tomohiro Nakamura, Jun Nishioka, Naoto Ebuchi
    JOURNAL OF OCEANOGRAPHY 70 (4) 377 - 387 0916-8370 2014/08 [Refereed][Not invited]
     
    In the southwestern Okhotsk Sea, the cold water belt (CWB) is frequently observed on satellite images offshore of the Soya Warm Current flowing along the northeastern coast of Hokkaido, Japan, during summertime. It has been speculated that the CWB is upwelling cold water that originates from either subsurface water of the Japan Sea off Sakhalin or bottom water of the Okhotsk Sea. Hydrographic and chemical observations (nutrients, humic-type fluorescence intensity, and iron) were conducted in the northern Japan Sea and southwestern Okhotsk Sea in early summer 2011 to clarify the origin of the CWB. Temperature-salinity relationships, vertical distributions of chemical components, profiles of chemical components against density, and the (NO3 + NO2)/PO4 relationship confirm that water in the CWB predominantly originates from Japan Sea subsurface water.
  • Naoya Kanna, Takenobu Toyota, Jun Nishioka
    PROGRESS IN OCEANOGRAPHY 126 44 - 57 0079-6611 2014/08 [Refereed][Not invited]
     
    To elucidate the roles of sea ice in biogeochemical cycles in the Sea of Okhotsk, the concentrations of macro-nutrients (NO3 + NO2, PO4, SiO2, and NH4) and trace elements (Fe, Al) were measured in samples of sea ice, overlying snow, and seawater. The oxygen isotope ratio (delta O-18) in the sea ice was used to distinguish between snow ice and seawater-origin ice. Except for NH4, the macro-nutrient concentrations were lower in sea ice than in surface water in the ice-covered area. A linear relationship between salinity and concentrations of NO3 + NO2, PO4, and SiO2 in the sea ice indicated that these macro-nutrients originated mainly from seawater. The Fe concentrations in sea ice were variable and several orders of magnitude higher than those in surface water in the ice-covered area. The Fe concentrations in the sea ice were positively correlated with Al concentrations, the suggestion being that the Fe contained in the sea ice originated mainly from lithogenic mineral particles. The annual Fe flux into the surface water from sea ice melting in the southern Sea of Okhotsk was estimated to be similar to 740 mu mol Fe m(-2) yr(-1). This flux is comparable to the reported annual atmospheric Fe flux (267-929 mu mol Fe m(-2) yr(-1)) in the western North Pacific. In spring, sea ice melting may slightly dilute macro-nutrient concentrations but increase Fe concentrations in surface water. These results suggest that sea ice may contribute to phytoplankton growth by release of Fe into the water column and have a large impact on biogeochemical cycles in the Sea of Okhotsk. Crown Copyright (C) 2014 Published by Elsevier Ltd. All rights reserved.
  • Sachihiko Itoh, Yuki Tanaka, Satoshi Osafune, Ichiro Yasuda, Masahiro Yagi, Hitoshi Kaneko, Shunsuke Konda, Jun Nishioka, Yuri. N. Volkov
    PROGRESS IN OCEANOGRAPHY 126 109 - 120 0079-6611 2014/08 [Refereed][Not invited]
     
    Observations and model data have been analyzed for the Urup Strait, one of the Kuril Straits, focusing on processes driving the extremely strong vertical mixing. Observations over 1 day on the Pacific side of the sill of the Urup Strait show the generation of a large-amplitude internal wave (LAIW), characterized by the depression of potential density surfaces caused by a strong off-sill flow. When the LAIW developed, the potential density surface of 26.7 sigma(theta) was depressed downward by more than 300 m, and vertical mixing was markedly elevated within the 400-m-thick layer 26.6-26.7 sigma(theta), with a layer-mean (maximum) energy dissipation rate and vertical diffusivity of 1.2 x 10(-6) (7.9 x 10(-6) W kg(-1)) and 4.0 x 10(-1) (2.8 m(2) s(-1)), respectively. A three-dimensional model simulation of the Kuril Straits driven by the diurnal barotropic tide reproduces well the observed features relevant to the generation of LAIWs in and around the Urup Strait. The structure and generation pattern of LAIWs in the Urup Strait are consistent with those of the arrested lee waves observed in the Hawaiian ridge and Luzon Strait, although the depth at the top of the sill is far shallower in the Urup Strait. It is suggested that barotropic tidal energy with diurnal frequency is converted into LAIWs through the excitation of topographically trapped waves with hydraulically supercritical velocity near the sill top, and it is these waves that are responsible for the extremely strong mixing on the sill-flanks of the shallow Kuril Straits. (C) 2014 Elsevier Ltd. All rights reserved.
  • K. Suzuki, A. Hattori-Saito, Y. Sekiguchi, J. Nishioka, M. Shigemitsu, T. Isada, H. Liu, R. M. L. McKay
    BIOGEOSCIENCES 11 (9) 2503 - 2517 1726-4170 2014 [Refereed][Not invited]
     
    The Sea of Okhotsk is known as one of the most biologically productive regions among the world's oceans, and its productivity is supported in part by the discharge of iron (Fe)-rich water from the Amur River. However, little is known about the effect of riverine-derived Fe input on the physiology of the large diatoms which often flourish in surface waters of the productive continental shelf region. We conducted diatom-specific immunochemical ferredoxin (Fd) and flavodoxin (Fld) assays in order to investigate the spatial variability of Fe nutritional status in the microplanktonsized (20-200 mu m; hereafter micro-sized) diatoms. The Fd index, defined as the proportion of Fd to the sum of Fd plus Fld accumulations in the cells, was used to assess their Fe nutritional status. Additionally, active chlorophyll fluorescence measurements using pulse-amplitude-modulated (PAM) fluorometry were carried out to obtain the maximum photochemical quantum efficiency (F-v / F-m) of photosystem II for the total micro-sized phytoplankton assemblages including diatoms. During our observations in the summer of 2006, the micro-sized diatoms were relatively abundant (> 10 mu g C L-1) in the neritic region, and formed a massive bloom in Sakhalin Bay near the mouth of the Amur River. Values of the Fd index and F-v / F-m were high (> 0.9 and > 0.65, respectively) near the river mouth, indicating that Fe was sufficient for growth of the diatoms. However, in oceanic waters of the Sea of Okhotsk, the diatom Fd index declined as cellular Fld accumulation increased. These results suggest that there was a distinct gradient in Fe nutritional status in the micro-sized diatoms from near the Amur River mouth to open waters in the Sea of Okhotsk. A significant correlation between dissolved Fe (D-Fe) concentration and the Fd index was found in waters off Sakhalin Island, indicating that D-Fe was a key factor for the photophysiology of this diatom size class. In the vicinity of the Kuril Islands between the Sea of Okhotsk and the Pacific Ocean, micro-sized diatoms only accumulated Fld (i.e., Fd index = 0), despite strong vertical mixing consistent with elevated surface D-Fe levels (> 0.4 nM). Since higher Fe quotas are generally required for diatoms growing under low-light conditions, the micro-sized diatoms off the Kuril Islands possibly encountered Fe and light co-limitations. The differential expressions of Fd and Fld in micro-sized diatoms helped us to understand how these organisms respond to Fe availability in the Sea of Okhotsk in connection with the Amur River discharge.
  • Obata Hajime, Nishioka Jun, Kim Taejin, Norisuye Kazuhiro, Takeda Shigenobu, Wakuta Yohei, Gamo Toshitaka
    Abstracts of Annual Meeting of the Geochemical Society of Japan 日本地球化学会 61 67 - 67 2014 
    国際GEOTRACES計画の進行に伴い、海洋における微量元素の研究は急速に進みつつある。日本では、2013年度から運用を開始した学術研究船新青丸に国際GEOTRACES計画レベルのクリーン採水が可能なシステムが導入された。この新しい採水システムを用いて、実際にクリーンな海水が採取できるかどうかを検討した。さらに、相模湾、伊豆小笠原海溝域で得られた海水中の溶存鉄・亜鉛の結果から、日本近海における鉄・亜鉛の挙動についての議論を行った。
  • Kondo Yoshiko, Obata Hajime, Ooki Atsushi, Yamashita Youhei, Nishioka Jun, Kuma Kenshi
    Abstracts of Annual Meeting of the Geochemical Society of Japan 日本地球化学会 61 199 - 199 2014 
    過酸化水素は、大気中ではその酸性度を高める物質として、海水中では微量金属の酸化還元反応や溶存有機物の循環および微生物群集の生物生産に影響を与える因子として重要な物質である。海水中の過酸化水素のソースとシンクは光化学反応や生物活動と密接に関わるためその挙動は動的で一様ではない。これまで、海洋における過酸化水素の分布は外洋域・沿岸域様々な海域で報告されているが、北極海におけるデータは限られている。本発表では、これまでの海洋における過酸化水素の分布動態に関するレビューを含め、GRENE北極海研究プロジェクト（http://www.nipr.ac.jp/grene/）の一環として2013年夏期に実施された北海道大学練習船おしょろ丸航海で得られた北極海（チャクチ海）における過酸化水素の分布について紹介する。
  • Nomura Daiki, Nishioka Jun, Kawai Michiyo, Ooki Atsushi, Tamura Takeshi
    Abstracts of Annual Meeting of the Geochemical Society of Japan 日本地球化学会 61 132 - 132 2014 
    我が国におけるこれまでの海氷研究は、主に物理過程を明らかにすることに主眼が置かれ、海氷の物理的動態を実測や衛星観測、モデリングから明らかにする研究が精力的に進められてきた。一方で、海氷に関わる化学過程や生物過程に関しては、これまで十分な研究が展開されていなかった。海氷の生成が生み出す独特の海洋循環を介して、他海域の物質循環にまで多大な影響を与えているシステムの存在が指摘されており、これらのシステムを理解するためには、海氷の存在する時期から消失していくまでの詳細な時系列観測、ポリニヤにおける海氷の生成が起こっている現場での観測が欠かせない。また、地球規模で起こっている環境変動を理解するためには、通年観測する必要があり、海洋研究プラットフォームを生物地球化学分野で最大限活用すると同時に、厳冬期でも観測が続けられる本格的な観測研究用の砕氷船が欠かせないだろう。
  • Yasuda Tomoki, Asahara Yoshihiro, Ichikawa Ryo, Nakatsuka Takeshi, Nishioka Jun, Minami Hideki, Nagao Seiya
    Abstracts of Annual Meeting of the Geochemical Society of Japan 日本地球化学会 61 88 - 88 2014 
    オホーツク海は、生物生産の高い海域として知られている。中でも特に、北西陸棚域及びサハリン島東岸において高い生物生産を誇っており、世界最大級の河川であるアムール川から供給される栄養塩及び溶存態鉄の供給によってこの海域の生物生産が支えられていることが指摘されてきた。しかしながら、アムール川供給物質がオホーツク海をどのように輸送され、分布しているかは明らかにされていないこなかった。そこで、本研究ではアムール川懸濁物及びオホーツク海懸濁物、表層堆積物に含まれる珪酸塩砕屑物成分のSr、Nd同位体比及び微量・希土類元素組成分析からを行うことで、アムール川供給砕屑物の分布及び輸送過程の解明を試みた。本研究の結果、オホーツク海北西陸棚域の砕屑物は3つの供給源物質によって構成されており、また北西陸棚域のSakhalinsky Bay及びサハリン島東岸の生物生産の高い海域にアムール川供給物質が輸送されていることが明らかになった。
  • Masahito Shigemitsu, J. Nishioka, Y. W. Watanabe, Y. Yamanaka, T. Nakatsuka, Y. N. Volkov
    Marine Chemistry 157 41 - 48 0304-4203 2013/12/20 [Refereed][Not invited]
     
    We have found that the ratio of Fe to Al in suspended particulate matter (SPM) in the intermediate layer of the Okhotsk Sea increases to the open ocean from the dense shelf water on the northwestern continental shelf, which is the source of southward-flowing Okhotsk Sea Intermediate Water (OSIW). The SPM concentration and the Al content of the SPM decrease along the course of the OSIW, whereas the ratios of Ba and Mn to Al increase. The SPM samples on the continental shelf were collected in waters characterized by low N* (the deviation from the stoichiometric relationship between nitrogenous nutrients and phosphate). These low N* values imply that sedimentary denitrification occurs on the continental shelf, and both Fe and Mn are concomitantly reduced in the sediments and diffuse from the sediments to the overlying waters, where they are subsequently oxidized and precipitate. We assume that the SPM in the intermediate layer is influenced mainly by horizontal transport from the continental shelf to the ocean interior and that Ba can be used as a proxy for organic matter. Based on these assumptions, the results could reflect the following processes: 1) a gradual loss of denser lithogenic matter, evidenced by the decreases of the SPM concentration and of the Al content of the SPM, and 2) a relative increase of the Fe and Mn fractions associated with the organic matter in the SPM, which is derived from the continental shelf. The organic matter-related Fe and Mn could also be affected by the precipitation of both Fe and Mn just above the reduced sediments of the continental shelf. Our results show that the particulate Fe associated with organic matter is preferentially transported from the northwestern continental shelf to the open ocean. © 2013 Elsevier B.V.
  • K. Sugie, H. Endo, K. Suzuki, J. Nishioka, H. Kiyosawa, T. Yoshimura
    Biogeosciences Copernicus {GmbH} 10 (10) 6309  1726-4189 2013/10 [Refereed][Not invited]
  • Takeshi Yoshimura, Koji Suzuki, Hiroshi Kiyosawa, Tsuneo Ono, Hiroshi Hattori, Kenshi Kuma, Jun Nishioka
    JOURNAL OF OCEANOGRAPHY 69 (5) 601 - 618 0916-8370 2013/10 [Refereed][Not invited]
     
    Response of phytoplankton to increasing CO2 in seawater in terms of physiology and ecology is key to predicting changes in marine ecosystems. However, responses of natural plankton communities especially in the open ocean to higher CO2 levels have not been fully examined. We conducted CO2 manipulation experiments in the Bering Sea and the central subarctic Pacific, known as high nutrient and low chlorophyll regions, in summer 2007 to investigate the response of organic matter production in iron-deficient plankton communities to CO2 increases. During the 14-day incubations of surface waters with natural plankton assemblages in microcosms under multiple pCO(2) levels, the dynamics of particulate organic carbon (POC) and nitrogen (PN), and dissolved organic carbon (DOC) and phosphorus (DOP) were examined with the plankton community compositions. In the Bering site, net production of POC, PN, and DOP relative to net chlorophyll-a production decreased with increasing pCO(2). While net produced POC:PN did not show any CO2-related variations, net produced DOC:DOP increased with increasing pCO(2). On the other hand, no apparent trends for these parameters were observed in the Pacific site. The contrasting results observed were probably due to the different plankton community compositions between the two sites, with plankton biomass dominated by large-sized diatoms in the Bering Sea versus ultra-eukaryotes in the Pacific Ocean. We conclude that the quantity and quality of the production of particulate and dissolved organic matter may be altered under future elevated CO2 environments in some iron-deficient ecosystems, while the impacts may be negligible in some systems.
  • Jun Nishioka, Takeshi Nakatsuka, Yutaka W. Watanabe, Ichiro Yasuda, Kenshi Kuma, Hiroshi Ogawa, Naoto Ebuchi, Alexey Scherbinin, Yuri N. Volkov, Takayuki Shiraiwa, Masaaki Wakatsuchi
    GLOBAL BIOGEOCHEMICAL CYCLES 27 (3) 920 - 929 0886-6236 2013/09 [Refereed][Not invited]
     
    The subarctic Pacific is a high-nutrient low-chlorophyll (HNLC) region in which phytoplankton growth is broadly limited by iron (Fe) availability. However, even with Fe limitation, the western subarctic Pacific (WSP) has significant phytoplankton growth and greater seasonal variability in lower trophic levels than the eastern subarctic Pacific. Therefore, differences in Fe supply must explain the west-to-east decrease in seasonal phytoplankton growth. The Fe flux to the euphotic zone in the WSP occurs at a moderate value, in that it is significantly higher than its value on the eastern side, yet it is not sufficient enough to cause widespread macronutrient depletion, that is, HNLC status is maintained. Although we recognize several Fe supply processes in the WSP, the mechanisms that account for this moderate value of Fe supply have not previously been explained. Here we demonstrate the pivotal role of tidal mixing in the Kuril Islands chain (KIC) for determining the moderate value. A basin-scale meridional Fe section shows that Fe derived from sediments in the Sea of Okhotsk is discharged through the KIC into the intermediate water masses (similar to 800m) of the western North Pacific. The redistribution of this Fe-rich intermediate water by intensive mixing as it crosses the KIC is the predominant process determining the ratio of micronutrient (Fe) to macronutrients (e.g., nitrate) in subsurface waters. This ratio can quantitatively explain the differences in surface macronutrient consumption between the western and eastern subarctic, as well as the general formation and biogeochemistry of HNLC waters of the subarctic North Pacific.
  • Report of Working Group 22 on Iron Supply and its Impact on Biogeochemistry and Ecosystems in the North Pacific Ocean
    S. Takeda, F. Chai, J. Nishioka
    PICES Scientific Report (42) 1 - 60 2013/07 [Not refereed][Not invited]
  • Koji Sugie, Jun Nishioka, Kenshi Kuma, Yuri N. Volkov, Takeshi Nakatsuka
    Marine Chemistry 152 20 - 31 0304-4203 2013/05/20 [Refereed][Not invited]
     
    In a shipboard incubation study, we investigated the availability of particulate iron (Fe) to a Fe-starved phytoplankton community through the addition of suspended particulate matter (SPM > 1 μm) collected from the nepheloid layer in the coastal region of the Sea of Okhotsk. Surface seawater incubations were also conducted at three stations around the Bussol' Strait where the SPM that possibly originated from the coastal nepheloid layer could emerge to the surface mixed layer due to strong vertical mixing around the Kuril Islands. In the SPM-added experiment, the growth rate of phytoplankton was significantly enhanced by the addition of SPM compared to the unamended control. This result clearly indicates that Fe in the SPM collected from the nepheloid layer was available to marine phytoplankton. In addition, phytoplankton particularly coastal diatoms in the nepheloid layer were viable and showed healthy growth. In the surface seawater incubation experiments, phytoplankton growth and nutrient drawdown in unamended control conditions in two of the three stations may be supported by Fe from the particulate fraction (> . 0.22. μm), as estimated from stoichiometric calculation. We suggest that the bioavailable particulate Fe in SPM of the coastal region supports biological production and nutrient drawdown even after the depletion of dissolved Fe around the Kuril Islands, where strong vertical mixing occurs. © 2013 Elsevier B.V.
  • Tomonori Isada, Takahiro Iida, Hongbin Liu, Sei-Ichi Saitoh, Jun Nishioka, Takeshi Nakatsuka, Koji Suzuki
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 118 (4) 1995 - 2013 2169-9275 2013/04 [Refereed][Not invited]
     
    We investigated the photosynthetic parameters of phytoplankton in the Sea of Okhotsk during the late summer of 2006 to characterize their spatiotemporal variability and to test the hypothesis that discharge from the Amur River could influence the algal photophysiology. The highest maximum quantum yield of carbon fixation in photosynthesis (phi cmax; 0.098mol C mol photons-1) was found near the Amur River mouth, where nitrate was depleted. However, none of the photosynthetic parameters, including primary productivity (PP) at the surface, were correlated with temperature, daily photosynthetically available radiation (PAR), or ambient nutrient concentrations. Variations in phi cmax depended on the variations in not only the mean chlorophyll a specific absorption coefficient of phytoplankton (*ph) but also the slope index of the absorption coefficient of phytoplankton (aph slope), an indicator for the ratio of nonphotosynthetic carotenoids to photosynthetic carotenoids. These results indicated that the phytoplankton assemblages acclimated to the ambient light conditions by regulating their cellular pigments. Additionally, *ph and euphotic depth (Zeu) were significantly correlated with salinity, suggesting that photoacclimation of the phytoplankton assemblages observed in this study could be induced by discharge of Amur River. Because spatiotemporal variations in PP were concomitant with phi cmax, *ph, and the chlorophyll a concentration, PP models based on inherent optical property (IOP) were suitable for estimating PP in the Sea of Okhotsk. This study is the first to investigate the factors controlling phytoplankton photophysiology in the Sea of Okhotsk, one of the highest primary production areas in the world.
  • K. Sugie, H. Endo, K. Suzuki, J. Nishioka, H. Kiyosawa, T. Yoshimura
    Biogeosciences Discussions Copernicus {GmbH} 10 (3) 4331  1810-6285 2013/03 [Not refereed][Not invited]
  • Yoshiko Kondo, Shigenobu Takeda, Jun Nishioka, Mitsuhide Sato, Hiroaki Saito, Koji Suzuki, Ken Furuya
    JOURNAL OF OCEANOGRAPHY 69 (1) 97 - 115 0916-8370 2013/02 [Refereed][Not invited]
     
    The influence of organic ligands on natural phytoplankton growth was investigated in high-nitrate low-chlorophyll (HNLC) waters and during a phytoplankton bloom induced by a mesoscale iron enrichment experiment (SEEDS II) in the western subarctic Pacific. The growth responses of the phytoplankton in the treatments with iron complexed with model ligand were compared with those with inorganic iron or a control. Desferrioxamine B and protoporphyrin IX were used as models for hydroxamate-type siderophore and tetrapyrrole-type cell breakdown ligand, respectively. In the HNLC water, iron associated with protoporphyrin IX especially stimulated smaller phytoplankton (< 10 mu m) growth, 1.5-fold more than did inorganic iron. Surprisingly, only the addition of protoporphyrin IX stimulated small phytoplankton growth, suggesting that these cell breakdown ligands might be more bioavailable for them. The protoporphyrin IX's stimulatory effect on small phytoplankton was not observed during bloom decline phase. The growth of phytoplankton was inhibited in the treatment with desferrioxamine B-complexed iron, suggesting its low bioavailability for the natural phytoplankton community. Its inhibitory effects were particularly pronounced in pico-eukaryotic phytoplankton. During the iron-induced bloom, the phytoplankton's iron-stress response gradually increased with the desferrioxamine B concentration, suggesting that the competition for iron complexation between natural ligands and desferrioxamine B affected phytoplankton growth. However, the pico-eukaryotes did seem better able to utilize the desferrioxamine B-complexed iron during the bloom-developing phase. These results indicate that the iron bioavailability for phytoplankton differs between bloom-developing and bloom-decline phases.
  • Tomonori Isada, Takahiro Iida, Hongbin Liu, Sei-Ichi Saitoh, Jun Nishioka, Takeshi Nakatsuka, Koji Suzuki
    Journal of Geophysical Research: Oceans 118 (4) 1995 - 2013 2169-9291 2013 [Refereed][Not invited]
     
    We investigated the photosynthetic parameters of phytoplankton in the Sea of Okhotsk during the late summer of 2006 to characterize their spatiotemporal variability and to test the hypothesis that discharge from the Amur River could influence the algal photophysiology. The highest maximum quantum yield of carbon fixation in photosynthesis (Φcmax 0.098 mol C mol photons -1) was found near the Amur River mouth, where nitrate was depleted. However, none of the photosynthetic parameters, including primary productivity (PP) at the surface, were correlated with temperature, daily photosynthetically available radiation (PAR), or ambient nutrient concentrations. Variations in Φcmax depended on the variations in not only the mean chlorophyll a specific absorption coefficient of phytoplankton (ā* ph) but also the slope index of the absorption coefficient of phytoplankton (aph slope), an indicator for the ratio of nonphotosynthetic carotenoids to photosynthetic carotenoids. These results indicated that the phytoplankton assemblages acclimated to the ambient light conditions by regulating their cellular pigments. Additionally, ā*ph and euphotic depth (Zeu) were significantly correlated with salinity, suggesting that photoacclimation of the phytoplankton assemblages observed in this study could be induced by discharge of Amur River. Because spatiotemporal variations in PP were concomitant with Φcmax, ā*ph, and the chlorophyll a concentration, PP models based on inherent optical property (IOP) were suitable for estimating PP in the Sea of Okhotsk. This study is the first to investigate the factors controlling phytoplankton photophysiology in the Sea of Okhotsk, one of the highest primary production areas in the world. Key Points Photoacclimation of phytoplankton influenced the efficiency of carbon fixation. Amur River discharge could affect the algal photophysiology in the Okhotsk Sea. This study is the first report on photosynthetic parameters in the Okhotsk Sea. © 2013. American Geophysical Union. All Rights Reserved.
  • K. Sugie, H. Endo, K. Suzuki, J. Nishioka, H. Kiyosawa, T. Yoshimura
    Biogeosciences 10 (10) 6309 - 6321 1726-4170 2013 [Not refereed][Not invited]
     
    Little is known concerning the effect of CO2 on phytoplankton ecophysiological processes under nutrient and trace element-limited conditions, because most CO2 manipulation experiments have been conducted under elements-replete conditions. To investigate the effects of CO2 and iron availability on phytoplankton ecophysiology, we conducted an experiment in September 2009 using a phytoplankton community in the iron limited, high-nutrient, low-chlorophyll (HNLC) region of the Bering Sea basin . Carbonate chemistry was controlled by the bubbling of the several levels of CO2 concentration (180, 380, 600, and 1000 ppm) controlled air, and two iron conditions were established, one with and one without the addition of inorganic iron. We demonstrated that in the iron-limited control conditions, the specific growth rate and the maximum photochemical quantum efficiency (< i> F< /i> v/< i> F< /i> m) of photosystem (PS) II decreased with increasing CO2 levels, suggesting a further decrease in iron bioavailability under the high-CO2 conditions. In addition, biogenic silica to particulate nitrogen and biogenic silica to particulate organic carbon ratios increased from 2.65 to 3.75 and 0.39 to 0.50, respectively, with an increase in the CO2 level in the iron-limited controls. By contrast, the specific growth rate, < i> F< /i> v/< i> F< /i> m values and elemental compositions in the iron-added treatments did not change in response to the CO2 variations, indicating that the addition of iron canceled out the effect of the modulation of iron bioavailability due to the change in carbonate chemistry. Our results suggest that high-CO2 conditions can alter the biogeochemical cycling of nutrients through decreasing iron bioavailability in the iron-limited HNLC regions in the future. © Author(s) 2013.
  • Jun Nishioka, Hajime Obata, Daisuke Tsumune
    EARTH AND PLANETARY SCIENCE LETTERS 361 26 - 33 0012-821X 2013/01 [Refereed][Not invited]
     
    Ocean scale transect observations covering the entire water depth enable a comprehensive picture of the chemistry including the circulation and biogeochemical cycling of elements in seawater. The large-scale investigation of dissolved iron (Fe) took place through the Japanese-GEOTRACES study and here we report a basin-scale full-depth section profile of dissolved Fe in the Indian Ocean, from the Arabian Sea to the Southern Ocean. The data clearly shows the hydrothermal Fe distributed over 3000 km distance in the deep layer centered at a depth of approximately 3000 m, around the Central Indian Ridge segment, and a large part of the dissolved Fe from the hydrothermal sources was in the real soluble fraction rather than the colloidal fraction. In the intermediate water in the north Arabian Sea, another dissolved Fe rich water mass existed where Fe was enriched by remineralization processes from settling particles and/or adjacent reducing sediments, and preserved in the suboxic water. The basin-scale section profile indicates that there are several sources supplying dissolved Fe to deep waters, such as the hydrothermal sources and terrestrial Fe input with a persistent condition in the oxygen minimum zone (OMZ), between the northern-subtropical section, though few Fe sources were apparent in the Southern Ocean. Combining our size-fractionated Fe data with numerical modeling study suggests that the Fe physical chemical form in seawater differs between the sources and is a key factor for controlling residence time and explaining the large scale distributed hydrothermal Fe. (C) 2012 Elsevier B.V. All rights reserved.
  • Yutaka W. Watanabe, Jun Nishioka, Takeshi Nakatsuka
    GEOPHYSICAL RESEARCH LETTERS 40 (2) 322 - 326 0094-8276 2013/01 [Refereed][Not invited]
     
    Comparing the data sets of chemical transient tracers (trichlorofluoromethane (CFC-11), dichlorodifluoromethane (CFC-12), sulfur hexafluoride (SF6)) among three reobservations from 1993 to 2006 in the Okhotsk Sea, we found the remarkable change in the efficiency of oceanic absorption of anthropogenic carbon (EF) based on the observed change of apparent tracer age. EF above 27.0 sigma(theta) increased by 16 % while EF below 27.0 sigma(theta) declined by 27 % during the period from 1993 to 2006. Consequently, EF in the entire water column decreased by 14 %. We concluded that the recent reinforcement of ocean stratification derived from the ocean warming caused the weakening of water ventilation in this region, indicating that the penetration of atmospheric anthropogenic carbon to the deep ocean interior is becoming difficult. Citation: Watanabe, Y. W., J. Nishioka, and T. Nakatsuka (2013), Decadal time evolution of oceanic uptake of anthropogenic carbon in the Okhotsk Sea, Geophys. Res. Lett., 40, 322-326, doi:10.1002/grl.50113.
  • M. Shigemitsu, T. Okunishi, J. Nishioka, H. Sumata, T. Hashioka, M. N. Aita, S. L. Smith, N. Yoshie, N. Okada, Y. Yamanaka
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 117 0148-0227 2012/06 [Refereed][Not invited]
     
    To investigate the iron cycle at Station A4 in the Oyashio region of the western subarctic Pacific, we developed a 1-D ecosystem model consisting of 14 components including the iron cycle. The parameters associated with the iron cycle were optimized by assimilating monthly averaged data from time series observations for depth-integrated net primary production, nitrate, silicate, dissolved and particulate iron within the surface mixed layer (ML) and at two depths (200 and 300 m depth). The model successfully reproduced the observations and demonstrated that (1) on an annual basis, winter mixing of subsurface water supplies more dissolved iron (Fe-d) to the ML than does dust dissolution, (2) Fe-d concentration in the ML rapidly declines to near-depletion during the peak period of the diatom bloom in spring, which results in an increasing consumption ratio of silicate to nitrogenous nutrients by diatoms as they become more iron-limited, causing a more rapid decrease of silicate compared to that of nitrogenous nutrients in the ML, followed by the silicate limitation of diatoms, and (3) Fe-d supplied to the ML by dust dissolution and desorption from particulate iron, by alleviating iron limitation of phytoplankton, supports their continuous utilization of nitrate from spring to fall even though Fe-d concentration in the ML remains low after the peak spring bloom. The model explained quantitatively the above behavior of Fe-d and other nutrients associated with Fe-d over the annual cycle in the Oyashio region.
  • T. Tanaka, I. Yasuda, K. Kuma, J. Nishioka
    GEOPHYSICAL RESEARCH LETTERS 39 0094-8276 2012/04 [Refereed][Not invited]
     
    To evaluate the impact of vertical turbulent iron flux on the summertime biological productivity in the Bering Sea Green Belt (GB), we conducted the concurrent observations of dissolved iron (D-Fe) and turbulence in the Bering Sea for the first time. We show that the GB can be sustained by iron supply from iron-rich, subsurface thick layer distributed along the southeastern shelf break where the GB is located, via strong turbulent vertical mixing. The flux ratio of D-Fe and nitrate was within the range of the N/Fe uptake ratio by GB phytoplankton, suggesting this flux of nutrients can sustain GB productivity. We also analyzed historical hydrographic data and suggest the thick subsurface layer along the GB is formed by the mixing of relatively warm water with some iron from the Aleutian Passes and iron-rich outer-shelf cold water in which the D-Fe derived from seafloor sediment is suspended due to strong vertical mixing. Citation: Tanaka, T., I. Yasuda, K. Kuma, and J. Nishioka (2012), Vertical turbulent iron flux sustains the Green Belt along the shelf break in the southeastern Bering Sea, Geophys. Res. Lett., 39, L08603, doi:10.1029/2012GL051164.
  • M. Shigemitsu, T. Okunishi, J. Nishioka, H. Sumata, T. Hashioka, M. N. Aita, S. L. Smith, N. Yoshie, N. Okada, Y. Yamanaka
    Journal of Geophysical Research: Oceans 117 (6) 2169-9291 2012 [Refereed][Not invited]
     
    To investigate the iron cycle at Station A4 in the Oyashio region of the western subarctic Pacific, we developed a 1-D ecosystem model consisting of 14 components including the iron cycle. The parameters associated with the iron cycle were optimized by assimilating monthly averaged data from time series observations for depth-integrated net primary production, nitrate, silicate, dissolved and particulate iron within the surface mixed layer (ML) and at two depths (200 and 300 m depth). The model successfully reproduced the observations and demonstrated that (1) on an annual basis, winter mixing of subsurface water supplies more dissolved iron (Fed) to the ML than does dust dissolution, (2) Fed concentration in the ML rapidly declines to near-depletion during the peak period of the diatom bloom in spring, which results in an increasing consumption ratio of silicate to nitrogenous nutrients by diatoms as they become more iron-limited, causing a more rapid decrease of silicate compared to that of nitrogenous nutrients in the ML, followed by the silicate limitation of diatoms, and (3) Fed supplied to the ML by dust dissolution and desorption from particulate iron, by alleviating iron limitation of phytoplankton, supports their continuous utilization of nitrate from spring to fall even though Fed concentration in the ML remains low after the peak spring bloom. The model explained quantitatively the above behavior of Fed and other nutrients associated with Fed over the annual cycle in the Oyashio region. © 2012 American Geophysical Union. All Rights Reserved.
  • K. Misumi, D. Tsumune, Y. Yoshida, K. Uchimoto, T. Nakamura, J. Nishioka, H. Mitsudera, F. O. Bryan, K. Lindsay, J. K. Moore, S. C. Doney
    Journal of Geophysical Research: Biogeosciences 116 (3) G03005  0148-0227 2011/09/01 [Refereed][Not invited]
     
    Mechanisms controlling the dissolved iron distribution in the North Pacific are investigated using the Biogeochemical Elemental Cycling (BEC) model with a resolution of approximately 1 in latitude and longitude and 60 vertical levels. The model is able to reproduce the general distribution of iron as revealed in available field data: surface concentrations are generally below 0.2 nM concentrations increase with depth and values in the lower pycnocline are especially high in the northwestern Pacific and off the coast of California. Sensitivity experiments changing scavenging regimes and external iron sources indicate that lateral transport of sedimentary iron from continental margins into the open ocean causes the high concentrations in these regions. This offshore penetration only appears under a scavenging regime where iron has a relatively long residence time at high concentrations, namely, the order of years. Sedimentary iron is intensively supplied around continental margins, resulting in locally high concentrations the residence time with respect to scavenging determines the horizontal scale of elevated iron concentrations. Budget analysis for iron reveals the processes by which sedimentary iron is transported to the open ocean. Horizontal mixing transports sedimentary iron from the boundary into alongshore currents, which then carry high iron concentrations into the open ocean in regions where the alongshore currents separate from the coast, most prominently in the northwestern Pacific and off of California. Copyright 2011 by the American Geophysical Union.
  • Sachihiko Itoh, Ichiro Yasuda, Masahiro Yagi, Satoshi Osafune, Hitoshi Kaneko, Jun Nishioka, Takeshi Nakatsuka, Yuri N. Volkov
    GEOPHYSICAL RESEARCH LETTERS 38 0094-8276 2011/08 [Refereed][Not invited]
     
    Microstructure measurements were conducted in one of the Kuril Straits in the summer of 2007. Over the course of 1 day of repeated observations across the Pacific side of the steep sill of the Urup Strait, extremely strong mixing was observed during periods of Pacific-ward (down-sill) flows and during the transition from Pacific-ward to Okhotskward (up-sill) flows, with a turbulent energy dissipation rate epsilon of 10(-6) to 10(-5) W kg(-1) and vertical diffusivity of 10(-1) to 5 x 10(-1) m(2) s(-1). During the period of strong mixing, we observed homogeneous layers with a thickness of 300-600 m and potential density of 26.6-26.7 sigma(theta), occupying the entire water column in one case. High values of epsilon within this layer indicate the injection of diapycnal flows from the upper and lower layers, possibly contributing to southward intrusion of intermediate water into the subtropical gyre. Citation: Itoh, S., I. Yasuda, M. Yagi, S. Osafune, H. Kaneko, J. Nishioka, T. Nakatsuka, and Y. N. Volkov (2011), Strong vertical mixing in the Urup Strait, Geophys. Res. Lett., 38, L16607, doi:10.1029/2011GL048507.
  • K. Misumi, D. Tsumune, Y. Yoshida, K. Uchimoto, T. Nakamura, J. Nishioka, H. Mitsudera, F. O. Bryan, K. Lindsay, J. K. Moore, S. C. Doney
    JOURNAL OF GEOPHYSICAL RESEARCH-BIOGEOSCIENCES 116 doi:10.1029/2010JG001541  2169-8953 2011/07 [Refereed][Not invited]
     
    Mechanisms controlling the dissolved iron distribution in the North Pacific are investigated using the Biogeochemical Elemental Cycling (BEC) model with a resolution of approximately 1 in latitude and longitude and 60 vertical levels. The model is able to reproduce the general distribution of iron as revealed in available field data: surface concentrations are generally below 0.2 nM; concentrations increase with depth; and values in the lower pycnocline are especially high in the northwestern Pacific and off the coast of California. Sensitivity experiments changing scavenging regimes and external iron sources indicate that lateral transport of sedimentary iron from continental margins into the open ocean causes the high concentrations in these regions. This offshore penetration only appears under a scavenging regime where iron has a relatively long residence time at high concentrations, namely, the order of years. Sedimentary iron is intensively supplied around continental margins, resulting in locally high concentrations; the residence time with respect to scavenging determines the horizontal scale of elevated iron concentrations. Budget analysis for iron reveals the processes by which sedimentary iron is transported to the open ocean. Horizontal mixing transports sedimentary iron from the boundary into alongshore currents, which then carry high iron concentrations into the open ocean in regions where the alongshore currents separate from the coast, most prominently in the northwestern Pacific and off of California.
  • K. Uchimoto, T. Nakamura, J. Nishioka, H. Mitsudera, M. Yamamoto-Kawai, K. Misumi, D. Tsumune
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 116 (C02034) doi:10.1029/2010JG001541  2169-9275 2011/02 [Refereed][Not invited]
     
    Ventilation of waters in and around the Sea of Okhotsk was investigated using simulations of chlorofluorocarbons (CFCs) in the northwestern North Pacific. We used an ocean general circulation model coupled with a sea ice model. The model reproduces the distributions of CFCs similar to observed values and indicates the importance of tidal mixing along the Kuril Islands and brine rejection to ventilation of waters in and around the Sea of Okhotsk. To clarify the role of each process, numerical experiments excluding one of the two processes were carried out. Results show that brine rejection transports CFCs into the intermediate layer as deep as 200-400 m along the path of dense shelf water in the western Sea of Okhotsk, but hardly to other areas and layers. On the other hand, tidal mixing transports CFCs into the intermediate and deeper layers throughout the Sea of Okhotsk. We conclude that the tidal mixing has a greater influence than brine rejection on the ventilation of layers below the winter mixed layer.
  • Jun Nishioka, Tsuneo Ono, Hiroaki Saito, Keiichiro Sakaoka, Takeshi Yoshimura
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 116 2169-9275 2011/02 [Refereed][Not invited]
     
    Multiyear (2003-2008) time series observations along the A line provided information on the temporal variability of the dissolved iron (diss-Fe) concentration in the Oyashio region of the western subarctic Pacific, and the data indicated that there was an annual cycle in the concentration of surface diss-Fe occurring every year. Diss-Fe was supplied into the surface water in this region every winter and supports the spring phytoplankton bloom after development of the thermocline. The diss-Fe concentration was drawn down during the phytoplankton bloom period and was depleted in summer in some water masses. Then diss-Fe increased from autumn to winter with the increasing depth of the surface mixed layer. The high diss-Fe concentrations in the surface layer in winter were controlled by mesoscale oceanic intrinsic processes, such as vertical winter mixing and horizontal Fe-rich intermediate water transport. Difference in magnitude of the winter mixing processes among different water masses caused the heterogeneous distribution of diss-Fe concentration in the surface layer. Moreover, the vertical section profiles along a cross-Oyashio transect showed the occurrence of Fe-rich intermediate water, and upward transport of materials from the intermediate water to the surface layer via tidal and winter mixing processes are important mechanisms to explain the high winter surface diss-Fe concentrations. Additionally, the substantially higher diss-Fe/NO3 ratio in the winter surface layer in this studied area other than the high-nutrient low-chlorophyll region indicates that the winter surface water in the Oyashio and the Oyashio-Kuroshio transition zone has a high potential to stimulate phytoplankton growth.
  • 気候工学（ジオエンジニアリング)
    杉山昌広, 西岡 純, 藤原正智
    天気 58 (7) 3 - 24 2011 [Refereed][Not invited]
  • Nishioka Jun, Obata Hajime, Tsumune Daisuke
    Abstracts of Annual Meeting of the Geochemical Society of Japan 日本地球化学会 58 160 - 160 2011 
    海洋における微量金属元素は、供給過程、物理的な海洋循環と混合、生物生産、粒状・溶存物質などと密接に関連し、分布や循環が支配されている。また、観測には厳密なクリーン技術と分析技術が必要となる。これまで実施されてきた微量金属元素の観測は、その困難さから、十分な空間スケールを網羅しておらず、各大洋における微量金属の分布の全貌を捉えるには至っていない。GEOTRACES計画では、大洋縦断（横断）観測を実施し、微量元素および同位体の全海洋の分布とそれを支配する要因を明らかにすることを目的としている。2009年11月から2010年1月に、国際GEOTRACES計画の一環として、白鳳丸KH09-5次航海でインド洋の縦断観測が実施された。本報告では、Key Parameterの一つである溶存鉄濃度の船上分析の結果を報告する。
  • K. Uchimoto, T. Nakamura, J. Nishioka, H. Mitsudera, M. Yamamoto-Kawai, K. Misumi, D. Tsumune
    Journal of Geophysical Research: Oceans 116 (2) 2169-9291 2011 [Refereed][Not invited]
     
    Ventilation of waters in and around the Sea of Okhotsk was investigated using simulations of chlorofluorocarbons (CFCs) in the northwestern North Pacific. We used an ocean general circulation model coupled with a sea ice model. The model reproduces the distributions of CFCs similar to observed values and indicates the importance of tidal mixing along the Kuril Islands and brine rejection to ventilation of waters in and around the Sea of Okhotsk. To clarify the role of each process, numerical experiments excluding one of the two processes were carried out. Results show that brine rejection transports CFCs into the intermediate layer as deep as 200-400 m along the path of dense shelf water in the western Sea of Okhotsk, but hardly to other areas and layers. On the other hand, tidal mixing transports CFCs into the intermediate and deeper layers throughout the Sea of Okhotsk. We conclude that the tidal mixing has a greater influence than brine rejection on the ventilation of layers below the winter mixed layer. Copyright 2011 by the American Geophysical Union.
  • Jun Nishioka, Tsuneo Ono, Hiroaki Saito, Keiichiro Sakaoka, Takeshi Yoshimura
    Journal of Geophysical Research: Oceans 116 (2) 2169-9291 2011 [Refereed][Not invited]
     
    Multiyear (2003-2008) time series observations along the A line provided information on the temporal variability of the dissolved iron (diss-Fe) concentration in the Oyashio region of the western subarctic Pacific, and the data indicated that there was an annual cycle in the concentration of surface diss-Fe occurring every year. Diss-Fe was supplied into the surface water in this region every winter and supports the spring phytoplankton bloom after development of the thermocline. The diss-Fe concentration was drawn down during the phytoplankton bloom period and was depleted in summer in some water masses. Then diss-Fe increased from autumn to winter with the increasing depth of the surface mixed layer. The high diss-Fe concentrations in the surface layer in winter were controlled by mesoscale oceanic intrinsic processes, such as vertical winter mixing and horizontal Fe-rich intermediate water transport. Difference in magnitude of the winter mixing processes among different water masses caused the heterogeneous distribution of diss-Fe concentration in the surface layer. Moreover, the vertical section profiles along a cross-Oyashio transect showed the occurrence of Fe-rich intermediate water, and upward transport of materials from the intermediate water to the surface layer via tidal and winter mixing processes are important mechanisms to explain the high winter surface diss-Fe concentrations. Additionally, the substantially higher diss-Fe/NO3 ratio in the winter surface layer in this studied area other than the high-nutrient low-chlorophyll region indicates that the winter surface water in the Oyashio and the Oyashio-Kuroshio transition zone has a high potential to stimulate phytoplankton growth. Copyright © 2011 by the American Geophysical Union.
  • Uchimoto, K, T. Nakamura, J. Nishioka, H. Mitsudera, M. Yamamoto-Kawai, K. Misumi, D. Tsumune
    Journal of Geophysical Research 116 (C2) C02034  0148-0227 2011 [Refereed][Not invited]
  • Takeshi Yoshimura, Jun Nishioka, Takeshi Nakatsuka
    DEEP-SEA RESEARCH PART I-OCEANOGRAPHIC RESEARCH PAPERS 57 (11) 1454 - 1464 0967-0637 2010/11 [Refereed][Not invited]
     
    To clarify the iron (Fe) nutritional status of the phytoplankton assemblage in the Okhotsk Sea we conducted incubation experiments in summer 2006 Replicate surface seawater samples with the natural plankton community were incubated with three treatments Fe enrichment addition of the strong Fe chelator siderophore desferriferrioxime B (DFB) which strips Fe from the biologically accessible pool and as a control no addition To prevent macronutrient limitation we added surplus nutrients to all treatments At all 4 stations in Sakhalin Bay near the mouth of the Amur River and around the east of Sakhalin Island net specific growth rate showed no significant difference between the control and +Fe treatment and was repressed in +DFB treatment both in large- and small-sized phytoplankton These findings indicate that these waters contain sufficient bioavailable Fe and that the Amur River plume which is transported by the east Sakhalin current is a major source of the Fe In the Bussol Strait net specific growth rate in the control was significantly higher than +DFB treatment suggesting a supply of bioavailable Fe through intense vertical mixing at this site Iron enrichment treatment stimulated the net specific growth rate of large-sized phytoplankton indicating that Fe still limits the growth for the large-sized phytoplankton assemblage but not for small-sized phytoplankton in this area An Index of Fe availability was defined to quantify the degree of ambient Fe availability in each station and it revealed the spatial variability of ambient Fe availabilities among the sites (C) 2010 Elsevier Ltd All rights reserved
  • Takeshi Yoshimura, Jun Nishioka, Koji Suzuki, Hiroshi Hattori, Hiroshi Kiyosawa, Yutaka W. Watanabe
    JOURNAL OF EXPERIMENTAL MARINE BIOLOGY AND ECOLOGY 395 (1-2) 191 - 198 0022-0981 2010/11 [Refereed][Not invited]
     
    Increasing CO2 in seawater (i.e. ocean acidification) may have various and potentially adverse effects on phytoplankton dynamics and hence the organic carbon dynamics. We conducted a CO2 manipulation experiment in the Sea of Okhotsk in summer 2006 to investigate the response of the organic carbon dynamics. During the 14-day incubation of nutrient depleted and 200 mu atm in situ pCO(2) surface water with a natural plankton assemblage under 150, 280, 480, and 590 mu atm pCO(2), the amount of net dissolved organic carbon accumulation was significantly lower at >480 mu atm pCO(2) than at 150 mu atm pCO(2), while differences in net particulate organic carbon accumulation between the treatments were small and did not show a clear relationship with the pCO(2). This is the first report to show a decreased net organic carbon production of natural plankton community under elevated pCO(2). Phytoplankton pigment analysis suggests that the relative contribution of fucoxanthin-containing phytoplankton such as diatoms to the phytoplankton biomass was lower at >280 mu atm pCO(2) than at 150 mu atm pCO(2). Different pCO(2) conditions may alter the organic carbon dynamics through changes in plankton processes. We conclude that the continuing increase in atmospheric CO2 in a time scale from the last half century to the end of this century has potential to affect the carbon cycle in nutrient depleted subpolar surface waters. (C) 2010 Elsevier B.V. All rights reserved.
  • Marie Boye, Jun Nishioka, Peter Croot, Patrick Laan, Klaas R. Timmermans, Volker H. Strass, Shigenobu Takeda, Hein J. W. de Baar
    MARINE CHEMISTRY 122 (1-4) 20 - 27 0304-4203 2010/10 [Refereed][Not invited]
     
    Vertical distributions of iron and iron binding ligands were determined in 2 size classes (dissolved <0.2 mu m, soluble <200 kDa, e.g., similar to 0.03 mu m) in the Southern Ocean. Colloidal iron and complexing capacity (>200 kDa-<0.2 mu m) were inferred as the difference between the dissolved and soluble fractions. Dissolved iron and ligands exist primarily in the soluble size range in the surface waters, although iron-complexing colloids still represent a significant portion of the dissolved pool and this fraction increases markedly with depth. This work presents evidence for the colloidal nature of a significant portion (37-51% on average) of the 'dissolved' organic Fe pool in these oceanic waters. From the data it was not possible to discern whether iron colloids exist as discrete organic complexes and/or inorganic amorphous colloids. Iron-complexing colloids are the most saturated with iron at the thermodynamic equilibrium, whereas soluble organic ligands occur in larger excess compared to soluble iron. It suggests that the exchangeable fraction for iron uptake through dissociation of Fe complexes likely occurs in the soluble fraction, and that soluble ligands have the potential to buffer iron inputs to surface waters whereas iron colloids may aggregate and settle. Expectations based on Fe diffusion rates, distributions and the stability of the soluble iron complexes and iron colloids also suggest that the weaker soluble Fe complexes may be more bio-available, while the strongest colloids may be a major route for iron removal from oceanic waters. Investigations of the size classes of the dissolved organic iron thus can significantly increase our understanding of the oceanic iron cycle. (C) 2010 Elsevier B.V. All rights reserved.
  • S. Itoh, I. Yasuda, T. Nakatsuka, J. Nishioka, Y. N. Volkov
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 115 (C08004) doi:10.1029/2009JC005629.  0148-0227 2010/08 [Refereed][Not invited]
     
    For the first time, we performed continuous observations of turbulent mixing upon the sill of the Urup Strait, Kuril Islands, for a period of 1 day during August of 2006. Time series of velocity profiles, water properties, and isopycnal depths show variability with a period of about 1 day; however, the water mass variability is not explained by simple water exchange between the Sea of Okhotsk and the North Pacific. Empirical orthogonal function (EOF) analysis of isopycnal coordinates extracted variability presumed to represent tidal flows as major EOFs. In profiles of the residual flow after subtracting the mean flow and major EOFs, we detected phase propagations consistent with internal gravity waves, with an approximate period of 9.1-11.4 h. The turbulent energy dissipation rate epsilon and vertical diffusivity K(rho) ranged from 1 x 10(-9) to 3 x 10(-8) W kg(-1) and from 3 x 10(-5) to 2 x 10(-3) m(2) s(-1), respectively. Occurrences of high dissipation rates generally corresponded to either strong shear or weak stratification. A significant contribution of shear S to the dissipation rate epsilon was indicated with a relationship of epsilon similar to S(1.4)-S(1.8), probably related to propagating internal waves. A strong mixing event was observed within the water column, with extremely weak stratification and a Richardson number below the critical value of 0.25, when the flow direction changed from up-sill to down-sill.
  • Youhei Yamashita, Rose M. Cory, Jun Nishioka, Kenshi Kuma, Eiichiro Tanoue, Rudolf Jaffe
    DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 57 (16) 1478 - 1485 0967-0645 2010/08 [Not refereed][Not invited]
     
    Fluorescent dissolved organic matter (DOM), a fraction of chromophoric DOM, is known to be produced in the deep ocean and is considered to be bio-refractory. However, the factors controlling fluorescence properties of DOM in the deep ocean are still not well understood. In this study, we determined the fluorescence properties of DOM in the deep waters of the Okhotsk Sea and the northwestern North Pacific Ocean using excitation-emission matrix (EEM) fluorescence and parallel factor analysis (PARAFAC). One protein-like, two humic-like components, and one uncertain component, which might be derived from a fluorometer artifact, were identified by EEM-PARAFAC. Fluorescence intensity levels of the protein-like component were highest in the surface waters, decreased with depth, but did not change systematically in the bathypelagic layer (1000 m - bottom). Fluorescence characteristics of the two humic-like components were similar to those traditionally defined as marine and terrestrial humiclike fluorophores. The fluorescence intensity levels of the two humic-like components were lowest in the surface waters, increased with depth in the mesopelagic layer (200 - 1000 m), and then slightly decreased with depth in the bathypelagic layer. The ratio of the two humic-like components remained in a relatively narrow range in the bathypelagic layer compared to that in the surface layer, suggesting a similar composition of humic-like fluorophores in the bathypelagic layer. In addition, the fluorescence intensities of the two humic-like components were linearly correlated to apparent oxygen utilization (AOU) in the bathypelagic layer, suggesting that both humic-like components are produced in situ as organic matter is oxidized biologically. These findings imply that optical characteristics of humic-like fluorophores once formed might not be altered further biologically or geochemically in the deep ocean. On the other hand, relationships of fluorescence intensities with AOU and Fe(III) solubility were different between the two humic-like components in the mesopelagic layer, suggesting different environmental dynamics and biogeochemical roles for the two humic-like components. (C) 2010 Elsevier Ltd. All rights reserved.
  • Status and trends of the Sea of Okhotsk region, 2003-2008
    Radchenko, V.I, Dulepova, E.P, Figurkin, A.L, Katugin, O.N, Ohshima, K, Nishioka, J, McKinnell, S.M, Tsoy, A.T
    Marine Ecosystems of the North Pacific Ocean, 2003-2008. PICES Special Publication 4, 393 p.  - 393 2010/05 [Refereed][Not invited]
  • Daiki Nomura, Jun Nishioka, Mats A. Granskog, Andreas Krell, Sumito Matoba, Takenobu Toyota, Hiroshi Hattori, Kunio Shirasawa
    MARINE CHEMISTRY 119 (1-4) 1 - 8 0304-4203 2010/04 [Refereed][Not invited]
     
    Samples of first-year sea ice, snow and under-ice water were collected in the southern Sea of Okhotsk in mid-February 2007 and 2008 to elucidate the processes controlling nutrient concentrations in sea ice. Temperature, salinity, oxygen isotopic ratio (delta O-18) and inorganic nutrient concentrations (NO3, NH4, NO2, PO4 and SiO2) were measured. Sea ice was categorized into four types; snow-ice, frazil ice, columnar ice and a mixture of granular and columnar ice, based on delta O-18 composition and ice texture. Frazil ice dominated the total ice thickness (52.8%), and columnar ice was sandwiched between frazil ice layers, indicating dynamic ice-growth processes such as rafting and ridging. Furthermore, the ice was banded by layers of particulate materials (sediment layers), which were frequently encountered during cruises. High NO3 and NH4 concentrations were found in snow and snow-ice implying that these were supplied from the atmosphere with snowfall and incorporated into the sea ice through snow-ice formation. In the sediment-laden layers, which were categorized as frazil ice, NO2, PO4 and SiO2 concentrations were highest of all the ice types and considerably enriched compared to parent seawater, suggesting the remineralization of the particulate organic matter. On the other hand, NO3 concentrations in sediment layers were low (depleted), leading to extremely low N (NO3 + NH4 + NO2): P ratios in sediment layers, from 0.2 to 0.8, with respect to that of under-ice water or Redfield ratio. These results suggest that in part of sediment-laden layers fixed-nitrogen was removed partially as molecular nitrogen (N-2) from the sea ice environment by anaerobic nitrate reduction processes (denitrification) by denitrifying bacteria while adding phosphate from associated remineralization of organic phosphorus. The effect of melting of snow and sea ice is dilution for salinity, NO3 and SiO2, no change in NO2 and PO4, and a minor enrichment for NH4 in the mixed layer in spring and early summer. This suggests that snow/ice meltwater with different nutrient ratios than in under-ice water/Redfield ratio is supplied to under-ice water during melt season in April/May in southern Sea of Okhotsk. However, the impact of sediment-laden sea ice cannot be assessed at this point. (C) 2009 Elsevier B.V. All rights reserved.
  • Ai Hattori-Saito, Jun Nishioka, Tsuneo Ono, R. Michael L. Mckay, Koji Suzuki
    JOURNAL OF OCEANOGRAPHY 66 (1) 105 - 115 0916-8370 2010/02 [Refereed][Not invited]
     
    In order to detect iron (Fe) stress in micro-sized (20-200 mu m) diatoms in the Oyashio region, western subarctic Pacific during spring, immunological ferredoxin/navodoxin assays were applied to samples collected from the surface layer in May 2005. Concomitantly, the community composition of the micro-sized phytoplankton and hydrographic conditions, including dissolved Fe and macronutrient concentrations, were also examined. Chlorophyll (Chl) a concentrations were <2 mg m(-3) at all sampling stations, except at a station where the Chi a level was 9.0 mg m(-3) and a microsized diatom bloom occurred. A high abundance of ferredoxin in micro-sized diatoms was detected only at a rather near-shore station where dissolved Fe and macronutrient concentrations were higher, indicating that the micro-sized diatoms did not suffer from iron deficiency. On the other hand, flavodoxin in micro-sized diatoms was often observed at the other stations, including the bloom station, where macronutrients were replete but dissolved Fe concentration was low (0.31 nM). A significant amount of chlorophyllide a, a degradation product of Chi a, was also observed at the bloom station, suggesting a decline of the diatom bloom. The micro-sized phytoplankton species at all the stations were mainly composed of the diatoms Thalassiosira, Chaetoceros, and Fragilariopsis spp. Our study indicates that micro-sized diatoms were stressed by Fe bioavailability during the spring season in the Oyashio region.
  • Atsushi Tsuda, Kimio Fukami, Hiroshi Kiyosawa, Koji Suzuki, Shigenobu Takeda, Jun Nishioka, Masayuki Takahashi, Keith Johnson, Chi-Shing Wong
    Plankton and Benthos Research 5 (4) 144 - 155 1880-8247 2010 [Not refereed][Not invited]
     
    The roles of heterotrophic organisms (microzooplankton, mesozooplankton, bacteria and heterotrophic nanoflagellates) were examined during a nutrient enrichment experiment using a mesocosm in Saanich Inlet, British Columbia, Canada. Grazing rates of microzooplankton, copepods, and Noctiluca scintillans were respectively estimated by the dilution method, from the egg production, and the apparent growth rate. The primary production increased by about 11 times during the initial 3 days, and the grazing rate by zooplankton also increased by 7.4 times. The primary production exceeded the grazing rate during the initial 5 days, after that, almost balanced rates were observed. Biomass peaks of bacteria and HNFs (heterotrophic nanoflagellates) were observed after the decline of the phytoplankton bloom. Bacterial production and HNF bacterivory gradually increased from the beginning to the end of the experiment. Microzooplankton consistently removed about half of the primary production. The contribution of microzooplankton to grazing was largest during the initial 7 days. Heterotrophic dinoflagellates were the most dominant component of the microzooplankton, but oligotrich ciliates showed the fastest growth response to phytoplankton production. Noctiluca scintillans became an important grazer after the bloom. Overall, the contribution of microzooplankton grazing was the largest of the processes through which phytoplankton were lost. Cell sinking was a minor component contributing to loss of phytoplankton. Thus, oligotrich ciliates and heterotrophic dinoflagellates were the most plausible organisms contributing to the steady state of phytoplankton concentrations. © The Plankton Society of Japan.
  • Ai Hattori-Saito, Jun Nishioka, Tsuneo Ono, R. Michael L. McKay, Koji Suzuki
    Journal of Oceanography 66 (1) 105 - 115 0916-8370 2010/01 [Not refereed][Not invited]
     
    In order to detect iron (Fe) stress in micro-sized (20-200 μm) diatoms in the Oyashio region, western subarctic Pacific during spring, immunological ferredoxin/flavodoxin assays were applied to samples collected from the surface layer in May 2005. Concomitantly, the community composition of the micro-sized phytoplankton and hydrographic conditions, including dissolved Fe and macronutrient concentrations, were also examined. Chlorophyll (Chl) a concentrations were < 2 mg m-3 at all sampling stations, except at a station where the Chl a level was 9.0 mg m-3 and a micro-sized diatom bloom occurred. A high abundance of ferredoxin in micro-sized diatoms was detected only at a rather near-shore station where dissolved Fe and macronutrient concentrations were higher, indicating that the micro-sized diatoms did not suffer from iron deficiency. On the other hand, flavodoxin in micro-sized diatoms was often observed at the other stations, including the bloom station, where macronutrients were replete but dissolved Fe concentration was low (0.31 nM). A significant amount of chlorophyllide a, a degradation product of Chl a, was also observed at the bloom station, suggesting a decline of the diatom bloom. The micro-sized phytoplankton species at all the stations were mainly composed of the diatoms Thalassiosira, Chaetoceros, and Fragilariopsis spp. Our study indicates that micro-sized diatoms were stressed by Fe bioavailability during the spring season in the Oyashio region © Springer Science+Business Media B.V. 2010.
  • S. Itoh, I. Yasuda, T. Nakatsuka, J. Nishioka, Y. N. Volkov
    Journal of Geophysical Research: Oceans 115 (8) 2169-9291 2010 [Refereed][Not invited]
     
    For the first time, we performed continuous observations of turbulent mixing upon the sill of the Urup Strait, Kuril Islands, for a period of 1 day during August of 2006. Time series of velocity profiles, water properties, and isopycnal depths show variability with a period of about 1 day however, the water mass variability is not explained by simple water exchange between the Sea of Okhotsk and the North Pacific. Empirical orthogonal function (EOF) analysis of isopycnal coordinates extracted variability presumed to represent tidal flows as major EOFs. In profiles of the residual flow after subtracting the mean flow and major EOFs, we detected phase propagations consistent with internal gravity waves, with an approximate period of 9.1-11.4 h. The turbulent energy dissipation rate ε and vertical diffusivity Kρ ranged from 1 × 10-9 to 3 × 10-8 W kg-1 and from 3 × 10-5 to 2 × 10-3 m2s -1, respectively. Occurrences of high dissipation rates generally corresponded to either strong shear or weak stratification. A significant contribution of shear S to the dissipation rate ε was indicated with a relationship of Ε ∼ S1.4-S1.8, probably related to propagating internal waves. A strong mixing event was observed within the water column, with extremely weak stratification and a Richardson number below the critical value of 0.25, when the flow direction changed from up-sill to down-sill. Copyright 2010 by the American Geophysical Union.
  • Nishioka J, Ono T, Saito H, Sakaoka K, Yoshimura T, Matoba
    AGU Fall Meeting Abstracts 1 0331  2010 [Refereed][Not invited]
  • Hongmei Jing, Hongbin Liu, Koji Suzuki, Rumi Sohrin, Jun Nishioka
    AQUATIC MICROBIAL ECOLOGY 61 (2) 191 - 204 0948-3055 2010 [Refereed][Not invited]
     
    We investigated spatial variations of the bacterial and archaeal community composition in surface waters of the Sea of Okhotsk during summer of 2006, using denaturing gradient gel electrophoresis (DGGE). We determined phylogenetic assemblages, including Actinobacteria, Bacteroidetes, Cyanobacteria, Alpha-and Gammaproteobacteria, and Marine Group II Euryarchaea, and successfully revealed their spatial distribution patterns. At most stations, Bacteria had higher diversity than Archaea, indicated by the relatively higher Shannon-Weaver indexes and numbers of operational taxonomic units (OTUs). Canonical correspondence analysis (CCA) and redundancy analysis (RDA) demonstrated that the community compositions of bacterial populations in the Sea of Okhotsk were closely associated with total prokaryote abundance, while the spatial distributions of bacterial phylogenetic groups were more related to both the concentration of dissolved organic nitrogen (DON) and the ratio of DON versus dissolved organic carbon concentration (DOC:DON). On the other hand, the spatial variation of the community composition of Archaea had no significant association with environmental parameters.
  • Naoki Yoshie, Koji Suzuki, Akira Kuwata, Jun Nishioka, Hiroaki Saito
    MARINE ECOLOGY PROGRESS SERIES 399 39 - 52 0171-8630 2010 [Refereed][Not invited]
     
    Variations in photosynthetic parameters of phytoplankton during the spring bloom were investigated in the western subarctic Pacific. Light-saturated maximum photosynthesis rate (P-max(B)) and light-limited slope (alpha(B)) in the photosynthesis-irradiance (P-E) curve were almost constant in the first half of the bloom and then concurrently increased by 5 times in the latter half with a constant light-saturation index (E-k = p(max)(B)/alpha(B)). In contrast, the maximum photochemical quantum efficiency (F-v/F-m) of the algal photosystem II increased in the first half and thereafter decreased. Water temperature significantly affected the covariation in P-max(B) and alpha(B) (i.e. E-k-independent variability), suggesting that dark reaction processes in photosynthesis mainly regulated the changes in the P-E parameters. Although influences of nitrate, silicic acid, and dissolved iron on the P-E parameters were not observed, the elevated ammonium levels found in the latter half of the bloom could contribute to the E-k-independent variability. There was no significant relationship among algal net growth rate, P-max(B), and alpha(B) throughout the observations. Since most ecosystem models have used a constant alpha(B), parallel changes in P-max(B) and alpha(B) should be taken into account in models reproducing ecosystem dynamics during spring diatom blooms.
  • Daisuke Tsumune, Jun Nishioka, Akifumi Shimamoto, Yutaka W. Watanabe, Takafumi Aramaki, Yukihiro Nojiri, Shigenobu Takeda, Atsushi Tsuda, Takaki Tsubono
    DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 56 (26) 2948 - 2957 0967-0645 2009/12 [Refereed][Not invited]
     
    Sulfur hexafluoride (SF(6)) tracer release experiments were carried out to trace the iron-fertilized water mass during the iron-fertilization experiments in the western North Pacific of Subarctic Pacific iron Experiment for Ecosystem Dynamics Study II (SEEDS II) in 2004. A solution of Fe and SF(6) tracer was released into the surface mixed layer over an 8 x 8 km area, and the fertilized patch was traced by onboard SF(6) analysis for 12 days during each experiment. A Lagrangian frame of reference was maintained by the use of a drogued GPS buoy released at the center of the patch to reduce the advection effect on observations. The patch moved along the contour of sea-surface height (SSH) of a clockwise mesoscale eddy for 4 days after release. Then strong easterly winds dragged the patch across the contour of SSH. The patch behavior was affected by both the mesoscale eddy and surface winds. Apparent horizontal diffusivities were determined by the change of the distribution of SF(6) concentrations. The averaged apparent horizontal diffusivity was about 49 m(2) s(-1) during SEEDS II. It was larger than the one in SEEDS. Mixed-layer depth (MLD) was 8.5-18 m during SEEDS, and 12-33 In during SEEDS II. The larger horizontal diffusivity and deeper MLD in SEEDS II were disadvantages to maintain a high iron concentration in the surface layer compared to SEEDS. Temporal change of the MLD corresponded to the temporal change of chlorophyll-a concentration. Temporal change in the surface MLD was also important for the response of phytoplankton by iron fertilization. (C) 2009 Elsevier Ltd. All rights reserved.
  • Seiji Nakatsuka, Kei Okamura, Shigenobu Takeda, Jun Nishioka, M. Lutfi Firdaus, Kazuhiro Norisuye, Yoshiki Sohrin
    DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 56 (26) 2822 - 2838 0967-0645 2009/12 [Refereed][Not invited]
     
    During mesciscale Fe enrichment (SEEDS II) in the western North Pacific ocean, we investigated dissolved and particulate Co, Ni, Cu, Zn, Cd and Pb in seawater from both field observation and shipboard bottle incubation of a natural phytoplankton assemblage with Fe addition. Before the Fe enrichment, strong correlations between dissolved trace metals (Ni, Zn and Cd) and pO(4)(3-), and between particulate trace metals (Ni, Zn and Cd) and chlorophyll were obtained suggesting that biogeochemical cycles mainly control the distributions of Ni, Zn and Cd in the study area. Average concentrations of dissolved Co, Ni, Cu, Zn, Cd and Pb in the surface mixed layer (0-20 m) were 70 pM, 4.9, 2.1, 1.6, 0.48 nM and 52 pM, respectively, and those for the particulate species were 1.7 pM, 0.052, 0.094,0.46, 0.037 nM and 5.2 pM, respectively. After Fe enrichment, chlorophyll-a increased 3 fold (up to 3 mu g L(-1)) during developing phases of the bloom (< 12 days). Mesozooplankton biomass also increased. Particulate Co, Ni, Cu and Cd inside the patch hinted at an increase in the concentrations, but there were no analytically significant differences between concentrations inside and outside the patch. The bottle incubation with Fe addition (1 nM) showed an increase in chlorophyll-a (8.9 mu g L(-1)) and raised the particulate fraction up to 3-45% for all the metals, accompanying changes in Si/P, Zn/P and Cd/P. These results suggest that Fe addition lead to changes in biogeochemical cycling of trace metals. The comparison between the mesoscale Fe enrichment and the bottle incubation experiment suggests that although Fe was a limiting factor for the growth of phytoplankton, the enhanced biomass of mescizooplankton also limited the growth of phytoplankton and the transformation of trace metal speciation during the mesoscale Fe enrichment. Sediment trap data and the elemental ratios taken up by phytoplankton suggest that export loss was another reason that no detectable change in the concentrations of particulate trace metals was observed during the mesciscale Fe enrichment. (C) 2009 Elsevier Ltd. All rights reserved.
  • Koji Suzuki, Hiroaki Saito, Tomonori Isada, Ai Hattori-Saito, Hiroshi Kiyosawa, Jun Nishioka, R. Michael. L. McKay, Akira Kuwata, Atsushi Tsuda
    DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 56 (26) 2733 - 2744 0967-0645 2009/12 [Refereed][Not invited]
     
    Temporal changes in the abundance, community composition, and photosynthetic physiology of phytoplankton in surface waters were investigated during the second in situ iron (Fe) fertilization experiment in the NW subarctic Pacific (SEEDS-II). Surface chlorophyll a concentration was 0.75 mg m(-3) on the day before the first Fe enrichment (i.e. Day 0), increased ca. 3-fold until Day 13 after two Fe additions, and thereafter declined with time. The photochemical quantum efficiency (F-v/F-m) and functional absorption cross-section (sigma(PSII)) of photosystem II for total phytoplankton in surface waters increased and decreased inside the Fe-enriched patch through Day 13, respectively. These results indicate that the photosynthetic physiological condition of the phytoplankton improved after the Fe infusions. However, the maximum F-v/F-m value of 0.43 and the maximum quantum yield of carbon fixation (phi(max)) of 0.041 mol C (mol photon)(-1) during the development phase of the bloom were rather low, compared to their theoretical maximum of ca. 0.65 and 0.10 mol C (mol photon)(-1), respectively. Diatoms, which were mainly composed of oceanic species, did not bloom, and autotrophic nanoflagellates such as cryptophytes and prasinophytes became predominant in the phytoplankton community inside the Fe-enriched patch. In ferredoxin/flavodoxin assays for micro-sized (20-200 mu m in cell length) diatoms, ferredoxin was not detected but flavodoxin expressions consistently occurred with similar levels both inside and outside the Fe-enriched patch, indicating that the large-sized diatoms were stressed by Fe bioavailability inside the Fe-enriched patch even after the Fe enrichments. Our data suggest that the absence of a Fe-induced large-sized diatom bloom could be partly due to their Fe stress throughout SEEDS-II. (C) 2009 Elsevier Ltd. All rights reserved.
  • Takeshi Yoshimura, Hiroshi Ogawa, Keiri Imai, Takafumi Aramaki, Yukihiro Nojiri, Jun Nishioka, Atsushi Tsuda
    DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 56 (26) 2863 - 2874 0967-0645 2009/12 [Refereed][Not invited]
     
    The dynamics of organic carbon (C), nitrogen (N), and phosphorus (P) were examined during an in situ mesoscale iron-enrichment experiment in the western North Pacific in the summer of 2004. We separately determined the production of particulate organic matter (POM) and dissolved organic matter (DOM) and their subsequent removal during the bloom decline. As the iron-induced phytoplankton bloom progressed (days 0-14), POM increased in the surface mixed layer, while DOM did not increase significantly. The molar ratios for C:N, C:P, and N:P of the newly produced POM were estimated to be 4.9,190, and 37 in the surface mixed layer, whereas the dissolved inorganic nitrogen to soluble reactive phosphorus drawdown ratio was 17. Preferential remineralization of P over C and N from the POM was postulated during the developing phytoplankton bloom. During the bloom decline (days 16-25), surface POM decreased with a similar C:N of 5.2. The N:P ratio of surface DOM increased during the bloom decline. Below the surface mixed layer, DOC and DON increased moderately after the peak of the bloom. The time-series variation of DOC and DON was not identical. The C, N, and P dynamics through the accumulation and removal of POM and DOM were complex. Grazing by mesozooplankton during the experiment may have played a significant role in the uncoupling of the dynamics of C, N, and P. (C) 2009 Elsevier Ltd. All rights reserved.
  • Hiroaki Saito, Atsushi Tsuda, Yukihiro Nojiri, Takafumi Aramaki, Hiroshi Ogawa, Takeshi Yoshimura, Keiri Imai, Isao Kudo, Jun Nishioka, Tsuneo Ono, Koji Suzuki, Shigenobu Takeda
    DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 56 (26) 2852 - 2862 0967-0645 2009/12 [Refereed][Not invited]
     
    Biogeochemical cycles of N and Si were examined in the surface mixed layer during the mesoscale iron-enrichment (IE) experiment in the high-nutrient low-chlorophyll (HNLC) western subarctic Pacific (SEEDS-II). Although the IEs increased nitrate uptake, silicic acid utilization was not stimulated. The nitrate drawdown in the iron-patch (IN-patch, 140.3 mmol m(-2) in the surface mixed layer, 0-30 in) was only 25% of the initial inventory, which was 1/3-2/5 of the previous IE experiments in the subarctic Pacific. This relatively weak response of nutrient drawdown to IEs was due to the high biomass of mesozooplankton (MZ) dominated by copepod Neocalanus; plumchrus. Feeding of MZ (247.2 mmol m(-2) during Day 0-21 from the first IE) in the IN-patch was higher than the nitrate drawdown and prevented further development of the phytoplankton bloom. In the later period of the experiment (Day 14-21) the increase in the feeding activity and resultant decrease in phytoplankton biomass induced the accumulation of dissolved organic nitrogen (DON) and ammonium. Among total growth of MZ (81.6 mmol N m(-2)), 89% (72.8 mmol N m(-2)) was transported to the depth by the ontogenetic downward migration of N. plumchrus. Although silicic acid drawdown was not increased by the IEs, Si export flux increased by 2.7 times. The increase in Si export was also due to the increase in MZ, which egested faecal pellets with higher Si:N ratio and faster sinking speed than diatoms. The export efficiency (78% of new production) and total amount of export flux (143.8 mmol N m(-2) 1392 mmol C m(-2)) were highest records within the IE experiments despite weak responses of nutrient drawdown to the IE. During SEEDS-II, the high biomass of MZ reduced the phytoplankton response and nutrient drawdown to the IEs but via grazing and ontogenetic vertical migration accelerated the export flux as well as accumulations of dissolved forms of N. Results of the present and previous IE experiments indicate that the ecosystem and biogeochemical responses to IEs in the HNLC region are quite sensitive to the ecosystem components, especially for grazers of diatoms such as copepods and heterotrophic dinoflagellates. More attention needs to be paid to the ecosystem components and their biogeochemical functions as well as physical and chemical properties of the ecosystems in order to hindcast or forecast the impacts of changes in atmospheric iron deposition. (C) 2009 Elsevier Ltd. All rights reserved.
  • Julie L. Higgins, Isao Kudo, Jun Nishioka, Atsushi Tsuda, Steven W. Wilhelm
    DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 56 (26) 2788 - 2795 0967-0645 2009/12 [Refereed][Not invited]
     
    Although the important role of viruses in marine biogeochemical cycles has been established in recent years, virus activity (including changes in this activity) has been largely ignored during mesoscale iron (Fe)-fertilization experiments relative to other processes. This is of particular interest as viruses have been shown to be critical to the transformation of Fe from the particulate (i.e., biological) to the dissolved pools. The goal of the present study was to evaluate changes in the virus-mediated lysis of heterotrophic bacterial cells following a shift in ecosystem trophic status brought about by a mesoscale Fe addition in the subarctic Pacific Ocean. Virus production rates, estimated by a reduction and reoccurrence assay, were coupled with transmission electron microscopy estimates of burst size and direct counts of virus and bacterial abundance. Fe fertilization of the upper mixed layer resulted in significant yet weak increases in virus production rates during the 12 days of observation immediately after fertilization, although the burst size (viruses produced per lytic event) and the percentage of visibly infected cells remained constant. The results imply that increases in virus production rates were most likely tied to a decreased lytic cycle length or the stimulation of lysogenized cells following the stimulation of primary and secondary productivity by the addition of Fe. The results also indicate that virus-induced cell-lysis regenerated an estimated nearly 200 pmol L(-1) Fe daily, providing a significant return of Fe back to the water column, which may be critical in the maintenance of this added Fe as resident. (C) 2009 Elsevier Ltd. All rights reserved.
  • Jun Nishioka, Shigenobu Takeda, Yoshiko Kondo, Hajime Obata, Takashi Doi, Daisuke Tsumune, C. S. Wong, W. Keith Johnson, N. Sutherland, Atsushi Tsuda
    DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 56 (26) 2796 - 2809 0967-0645 2009/12 [Refereed][Not invited]
     
    A patch of water in the western subarctic gyre (low iron concentration, < 0.02 nM) was fertilized twice with 322 and 159 kg of iron to induce a phytoplankton bloom. In order to understand the changes in iron distribution and bio-availability throughout the evolution and termination phase of the iron-induced bloom, iron concentrations were measured at stations inside and outside of the iron-fertilized patch, and shipboard culture experiments using iron and desferrioxamine B (DFB) inoculation to regulate iron availability were conducted 5 times with water collected from the center of the iron-fertilized patch on D2, D7, D11, D17 and D23. After the iron fertilization, we observed a significant increase in dissolved iron (1.38 nM at 5 m depth) at the center of the patch (D1). Dissolved iron concentrations subsequently decreased to an ambient level (similar to 0.08 nM) on D16-D17, despite the second iron fertilization made on D6. During the 4-day incubations of the shipboard culture experiments, excess DFB-inoculated treatment inhibited the phytoplankton growth compared to the controls for D2, D7 and D11 patch water. This indicated that available iron existed in the iron-fertilized patch at least until D11. Moreover, iron-inoculated treatments induced growth of large-sized phytoplankton with an accompanying silicate decrease for D7, D11 and D17 patch water, but not for D23 patch water. These results indicated that large diatoms, which can respond to additional iron inoculation, existed in the iron-fertilized patch in evolution and early termination phase of the iron-induced bloom (at least until D17); however, there was no significant amount of large diatoms, which could rapidly respond to iron, in late termination phase (D23) of the iron-induced phytoplankton bloom. (C) 2009 Elsevier Ltd. All rights reserved.
  • Yoshimura T., Nishioka J., Suzuki K., Hattori H., Kiyosawa H., Watanabe Y. W.
    Biogeosciences Discussions Copernicus Publications 6 (2) 4143 - 4163 1810-6285 2009/04/15 
    Impacts of the increasing CO2 in seawater (i.e. ocean acidification) on phytoplankton physiology may have various and potentially adverse effects on phytoplankton dynamics and the carbon cycle. We conducted a CO2 manipulation experiment in the Sea of Okhotsk in summer 2006 to investigate the response of the phytoplankton assemblage and dynamics of organic carbon. During the 14-day incubation of nutrient-depleted surface water with a natural phytoplankton assemblage under 150, 280, 480, and 590μatm pCO2, the relative abundance of fucoxanthin-containing phytoplankton such as diatoms and prymnesiophytes decreased with increasing pCO2. The amount of DOC accumulation also decreased with increasing pCO2, while differences in POC accumulation between the treatments were small and did not show a clear trend with the pCO2. Change in the phytoplankton community composition under different pCO2 conditions will alter the organic carbon dynamics as found in the present experiment. Compared to results in the literature from nutrient-replete conditions indicating a potential enhancement of phytoplankton production with elevated pCO2, the present results indicated a different physiological response of phytoplankton under nutrient-depleted conditions. These results indicate that the continuing increase in atmospheric CO2 can significantly affect the structure of marine ecosystems and carbon cycle in nutrient-depleted subpolar surface waters.
  • Hongbin Liu, Koji Suzuki, Jun Nishioka, Rumi Sohrin, Takeshi Nakatsuka
    DEEP-SEA RESEARCH PART I-OCEANOGRAPHIC RESEARCH PAPERS 56 (4) 561 - 570 0967-0637 2009/04 [Not refereed][Not invited]
     
    The Sea of Okhotsk is one of the most productive marine basins in the world ocean and plays an important role in transport of organic carbon and iron to the western subarctic Pacific. We report the first measurements of phytoplankton growth and microzooplankton grazing rates in the Sea of Okhotsk, in late summer of 2006. The study area can be divided into two areas: nutrient-sufficient waters on the continental shelf along the east coast of Sakhalin Island and in the vicinity of Bussol Strait, and surface nutrient-depleted waters beyond the shelf break and in the vicinity of Sakhalin Bay. Phytoplankton growth rate in the studied area was strongly affected by nutrient availability, with high phytoplankton growth rate (0.55 +/- 0.14 d(-1)) in the nutrient-replete region and severely depressed growth (0.03 +/- 0.05 d(-1)) in the nutrient-depleted region. On the other hand, microzooplankton grazing rates in both the nutrient-replete and nutrient-depleted regions were approximately the same (0.26 +/- 0.20 d(-1) vs. 0.27 +/- 0.24 d(-1)). Consequently, microzooplankton grazing consumed <50% of the phytoplankton growth in nutrient-rich waters but >3 times the phytoplankton growth in nutrient-depleted waters. Phytoplankton physiological condition as measured by the maximum photochemical quantum efficiency (F-v/F-m.) of algal photosystem II (PS II) showed a general trend in agreement with the in situ growth rate of phytoplankton. In contrast to the phytoplankton community, picophytoplankton, especially the cyanobacteria Synechococcus, showed no nutrient effect on their growth, and the growth and mortality rates were well balanced, suggesting that they have a low nutrient requirement and their biomass was controlled principally by microzooplankton grazing. (C) 2008 Elsevier Ltd. All rights reserved.
  • Atsushi Ooki, Jun Nishioka, Tsuneo Ono, Shinichiro Noriki
    Journal of Geophysical Research Atmospheres 114 (3) 0148-0227 2009/02/16 [Refereed][Not invited]
     
    Asian mineral dust was sampled at Hokkaido, northern Japan, in spring 2004 and 2006. Iron solubility of the bulk aerosol, the size-segregated aerosol (0.45 < D < 11 μm), the snow containing a lot of mineral dust, and a simulated Asian dust standard (CJ-2) were measured by an iron dissolution experiment using a newly developed continuous leaching method. The iron solubility of the bulk aerosol samples was 1.2-2.2%. Within the 1.1 < D < 11 μm size range, iron solubility (0.52-8.2%) was higher in the smaller fractions of the size-segregated aerosol samples. We considered that the preferential removal of larger mineral dust particles from the air by snow resulted in the low iron solubility of the snow samples. Iron solubility of mineral dust was relatively lower in the 4.7 < D < 11 μm fraction of the size-segregated aerosol samples (0.52%), in the snow samples (0.20-0.57%), and in the CJ-2 standard (0.33%), which are dominated by large size particles (D > 4.7 μm). We suggest that an iron solubility of around 0.4% is typical for Asian mineral dust of large particles transported to Hokkaido. In the high-nutrient low-chlorophyll region of the western subarctic North Pacific near the Asian continent, where the mineral dust deposition is dominated by large particles, the iron solubility of the mineral dust entering the ocean is around 0.4%. Copyright 2009 by the American Geophysical Union.
  • Atsushi Ooki, Jun Nishioka, Tsuneo Ono, Shinichiro Noriki
    JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES 114 (D03202) doi:10.1029/2008JD010804--  2169-897X 2009/02 [Not refereed][Not invited]
     
    Asian mineral dust was sampled at Hokkaido, northern Japan, in spring 2004 and 2006. Iron solubility of the bulk aerosol, the size-segregated aerosol (0.45 < D < 11 mu m), the snow containing a lot of mineral dust, and a simulated Asian dust standard (CJ-2) were measured by an iron dissolution experiment using a newly developed continuous leaching method. The iron solubility of the bulk aerosol samples was 1.2-2.2%. Within the 1.1 < D < 11 mu m size range, iron solubility (0.52-8.2%) was higher in the smaller fractions of the size-segregated aerosol samples. We considered that the preferential removal of larger mineral dust particles from the air by snow resulted in the low iron solubility of the snow samples. Iron solubility of mineral dust was relatively lower in the 4.7 < D < 11 mu m fraction of the size-segregated aerosol samples (0.52%), in the snow samples (0.20-0.57%), and in the CJ-2 standard (0.33%), which are dominated by large size particles (D > 4.7 mu m). We suggest that an iron solubility of around 0.4% is typical for Asian mineral dust of large particles transported to Hokkaido. In the high-nutrient low-chlorophyll region of the western subarctic North Pacific near the Asian continent, where the mineral dust deposition is dominated by large particles, the iron solubility of the mineral dust entering the ocean is around 0.4%.
  • Increases of alkalinity and pH in the Okhotsk Sea during the 1990s and 2000s
    Watanabe, Y. W,, J, Nishioka, S, Shigemitsu, A, Mimura, T. Nakatsuka
    Geophys. Res. Lett doi:10.1029/2009GL038520  2009 [Refereed][Not invited]
  • Yoshiko Kondo, Shigenobu Takeda, Jun Nishioka, Hajime Obata, Ken Furuya, William Keith Johnson, C. S. Wong
    GEOPHYSICAL RESEARCH LETTERS 35 (12) doi:10.1029/2008GL033354--  0094-8276 2008/06 [Not refereed][Not invited]
     
    Complexation of iron (III) with natural organic ligands was investigated during a mesoscale iron enrichment experiment in the western subarctic North Pacific (SEEDS II). After the iron infusions, ligand concentrations increased rapidly with subsequent decreases. While the increases of ligands might have been partly influenced by amorphous iron colloids formation (12-29%), most in-situ increases were attributable to the < 200 kDa fraction. Dilution of the fertilized patch may have contributed to the rapid decreases of the ligands. During the bloom decline, ligand concentration increased again, and the high concentrations persisted for 10 days. The conditional stability constant was not different between inside and outside of the fertilized patch. These results suggest that the chemical speciation of the released iron was strongly affected by formation of the ligands; the production of ligands observed during the bloom decline will strongly impact the iron cycle and bioavailability in the surface water.
  • Atsushi Tsuda, Shigenobu Takeda, Hiroaki Saito, Jun Nishioka, Isao Kudo, Yukihiro Nojiri, Koji Suzuki, Mitsuo Uematsu, Mark L. Wells, Daisuke Tsumune, Takeshi Yoshimura, Tatsuo Aono, Takafumi Aramaki, William P. Cochlan, Maki Hayakawa, Keiri Imai, Tomoshi Isada, Yoko Iwamoto, William K. Johnson, Sohiko Kameyama, Shungo Kato, Hiroshi Kiyosawa, Yoshiko Kondo, Maurice Levasseur, Ryuji J. Machida, Ippei Nagao, Fumiko Nakagawa, Takahiro Nakanish, Seiji Nakatsuka, Akira Narita, Yoshifumi Noiri, Hajime Obata, Hiroshi Ogawa, Kenji Oguma, Tsuneo Ono, Tomofumi Sakuragi, Motoki Sasakawa, Mitsuhide Sato, Akifumi Shimamoto, Hyoe Takata, Charles G. Trick, Yutaka W. Watanabe, Chi Shing Wong, Naoki Yoshie
    JOURNAL OF OCEANOGRAPHY 63 (6) 983 - 994 0916-8370 2007/12 [Not refereed][Not invited]
     
    mesoscale iron-enrichment study (SEEDS II) was carried out in the western subarctic Pacific in the summer of 2004. The iron patch was traced for 26 days, which included observations of the development and the decline of the bloom by mapping with sulfur hexafluoride. The experiment was conducted at almost the same location and the same season as SEEDS (previous iron-enrichment experiment). However, the results were very different between SEEDS and SEEDS II. A high accumulation of phytoplankton biomass (similar to 18 mg chl m(-3)) was characteristic of SEEDS. In contrast, in SEEDS II, the surface chlorophyll-a accumulation was lower, 0.8 to 2.48 mg m(-3), with no prominent diatom bloom. Photosynthetic competence in terms of F-v/F-m for the total phytoplankton community in the surface waters increased after the iron enrichments and returned to the ambient level by day 20. These results suggest that the photosynthetic physiology of the phytoplankton assemblage was improved by the iron enrichments and returned to an iron-stressed condition during the declining phase of the bloom. Pico-phytoplankton (< 2 mu m) became dominant in the chlorophyll-a size distribution after the bloom. We observed a nitrate drawdown of 3.8 mu M in the patch (day 21), but there was no difference in silicic acid concentration between inside and outside the patch. Mesozooplankton (copepod) biomass was three to five times higher during the bloom-development phase in SEEDS II than in SEEDS. The copepod biomass increased exponentially. The grazing rate estimation indicates that the copepod grazing prevented the formation of an extensive diatom bloom, which was observed in SEEDS, and led to the change to a pico-phytoplankton dominated community towards the end of the experiment.
  • Jun Nishioka, Tsuneo Ono, Hiroaki Saito, Takeshi Nakatsuka, Shigenobu Takeda, Takeshi Yoshimura, Koji Suzuki, Kenshi Kuma, Shigeto Nakabayashi, Daisuke Tsumune, Humio Mitsudera, W. Keith Johnson, Atsushi Tsuda
    Journal of Geophysical Research: Oceans 112 (10) 2169-9291 2007/10/08 [Refereed][Not invited]
     
    Iron is an essential nutrient and plays an important role in the control of phytoplankton growth (Martin et al., 1989). Atmospheric dust has been thought to be the most important source of iron, supporting annual biological production in the western subarctic Pacific (WSP) (Duce and Tindale, 1991 Moore et al., 2002). We argue here for another source of iron to the WSP. We found extremely high concentrations of dissolved and particulate iron in the Okhotsk Sea Intermediate Water (OSIW) and the North Pacific Intermediate Water (NPIW), and water ventilation processes in this region probably control the transport of iron through the intermediate water layer from the continental shelf of the Sea of Okhotsk to wide areas of the WSP. Additionally, our time series data in the Oyashio region of the WSP indicate that the pattern of seasonal changes in dissolved iron concentrations in the surface-mixed layer was similar to that of macronutrients, and that deep vertical water mixing resulted in higher winter concentrations of iron in the surface water of this region. The estimated dissolved iron supply from the iron-rich intermediate waters to the surface waters in the Oyashio region was comparable to or higher than the reported atmospheric dust iron input and thus a major source of iron to these regions. Our data suggest that the consideration of this source of iron is essential in our understanding of spring biological production and biogeochemical cycles in the western subarctic Pacific and the role of the marginal sea. Copyright 2007 by the American Geophysical Union.
  • Jun Nishioka, Tsuneo Ono, Hiroaki Saito, Takeshi Nakatsuka, Shigenobu Takeda, Takeshi Yoshimura, Koji Suzuki, Kenshi Kuma, Shigeto Nakabayashi, Daisuke Tsumune, Humio Mitsudera, W. Keith Johnson, Atsushi Tsuda
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 112 (C10) 0148-0227 2007/10 [Refereed][Not invited]
     
    Iron is an essential nutrient and plays an important role in the control of phytoplankton growth ( Martin et al., 1989). Atmospheric dust has been thought to be the most important source of iron, supporting annual biological production in the western subarctic Pacific (WSP) (Duce and Tindale, 1991; Moore et al., 2002). We argue here for another source of iron to the WSP. We found extremely high concentrations of dissolved and particulate iron in the Okhotsk Sea Intermediate Water (OSIW) and the North Pacific Intermediate Water (NPIW), and water ventilation processes in this region probably control the transport of iron through the intermediate water layer from the continental shelf of the Sea of Okhotsk to wide areas of the WSP. Additionally, our time series data in the Oyashio region of the WSP indicate that the pattern of seasonal changes in dissolved iron concentrations in the surface-mixed layer was similar to that of macronutrients, and that deep vertical water mixing resulted in higher winter concentrations of iron in the surface water of this region. The estimated dissolved iron supply from the iron-rich intermediate waters to the surface waters in the Oyashio region was comparable to or higher than the reported atmospheric dust iron input and thus a major source of iron to these regions. Our data suggest that the consideration of this source of iron is essential in our understanding of spring biological production and biogeochemical cycles in the western subarctic Pacific and the role of the marginal sea.
  • Takeshi Yoshimura, Jun Nishioka, Hiroaki Saito, Shigenobu Takeda, Atsushi Tsuda, Mark L. Wells
    MARINE CHEMISTRY 103 (1-2) 112 - 121 0304-4203 2007/01 [Refereed][Not invited]
     
    Several operationally defined fractions of phosphorus (P) were measured along a surface water transect in the North Pacific. The P content in all fractions was found to increase northward from the edge of the subtropical to the subarctic region. Particulate organic P (POP) concentrations increased from 9 to 110 nmol L-1, whereas the particulate inorganic P (PIP) concentrations increased from 1 to 13 nmol L-1. A significant correlation between POP, PIP and chlorophyll a suggested that these P pools are associated directly or indirectly with phytoplankton cells. PIP comprised 10-20% of the total particulate P pool across the transect, indicating it is an important component of the marine P cycle in this region. Dissolved non-reactive P (assumed to consist predominantly of non-reactive organic P compounds, thus referred to as DOP) concentration increased from 0.10 to 0.22 mu mol L-1, whereas soluble reactive P (SRP) concentration increased from 0.01 to 1.42 mu mol L-1 along the transect. The proportion of DOP and SRP varied widely, with a large proportion of DOP in areas with low total dissolved P concentrations in lower latitudes and a large proportion of SRP in areas with high total dissolved P concentrations in higher latitudes. High demand for DOP in the lower latitudinal region would diminish the concentration of this pool relative to higher latitudinal regions where SRP is more abundant and would be preferentially utilized. The availability of SRP could have a significant impact on the concentration and probably on the composition of DOP. We show that P fractionation provides an important insight for discussing the marine P cycle. (c) 2006 Elsevier B.V. All rights reserved.
  • Shouei Iwade, Kenshi Kuma, Yutaka Isoda, Masahiko Yoshida, Isao Kudo, Jun Nishioka, Koji Suzuki
    AQUATIC MICROBIAL ECOLOGY 43 (2) 177 - 191 0948-3055 2006/06 [Refereed][Not invited]
     
    The growth and iron uptake of the coastal marine diatom Chaetoceros sociale were experimentally measured in batch experiments at 10 degrees C to which an acidic Fe(III) stock solution was added. The direct input of Fe(III) into the culture media induced the highest iron uptake rate (similar to 3.4 to 4.2 x 10(-16) mol Fe cell(-1) d(-1)) by C. sociale during the first day of the incubation, resulting from the supply of bioavailable inorganic Fe(III) species at levels above its expected equilibrium value (similar to 0.1 nmol l(-1)) with solid amorphous Fe(III) hydroxide in seawater. The iron uptake rate during the first day of incubation in solid amorphous Fe(Ill) hydroxide medium aged for 1 d at 10 degrees C was approximately 50% lower than that in the direct Fe(III) input media. We used a modified approach in which further iron uptake by C. sociale from external iron in the direct Fe(III) input media was prevented by adding hydroxamate siderophore desferrioxamine B (DFB) during cultivation. After the addition of DFB, the highest growth rate (similar to 0.5 to 0.6 d(-1)) of C. sociale by intracellularly stored Fe in the direct Fe(III) input media was maintained for a few days since no iron uptake was observed after the DFB addition. The growth rate was independent of the amount of intracellularly stored Fe, However, the maximal cell yields appeared to be relatively dependent on the amount of intracellularly stored Fe, suggesting the presence of a critical concentration of intracellular Fe (minimum cellular Fe for growth) for phytoplankton growth (similar to 1 x 10(-16) mol Fe cell(-1) for C. sociale). In the present study, maximal and minimal Fe quotas were 3.4 to 4.2 x 10(-16) and similar to 1 x 10(-16) mol Fe cell(-1) (the maximal/minimal Fe ratio of 3.4 to 4.2), respectively. The high iron uptake and storage capacity in C. sociale allows this species to accumulate excess iron at high concentrations of bioavailable inorganic Fe species and to support up to 1.8 to 2.1 cell divisions without any additional iron uptake. In addition, we attempted to model the effect of luxury uptake on growth, as biodilution of cellular Fe eventually decreases the Fe quota to a critical threshold.
  • Hiroaki Saito, Takashi Ota, Koji Suzuki, Jun Nishioka, Atsushi Tsuda
    Geophysical Research Letters 33 (9) 0094-8276 2006/05/16 [Not refereed][Not invited]
     
    Iron enrichment to high-nutrient low-chlorophyll (HNLC) regions is being considered as a possible way of atmospheric CO2 sequestration to the deep sea. Mesoscale iron-enrichment to the HNLC subarctic Pacific induced a massive diatom bloom and led to a large decrease in pCO2. In response to the diatom bloom, the heterotrophic dinoflagellate Gyrodinium sp. increased and phagotrophically fed on the diatoms up to 12 times their length.Mathematical simulations show the carbon fixed by diatoms is mostly respired by Gyrodinium sp. in the sea surface. The emergence of initially rare species and their key biogeochemical roles were unexpected due to our limited understanding of food-web components. This indicates that the prediction of ecosystem responses to natural or anthropogenic perturbation remains a challenging issue. Effective carbon sequestration as a geoengineering technique may not be accomplished by purposeful ironenrichment, at least in the western subarctic Pacific where rapid-growth diatom grazers stand by. Copyright 2006 by the American Geophysical Union.
  • NISHIOKA Jun, Jun Nishioka, Central Research Institute of Electric Power Industry:(Present address)Pan-Okhotsk research center Institute of low temperature science Hokkaido University
    Oceanography in Japan 日本海洋学会 15 (1) 19 - 36 2186-3105 2006 [Refereed][Not invited]
     
    サイズ分画測定法を用いて外洋海域における鉄の存在状態を研究した。その結果, 従来"溶存態"と定義されてきた画分中にはコロイド状鉄が含まれており, 海水中の植物プランクトンによる鉄の利用過程や地球化学的鉄の循環を理解するために重要な画分であることを示した。また, 国際共同プロジェクトとして, 北太平洋亜寒帯域の西部および東部で現場鉄散布実験を行ない, 大気から供給される鉄が他海域より多いと考えられる西部海域においても, 鉄の不足が生物生産を制限する要因であることを明らかにした。さらに, 西部海域が鉄制限海域になるプロセスとして, 供給された鉄が, 速やかに植物プランクトンが利用しづらい形態に変化してしまうことが重要であることを示した。海洋における鉄の生物地球化学的な研究は, 自然海域における生物生産の諸過程を理解するためには欠かせない分野と成りつつある。本稿では, 著者がこれまでに展開してきた, 海水中の鉄の存在状態と鉄が生物生産に果たす役割に関する研究の一部を紹介した。A size-fractionation technique was used for observation and experimental studies to investigate iron speciation in oceanic seawater. The results showed that we need to consider the existence of colloidal Fe in the dissolved fraction to better understand Fe availability and the Fe biogeochemical cycle in the ocean. Moreover, an Fe-enrichment experiment in the western subarctic Pacific clearly demonstrated that iron availability controls phytoplankton growth in this region, even though there is a higher iron supply from atmospheric dust here than in the eastern region. Our results also suggested that Fe transformation is an important process in understanding how phytoplankton becomes iron limited in the western subarctic Pacific. Research on the biogeochemistry of Fe is becoming indispensable in understanding the processes of phytoplankton growth in a natural open-ocean environment. In this paper, the research results on Fe speciation and the role of Fe in phytoplankton growth in oceanic seawater are presented.
  • Effect of high iron level on iron uptake and growth of marine phytoplankton
    Iwade, S, K. Kuma, Y. Isoda, M. Yoshida, I. Kudo, J. Nishioka, K. Suzuki
    Aquatic Microbial Ecology 43 177 - 191 2006 [Refereed][Not invited]
  • Isao Kudo, Yoshifumi Noiri, Jun Nishioka, Yousuke Taira, Hiroshi Kiyosawa, Atsushi Tsuda
    DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 53 (20-22) 2201 - 2213 0967-0645 2006 [Refereed][Not invited]
     
    Ship-board iron enrichment bottle experiments were carried out with samples collected at the mesoscale iron fertilization experimental site (SERIES) in the subarctic NE Pacific in the summer of 2002. Samples were collected on Day 14 of the experiment outside the patch that was in a typical high nitrate and low chlorophyll (HNLC) condition. The iron concentration in the incubation bottles ranged from 0.1 to 2.0 nM by adding FeCl(3) solution. The increase in chlorophyll-a (chl-a) in the micro (> 10 mu m) and nano-sized (2-10 mu m) fraction was observed as a function of the added iron. Chl-a in the pico-sized fraction (0.7-2 pm) showed no increase with time. Nitrate and silicate were exhausted in the Fe-amended bottles, while those in the control bottle remained at the end of incubation. The relative consumption ratio of silicate to nitrate for the control bottles was significantly higher than that for the Fe-amended bottles. As a hyperbolic relation was found between iron concentration and the rate of increase in Chl-a (specific growth rate) for the micro and nano-sized fraction, the Monod equation was fit to obtain a maximum growth rate and a half-saturation constant for iron (K(Fe)). The mu(max) values were 0.72 and 0.48 d(-1) for the micro and nano-sized fraction, respectively. The K(Fe) values were 0.10 and 0.08 nM for the micro and nano-sized fraction, respectively. The mu(max) agreed with the rate of increase in Chl-a observed in situ for the mesoscale iron fertilization experiment. The P-a value for micro-sized fraction at 12 degrees C was half of that in the western subarctic Pacific Ocean (SEEDS experiment in 2001), indicating the Chl-a increase rate (potential growth rate) after iron enrichment was much higher in SEEDS than that in SERIES. The K(Fe) values were much lower than that in SEEDS, suggesting that the phytoplankton community in the NE subarctic Pacific Ocean acclimates to a lower ambient Fe concentration. This difference in K(Fe) between SERIES (NE) and SEEDS (NW) may reflect the previously suggested gradient in Fe flux to the subarctic Pacific Ocean. A temperature gradient was also applied to investigate the effect of temperature on the growth response of the phytoplankton community. No obvious effect of temperature increase to 16 degrees C was found in SERIES, while mu(max) and K(Fe) changed significantly with temperature in SEEDS. (c) 2006 Elsevier Ltd. All rights reserved.
  • Atsushi Tsuda, Hiroaki Saito, Jun Nishioka, Tsuneo Ono, Yoshifumi Noiri, Isao Kudo
    DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 53 (20-22) 2281 - 2296 0967-0645 2006 [Refereed][Not invited]
     
    A mesoscale iron-fertilization experiment was carried out in the eastern subarctic Pacific during summer 2002. The iron patch was traced for 26 days after the enrichment, and the abundance and behavior of mesozooplankton was compared with those outside of the patch during the first half of the experiment (days 2-18) by Sastri and Dower [2006. Mesozooplankton community response during the SERIES iron enrichment experiment in the subarctic NE Pacific. Deep-Sea Research Part II.) and during the post-enrichment diatom bloom and its period of decline (days 15-26; this paper). The surface chlorophyll-a concentration in the patch was high between days 15 and 17 (6 mg m(-3)) and decreased to 1.4 mg m(-3) at the end of the observation. Dominant zooplankton species in the upper 200 m were copepods: Eucalanus bungii, Pseudocalanus spp., Neocalanus plumchrus, N. cristatus, and Metridia pacifica. Species composition did not change significantly in the patch over the observation period. However, shallower distribution depths of E bungii, N. cristatus and M. pacifica were observed in the patch during and after the diatom bloom. Especially, E. bungii was mainly distributed in the subsurface layer outside of the patch, but it was mainly in the surface mixed layer inside the patch, where it also had an enhanced development rate and increased biomass. We also propose the accumulation mechanism of zooplankton in the patch due to the upward immigration. Moreover, the abundance of the first copepodite stage of E. bungii and calyptopis larvae of euphausiids increased several fold in the patch compared to the densities outside the patch. The increases in both species are considered to be due to lowered mortality during the egg and naupliar stages, which was caused by lowered relative importance of eggs and nauplii in the diets of the suspension-feeding omnivores in the patch due to increased diatom abundance during the diatom bloom. Gut-pigment contents of dominant copepods in the patch increased 6-8 times, and the maximum values were observed during the bloom peak. The grazing impact on phytoplankton was low during the bloom period, but increased in the declining period of the diatom bloom. (c) 2006 Elsevier Ltd. All rights reserved.
  • Hiroaki Saito, Atsushi Tsuda, Yukihiro Nojiri, Jun Nishioka, Shigenobu Takeda, Hiroshi Kiyosawa, Isao Kudo, Yoshifumi Noiri, Tsuneo Ono, Yousuke Taira, Koji Suzuki, Takeshi Yoshimura, Philip W. Boyd
    DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 53 (20-22) 2168 - 2181 0967-0645 2006 [Refereed][Not invited]
     
    This paper reports on the variations in nutrient concentrations and phytoplankton dynamics during the stationary and declining phases of a phytoplankton bloom induced by a mesoscale iron-enrichment conducted in the high-nutrient low-chlorophyll (HNLC) eastern subarctic Pacific. During the 26-d sampling period, the main pycnocline was located between 30 and 45 m with a shallow pycnocline developing at 10 m, 19 d after the first iron-enrichment. The iron-induced bloom dominated by diatoms peaked during days 15 and 18, a period of high chlorophyll a concentrations (ca. 5 mg m(-3)), and declined thereafter. Nitrogenous nutrients and phosphate were not depleted during the whole experiment. In contrast, silicic acid and iron concentrations became very low during the stationary phase of the diatom bloom (days 15-18) and F(v)/F(m) declined. These observations suggest that silicic acid and iron limitation probably prevented further development of the diatom bloom. The decline in chlorophyll a concentrations during days 19-24 was mostly due to the decrease in diatom abundance. On the other hand, cell abundances of pico- and nanophytoplankton exhibited little change until day 24. In the layer located between the main and the shallow pycnocline (10-30 m), ammonium and silicic acid concentration increased during days 19-26, suggesting recycling of these nutrients. The amount of silicic acid recycled during that period was estimated at 71.3-99.2 mmol m(-2), while the dissolution rate of biogenic silica (BSi) was estimated to be 5.9-9.2% d(-1) in the upper 50 m of the water column. These results show that the development of a shallow pycnocline during the experiment accelerated the iron and silicic acid depletion in the upper mixed layer and influenced the recycling of the organic matter assimilated during the iron-induced bloom in the ocean surface. (c) 2006 Elsevier Ltd. All rights reserved.
  • Hein J.W. de Baar, Philip W. Boyd, Kenneth H. Coale, Michael R. Landry, Atsushi Tsuda, Philipp Assmy, Dorothee C.E. Bakker, Yann Bozec, Richard T. Barber, Mark A. Brzezinski, Ken O. Buesseler, Marie Boyé, Peter L. Croot, Frank Gervais, Maxim Y. Gorbunov, Paul J. Harrison, William T. Hiscock, Patrick Laan, Christiane Lancelot, Cliff S. Law, Maurice Levasseur, Adrian Marchetti, Frank J. Millero, Jun Nishioka, Yukihiro Nojiri, Tim van Oijen, Ulf Riebesell, Micha J.A. Rijkenberg, Hiroaki Saito, Shingenobu Takeda, Klaas R. Timmermans, Marcel J.W. Veldhuis, Anya M. Waite, Chi-Shing Wong
    Journal of Geophysical Research C: Oceans 110 (9) 1 - 24 0148-0227 2005/09/08 [Refereed][Not invited]
     
    Comparison of eight iron experiments shows that maximum Chl a, the maximum DIC removal, and the overall DIC/Fe efficiency all scale inversely with depth of the wind mixed layer (WML) defining the light environment. Moreover, lateral patch dilution, sea surface irradiance, temperature, and grazing play additional roles. The Southern Ocean experiments were most influenced by very deep WMLs. In contrast, light conditions were most favorable during SEEDS and SERIES as well as during IronEx-2. The two extreme experiments, EisenEx and SEEDS, can be linked via EisenEx bottle incubations with shallower simulated WML depth. Large diatoms always benefit the most from Fe addition, where a remarkably small group of thriving diatom species is dominated by universal response of Pseudo-nitzschia spp. Significant response of these moderate (10-30 μm), medium (30-60 μm), and large (> 60 μm) diatoms is consistent with growth physiology determined for single species in natural seawater. The minimum level of "dissolved" Fe (filtrate < 0.2 μm) maintained during an experiment determines the dominant diatom size class. However, this is further complicated by continuous transfer of original truly dissolved reduced Fe(II) into the colloidal pool, which may constitute some 75% of the "dissolved" pool. Depth integration of carbon inventory changes partly compensates the adverse effects of a deep WML due to its greater integration depths, decreasing the differences in responses between the eight experiments. About half of depth-integrated overall primary productivity is reflected in a decrease of DIC. The overall C/Fe efficiency of DIC uptake is DIC/Fe ∼ 5600 for all eight experiments. The increase of particulate organic carbon is about a quarter of the primary production, suggesting food web losses for the other three quarters. Replenishment of DIC by air/sea exchange tends to be a minor few percent of primary CO2 fixation but will continue well after observations have stopped. Export of carbon into deeper waters is difficult to assess and is until now firmly proven and quite modest in only two experiments. Copyright 2005 by the American Geophysical Union.
  • M Boye, J Nishioka, PL Croot, P Laan, KR Timmermans, HJW de Baar
    MARINE CHEMISTRY 96 (3-4) 257 - 271 0304-4203 2005/09 [Refereed][Not invited]
     
    The speciation of strongly chelated iron during the 22-day course of an iron enrichment experiment in the Atlantic sector of the Southern Ocean deviates strongly from ambient natural waters. Three iron additions (ferrous sulfate solution) were conducted, resulting in elevated dissolved iron concentrations (Nishioka, J., Takeda, S., de l3aar, H.J.W., Croot, P.L., Boye, M., Laan, P., Timmermans, K.R., in press. Changes in the concentration of iron in different size fractions during an iron enrichment experiment in the open Southern Ocean. Marine Chemistry.) and significant Fe(II) levels (Croot, P.L., Laan, P., Nishioka, J., Strass, V, Cisewski, B., Boye, M., Timmermans, K.R., Bellerby, R.G., Goldson, L., Nightingale, P., de l3aar, H.J.W., in press. Spatial and Temporal distribution of Fe(II). and H2O2 during EisenEx, an open ocean mescoscale iron enrichment. Marine Chemistry.). Repeated vertical profiles for dissolved (filtrate < 0.2 mu m) Fe(Ill)-binding ligands indicated a production of chelators in the upper water column induced by iron fertilizations. Abiotic processes (chemical reactions) and an inductive biologically mediated mechanism were the likely sources of the dissolved ligands which existed either as inorganic amorphous phases and/or as strong organic chelators. Discrete analysis on ultra-filtered samples (< 200 kDa) suggested that the produced ligands would be principally colloidal in size (> 200 kDa-< 0.2 mu m), as opposed to the soluble fraction (< 200 kDa) which dominated prior to the iron infusions. Yet these colloidal ligands would exist in a more transient nature than soluble ligands which may have a longer residence time. The production of dissolved Fe-chelators was generally smaller than the overall increase in dissolved iron in the surface infused mixed layer, leaving a fraction (about 13-40%) of dissolved Fe not bound by these dissolved Fe-chelators. It is suggested that this fraction would be inorganic colloids. The unexpected persistence of such high inorganic colloids concentrations above inorganic Fe-solubility limits illustrates the peculiar features of the chemical iron cycling in these waters. Obviously, the artificial about hundred-fold increase of overall Fe levels by addition of dissolved inorganic Fe(II) ions yields a major disruption of the natural physical-chemical abundances and reactivity of Fe in seawater. Hence the ensuing responses of the plankton ecosystem, while in itself significant, are not necessarily representative for a natural enrichment, for example by dry or wet deposition of aeolian dust. Ultimately, the temporal changes of the Fe(Ill)-binding ligand and iron concentrations were dominated by the mixing events that occurred during EISENEX, with storms leading to more than an order of magnitude dilution of the dissolved ligands and iron concentrations. This had strongest impact on the colloidal size class (> 200 kDa-< 0.2 mu m) where a dramatic decrease of both the colloidal ligand and the colloidal iron levels (Nishioka, J., Takeda, S., de Baar, H.J.W., Croot, P.L., Boye, M., Laan, P., Timmermans, K.R., in press. Changes in the concentration of iron in different size fractions during an iron enrichment experiment in the open Southern Ocean. Marine Chemistry) was observed. (c) 2005 Elsevier B.V. All rights reserved.
  • J Nishioka, S Takeda, HJW de Baar, PL Croot, M Boye, P Laan, KR Timmermans
    MARINE CHEMISTRY 95 (1-2) 51 - 63 0304-4203 2005/05 [Refereed][Not invited]
     
    An in situ iron enrichment experiment was carried out in the Southern Ocean Polar Frontal Zone and fertilized a patch of water within an eddy of the Antarctic Circumpolar Current (EisenEx, Nov. 2000). During the experiment, a physical speciation technique was used for iron analysis in order to understand the changes in iron distribution and size-fractionations, including soluble Fe (< 200 kDa), colloidal Fe (200 kDa-0.2 mu m) and labile particle Fe (> 0.2 mu m), throughout the development of the phytoplankton bloom. Prior to the first infusion of iron, dissolved (< 0.2 mu m) iron concentrations in the ambient surface seawater were extremely low (0.06 +/- 0.015 nM) with colloidal iron being a minor fraction. For the iron addition, an acidified FeSO4 solution was released three times over a 23-day period to the eddy. High levels of dissolved iron concentrations (2.0 +/- 1.1 nM) were measured in the surface water until 4 days after the first iron infusion. After every iron infusion, when high iron concentrations were observed before storm events, there was a significant correlation between colloidal and dissolved iron concentrations ([Colloidal Fe]=0.7627 [Dissolved Fe]+0.0519, R-2=0.9346). These results indicate that a roughly constant proportion of colloidal vs. dissolved iron was observed after iron infusion (similar to 76%). Storm events caused a significant decrease in iron concentrations (< 0.61 nM in dissolved iron) and changed the proportions of the three iron size-fractions (soluble, colloidal and labile particle). The changes in each iron size-fraction indicate that colloidal iron was eliminated from surface mixed layer more easily than particulate and soluble fractions. Therefore, particle and soluble iron efficiently remain in the mixed layer, probably due to the presence of suspended particles and naturally dissolved organic ligands. Our data suggest that iron removal through colloidal aggregation during phytoplankton bloom should be considered in the oceanic iron cycle. (c) 2004 Elsevier B.V. All rights reserved.
  • PL Croot, P Laan, J Nishioka, Strass, V, B Cisewski, M Boye, KR Timmermans, RG Bellerby, L Goldson, P Nightingale, HJW de Baar
    MARINE CHEMISTRY 95 (1-2) 65 - 88 0304-4203 2005/05 [Refereed][Not invited]
     
    Measurements of Fe(II) and H2O2 were carried out in the Atlantic sector of the Southern Ocean during EisenEx, an iron enrichment experiment. Iron was added on three separate occasions, approximately every 8 days, as a ferrous sulfate (FeSO4) solution. Vertical profiles of Fe(II) showed maxima consistent with the plume of the iron infusion. While H2O2 profiles revealed a corresponding minima showing the effect of oxidation of Fe(II) by H2O2, observations showed detectable Fe(II) concentrations existed for up to 8 days after an iron infusion. H2O2 concentrations increased at the depth of the chlorophyll maximum when iron concentrations returned to pre-infusion concentrations (<< 80 pM) possibly due to biological production related to iron reductase activity. In this work, Fe(II) and dissolved iron were used as tracers themselves for subsequent iron infusions when no further SF6 was added. EisenEx was subject to periods of weak and strong mixing. Slow mixing after the second infusion allowed significant concentrations of Fe(II) and Fe to exist for several days. During this time, dissolved and total iron in the infusion plume behaved almost conservatively as it was trapped between a relict mixed layer and a new rain-induced mixed layer. Using dissolved iron, a value for the vertical diffusion coefficient K-z=6.7 &PLUSMN; 0.7 cm(2) s(-1) was obtained for this 2-day period. During a subsequent surface survey of the iron-enriched patch, elevated levels of Fe(H) were found in surface waters presumably from Fe(II) dissolved in the rainwater that was falling at this time. Model results suggest that the reaction between uncomplexed Fe(III) and O-2(-) was a significant source of Fe(II) during EisenEx and helped to maintain high levels of Fe(II) in the water column. This phenomenon may occur in iron enrichment experiments when two conditions are met: (i) When Fe is added to a system already saturated with regard to organic complexation and (ii) when mixing processes are slow, thereby reducing the dispersion of iron into under-saturated waters. © 2004 Elsevier B.V All rights reserved.
  • M Kinugasa, T Ishita, Y Sohrin, K Okamura, S Takeda, J Nishioka, A Tsuda
    PROGRESS IN OCEANOGRAPHY 64 (2-4) 129 - 147 0079-6611 2005 [Not refereed][Not invited]
     
    During the first iron-enrichment experiment in the northwest subarctic Pacific (SEEDS2001), the dynamics of dissolved and acid dissolvable trace metals were studied. Seawater samples were collected from the upper water column (5-70 m) of In- and Out-patch stations. Immediately after the collection, an aliquot of seawater for dissolved species was filtered through an 0.2 μ m filter and acidified to pH 2.2. An aliquot for acid dissolvable species was acidified without filtration and stored for 3 years at an ambient temperature. Before the Fe enrichment, the dissolved Fe concentration in the surface mixed layer was < 0.13 nM (the detection limit) and the acid dissolvable Fe concentration was 4.7 nM. The difference was a particulate fraction. Since the photochemical quantum efficiency of algal photosystem 11 was low, Fe in this fraction was not easily available to phytoplankton. After the Fe enrichment, acid dissolvable Fe in the patch decreased from 9.6 nM on day 2 to 4.6 nM on day 13, which was still &SIM; 3 nM higher than that at the Out-patch station. Dissolved Fe was 1.4 nM on day 2 and decreased exponentially to < 0.13 nM on day 11. The concentration ratio of the acid dissolvable fraction to the dissolved fraction for the other trace metals was lower than that for Fe. The acid dissolvable concentrations for Mn, Co, Ni, Cu, Zn and Cd did not show significant change during the observation. The dissolved concentrations for Co, Ni, Cu, Zn and Cd in the surface layer on day 2 were 0.039, 5.0, 1.7, 2.2 and 0.27 nM, respectively. They decreased exponentially to 0.013, 4.3, 1.2, 0.87 and 0.21 nM on day 13, respectively. The mole ratio of the concentration difference between days 2 and 13 was similar to the elemental ratio reported for phytoplankton. These are the first data showing that mesoscale iron fertilization alters the dynamics of dissolved Co, Ni, Cu and Zn. © 2005 Published by Elsevier Ltd.
  • D Tsumune, J Nishioka, A Shimamoto, S Takeda, A Tsuda
    PROGRESS IN OCEANOGRAPHY 64 (2-4) 111 - 127 0079-6611 2005 [Refereed][Not invited]
     
    The first iron (Fe) - fertilization experiment in the western North Pacific was carried out using SF6 to trace the Fe-fertilized water mass. A solution in 10,800 liters of seawater of 350 kg of Fe and 0.48 M of SF6 tracer was released into the mixed layer over a 8 x 10 km area. On the first underway transects through the patch after the Fe release, we observed a significant increase of dissolved Fe (ave. 2.89 nM). The fertilized patch was traced for 14 days by on-board SF6 analysis. A Lagrangian frame of reference was maintained by the use of a drogued GPS buoy released at the center of the patch. The patch moved westward at a rate of 6.8 km d(-1). Mixed layer depth increased from 8.5 to 15 m during the experiment. Horizontal diffusivity was determined by the change of SF6 concentration in the patch. The horizontal diffusivity increased during the experiment. We evaluate here the fate of Fe in a Fe-fertilized patch using the dilution rate determined from sulphur hexafluoride (SF6) concentration. Dissolved Fe concentrations subsequently decreased rapidly to &SIM; 0.15 nM on Day 13. However, the dissolved Fe half-life of &SIM; 43 h was relatively longer than in previous Fe-enrichment studies, and we observed a larger increase of the centric diatom standing stock and corresponding draw-down of macro-nutrients and carbon dioxide than in the previous studies. The most important reason for the larger response was the phytoplankton species in the western North Pacific. In addition, the smaller diffusivity and shallower mixed layer were effective to sustain the higher dissolved Fe concentration compared to previous experiments. This might be one reason for the larger response of diatoms in SEEDS. © 2005 Elsevier Ltd. All rights reserved.
  • N Ramaiah, S Takeda, K Furuya, T Yoshimura, J Nishioka, T Aono, Y Nojiri, K Imai, Kudo, I, H Saito, A Tsuda
    PROGRESS IN OCEANOGRAPHY 64 (2-4) 253 - 261 0079-6611 2005 [Refereed][Not invited]
     
    Dynamics of transparent exopolymer particles (TEP) was studied during the first in situ iron-enrichment experiment conducted in the western subarctic Pacific in July-August 2001, with the goal of evaluating the contribution of TEP to vertical flux as a result of increased primary production following iron enrichment in open ocean ecosystems. Subsequent to the enhancement of phytoplankton production, we observed increase in TEP concentration in the surface layer and sedimentation of organic matter beneath it. Vertical profiles of TEP, chlorophyll a (Chl a) and particulate organic carbon (POC) were obtained from six depths between 5 and 70 m, from a station each located inside and outside the enriched patch. TEP and total mass flux were estimated from the floating sediment traps deployed at 200 m depth. Chl a and TEP concentrations outside the patch varied from 0.2 to 1.9 μ g L-1 and 40-60 μ g XG equiv. L-1, respectively. Inside the patch, Chl a increased drastically from day 7 reaching the peak of 19.2 μ g L-1 on day 13, which coincided with the TEP peak of 189 μ g XG equiv. L-1. TEP flux in the sediment trap increased from 41 to 88 mg XG equiv. m(-2) d(-1), with 8-14% contribution of TEP to total mass flux. This forms the basic data set on ambient concentrations of TEP in the western subarctic Pacific, and evaluation of the effect of iron enrichment on TEP. © 2005 Elsevier Ltd. All rights reserved.
  • A Tsuda, H Kiyosawa, A Kuwata, M Mochizuki, N Shiga, H Saito, S Chiba, K Imai, J Nishioka, T Ono
    PROGRESS IN OCEANOGRAPHY 64 (2-4) 189 - 205 0079-6611 2005 [Refereed][Not invited]
     
    Phytoplankton species composition was analyzed inside and outside of the iron-enriched patch during the SEEDS experiment. Before the iron-enrichment, the phytoplankton community consisted of similar proportions of pico-, nano- and micro-sized phytoplankton, and the micro-phytoplankton was dominated by the pennate diatom Pseudo-nitzschia turgidula. Although all the diatoms, except the nano-sized Fragilariopsis sp., increased during the two weeks of the observation period, the flora in the patch dramatically changed with the increase of phytoplankton biomass to a centric diatom-dominated community. Neritic diatoms, especially Chaetoceros debilis, showed higher growth rates than other diatoms, without any delay in the initiation of growth after the enrichment, and accounted for 90% of the micro-phytoplankton after day 9. In contrast, the oceanic diatoms showed distinct delays in the initiation of growth. We conclude that the responses of the diatoms to the manipulation of iron concentration were different by species, and the fast and intensive response of the phytoplankton to iron-enrichment resulted from the presence of a small amount of neritic diatoms at the study site. The important factors that determine the dominant species in the bloom are the potential growth rates under an iron-replete condition and the growth lag. Abundant species in the patch are widely distributed in the North Pacific and their relative contributions in the Oyashio area and at Stn KNOT are high from spring to summer. However, a characteristic difference of species composition between the SEEDS bloom and natural blooms was the lack of Thalassiosira and Coscinodiscus species in the patch, which usually account for a major part of the phytoplankton community under blooming conditions in the western North Pacific. © 2005 Elsevier Ltd. All rights reserved.
  • Noiri,Y., I. Kudo, H. Kiyosawa, J. Nishioka, and A. Tsuda, (2005) Iron and Temperatue, two factors influencing phytoplankton species composition in the western subarctic Pacific Ocean. Progress in Oceanography 64. 149-166.*
    2005 [Not refereed][Not invited]
  • Y Noiri, Kudo, I, H Kiyosawa, J Nishioka, A Tsuda
    PROGRESS IN OCEANOGRAPHY 64 (2-4) 149 - 166 0079-6611 2005 [Refereed][Not invited]
     
    On-board bottle incubation experiments were carried out by adding different amounts of iron as part of the SEEDS (Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study) experiment to elucidate the phytoplankton community response to various iron concentrations. A temperature gradient from 5 to 18 &DEG; C was also applied to the incubation experiment because temperature also affects growth rate and metabolic functions such as enzyme reactions. Chlorophyll-a increased with increasing iron concentration, while the increase in chlorophyll-a in the control bottles (no iron addition) was nominal. Size-fractionated chlorophyll-a indicated that micro-phytoplankton (> 10 μ m) became dominant in the iron-enriched bottles below 13 &DEG; C, which was consistent with the results from in situ observations in the simultaneous mesoscale iron-enrichment experiment and similar experiments for the other high nitrate and low chlorophyll (HNLC) waters. Macronutrients (silicate, nitrate and phosphate) were consumed in the bottles below 13 &DEG; C. The consumption ratio of silicate to nitrate was around 1:1 for iron-enriched bottles, but this ratio was slightly higher for control bottles, indicating a physiological stress due to low iron concentration on the diatoms. In contrast, nanophytoplankton (< 10 μ m) were dominant in the iron-enriched bottles at 18 &DEG; C. There was no silicate utilization, but nitrate and phosphate were consumed in these bottles. Bacillariophyceae were dominant in the bottles below 13 &DEG; C, but Prymnesiophyceae became dominant at 18 &DEG; C. This is the first report demonstrating that temperature influences phytoplankton composition after iron addition in HNLC waters. The Chlorophyll-a specific increase rate for micro-plankton (> 10 μ m) doubled from 5 to 8 &DEG; C and was the highest between 8 and 13 &DEG; C at the same iron concentration. The surface mixed layer temperature was 8 &DEG; C at the beginning of iron fertilization, but it had been 5 &DEG; C just 2 weeks before the experiment. This dramatic increase in growth rate with temperature is hypothesized to be the reason for SEEDS 2001 biomass changes being the highest reported response to a mesoscale Fe-enrichment experiment in HNLC waters. © 2005 Elsevier Ltd. All rights reserved.
  • A Tsuda, H Saito, J Nishioka, T Ono
    PROGRESS IN OCEANOGRAPHY 64 (2-4) 237 - 251 0079-6611 2005 [Refereed][Not invited]
     
    A mesoscale iron-fertilization experiment was carried out in the western subarctic Pacific during summer 2001. The iron-patch was traced for 14 days after the fertilization, and the abundance and behavior of mesozooplankton were compared with those outside of the patch. The phytoplankton biomass in the patch rapidly increased to over 15 times the initial level by the later half of the observation period, and was composed of large-sized (> 10 mm), centric diatoms. Dominant zooplankton species in the upper 200-m depth were large copepods: Neocalanus plumchrus, Neocalanus cristatus, Eucalanus bungii and Metridia pacifica. Mesozoplankton biomass as well as species composition did not change significantly in the patch over the observation period. Furthermore, no changes of vertical distribution or diel vertical migration were observed for any species or stages of mesozooplankton throughout the observation period. However, the abundance of the first copepodite stages of N. plumchrus and E. bungii increased several fold in the patch after the diatom bloom formation compared to the densities outside the patch. The increases of both species are considered to be due to lowered mortality during the egg and nauplius stages. Spawning of N. plumchrus takes place at depth using lipid storage, while spawning of E. bungii takes place in the surface layer supported by grazing. These facts suggest that the relative importance of nauplii in the diets of the large copepods was decreased in the patch by the diatom bloom. Gut-pigment contents of dominant copepods in the patch increased 4-18 times, and the maximum values were observed during the bloom peak. However, the grazing impact on phytoplankton was low throughout the experiment, especially during the bloom period (< 6% of the primary production). © 2005 Elsevier Ltd. All rights reserved.
  • Kudo, I, Y Noiri, K Imai, Y Nojiri, J Nishioka, A Tsuda
    PROGRESS IN OCEANOGRAPHY 64 (2-4) 207 - 221 0079-6611 2005 [Refereed][Not invited]
     
    Primary productivity and uptake rates of nitrate and ammonium were measured during an iron enrichment-experiment in the western subarctic Pacific Ocean using stable C-13 and N-15 isotope methods with simulated, on-board, for 24-h incubations. Nitrate, phosphate and silicate before the iron enrichment in the surface mixed layer (0-10 m) were abundant at 17, 1.5 and 34 μ M, respectively. Primary productivity in the upper 20 m ranged from 10 to 30 mg C m(-3) d(-1) on Day 0 (the day of iron fertilization) and did not change Outside the iron patch during 13 days of the observation. Primary productivity in the iron patch increased more than 10-fold after Day 7 and the maximum of 310 mg C m(-3) d(-1) was observed on Day 9. The elevated primary productivity continued until Day 13 (end of the observation). The integrated primary productivity in the photic zone was about 2 g C m(-2) d(-1) after Day 7. During 13 days of the observation, 16.5 g C m(-2) was fixed in the patch, while C fixation was 4.5 g C m(-2) outside. Absolute nitrate uptake at 5 m depth was 0.2 μ M d(-1) on Day 0, almost same as the ammonium uptake rate. Nitrate uptake sharply increased 20 times after Day 7. The maximum rate of 4.2 μ M d(-1) was observed on Day 9. Total nitrate uptake at 5 m between Days 4 and 9 was 10.8 μ M, which was comparable to the net decrease of nitrate at 8.5 μ M at this depth. Uptake rate of ammonium was 0.2 μ M d(-1) on Day 0 and had not changed by Day 13. The f-ratio (a ratio of nitrate uptake to total nitrogen (nitrate and ammonium) uptake) was 0.4-0.6 before Day 4, but increased to 0.95 on Day 7 and remained high until Day 13. These results suggest that the nitrate assimilation pathway was induced by the iron enrichment to meet the high nitrogen demand from rapid growth of the dominant diatom Chaetoceros dedilis. Relative uptake rate among nutrients was estimated from their apparent concentration change at 5 m in the patch. Nitrate and phosphate decreased at a molar ratio of 15.5, close to Redfield's stoichiometry. Silicate to nitrate uptake ratio, however, changed sharply on Day 9 from 1.6 to 3.0, indicating physiological stress on growing diatoms. Photosynthetically active radiation (PAR) at 5 in decreased from 400 to < 100 μ mol m(-2) s(-1) after Day 8 due to the high biomass of phytoplankton within the patch. Dissolved iron concentration also decreased to less than 0.3 nM after Day 9. This concentration was below the half saturation level for iron (K-s) obtained from on-board iron-addition incubations. These results suggest that low light and low iron concentration caused the physiological stress on diatoms after Day 8. © 2005 Elsevier Ltd. All rights reserved.
  • T van Oijen, MJW Veldhuis, MY Gorbunov, J Nishioka, MA van Leeuwe, HJW de Baara
    MARINE CHEMISTRY 93 (1) 33 - 52 0304-4203 2005/01 [Refereed][Not invited]
     
    Storage carbohydrates (e.g., water-extractable beta-1,3-D-glucan in diatoms) are of key importance for phytoplankton growth in a variable light climate, because they facilitate continued growth of the cells in darkness by providing energy and carbon skeletons for protein synthesis. Here, we tested the hypothesis that synthesis of storage carbohydrates by phytoplankton in the Southern Ocean is reduced by low iron and light availability. During the EisenEx/CARbon dioxide Uptake by the Southern Ocean (CARUSO) in situ iron enrichment experiment in the Atlantic sector of the Southern Ocean in November 2000, we studied the dynamics of water-extractable carbohydrates in the particulate fraction over the period of 3 weeks following the iron release. The areal amount (integral between 0- and 100-m depth) of carbohydrates increased from 1400 to 2300 mg m(-2) inside the iron-enriched patch, while remaining roughly constant in the surrounding waters. Most of the increase inside the patch was associated with the fraction of large (>10 mum) phytoplankton cells, consistent with the shift in the community structure towards larger diatoms. Deck incubations at 60% of the ambient irradiance revealed that the diurnal chlorophyll a (Chl a)-specific production rates of water-extractable polysaccharides were significantly higher for "in-patch" than for "out-patch" samples (0.5 vs. 0.3 mug C [mug Chl a](-1) h(-1), respectively). Together with the higher photochemical efficiency of photosystent II (F-v/F-m), this indicates enhanced photosynthetic performance in response to iron fertilization. In addition, the nocturnal polysaccharide consumption rates were also enhanced by iron release, causing a striking increase in the diel dynamics of polysaccharide concentration. An iron-stimulated increase in diel dynamics was also observed in the fluorescence and size of pico- and nanophytoplankton cells (measured by flow cytometry) and is indicative of enhanced phytoplankton growth. Diurnal polysaccharide production by phytoplankton inside the patch was light-limited when they were incubated at intensities below ca. 200 mumol m(-2) s(-1) (daytime average). These irradiance levels correspond to those at 20- to 30-m depth in situ, whereas the upper mixed layer was frequently several-fold deeper due to storms. Therefore, these first measurements of phytoplankton carbohydrates during an in situ iron release experiment have revealed that both light and iron availability are the key factors controlling the synthesis of storage carbohydrates in phytoplankton and, hence, the development of diatom blooms in the Southern Ocean. (C) 2004 Elsevier B.V. All rights reserved.
  • H Takata, K Kuma, S Iwade, Y Yamajyoh, A Yamaguchi, S Takagi, K Sakaoka, Y Yamashita, E Tanoue, T Midorikawa, K Kimura, J Nishioka
    MARINE CHEMISTRY 86 (3-4) 139 - 157 0304-4203 2004/05 [Refereed][Not invited]
     
    Sources of natural Fe(III)-complexing organic ligands which control vertical distributions of Fe(III) hydroxide solubilities [Fe(III) solubility, < 0.025 mum] and labile dissolved Fe (< 0.22 mum) concentrations were studied at the subarctic and subtropical surface waters (5-300-m depth) in the northwestern North Pacific Ocean during June 2002. High Fe(III) solubility values (0.71.3 nM) were observed at the depth of high chlorophyll a (Chl a) concentrations in the surface mixed layer with no strong correlation between Fe(III) solubility and size-fractionated Chl a concentrations. However, a significant correlation was found between Fe(Ill) solubility values and heterotrophic bacteria abundance at 5 - 100-m depth in the subarctic coastal water. The high Fe(III) solubility observed in the surface mixed layer is probably due to the production of natural Fe(III)-complexing organic ligands, such as siderophores, which were possibly released by specific phytoplankton or bacteria species. Higher Fe(III) solubility values than the labile dissolved Fe concentrations in the surface waters indicate that natural Fe(III)-complexing organic ligands are in excess of labile dissolved Fe concentrations. The Fe(III) solubility levels appeared to increase with depth in mid-depth waters (100-300-m depth), especially in the subarctic waters, in association with higher iron levels (labile dissolved Fe and dissolvable Fe concentrations), nutrient concentrations and humic-type fluorescence intensity. The relatively strong linear correlations between labile dissolved Fe concentrations or Fe(III) solubility values and humic-type fluorescence intensity in mid-depth waters suggest that the labile dissolved Fe concentrations and Fe(III) solubility in mid-depth waters are primarily associated with humic-type fluorescent organic matter produced through the oxidative decomposition and transformation of sinking biogenic organic matter. Humic-type fluorescent organic matter (i.e., marine dissolved humic substances) may thus account for a significant fraction of the Fe(III)-complexing organic ligands, which control Fe(III) solubility and labile dissolved Fe concentrations in mid-depth waters. (C) 2004 Elsevier B.V. All rights reserved.
  • PW Boyd, CS Law, CS Wong, Y Nojiri, A Tsuda, M Levasseur, S Takeda, R Rivkin, PJ Harrison, R Strzepek, J Gower, RM McKay, E Abraham, M Arychuk, J Barwell-Clarke, W Crawford, D Crawford, M Hale, K Harada, K Johnson, H Kiyosawa, Kudo, I, A Marchetti, W Miller, J Needoba, J Nishioka, H Ogawa, J Page, M Robert, H Saito, A Sastri, N Sherry, T Soutar, N Sutherland, Y Taira, F Whitney, SKE Wong, T Yoshimura
    NATURE 428 (6982) 549 - 553 0028-0836 2004/04 [Refereed][Not invited]
     
    Iron supply has a key role in stimulating phytoplankton blooms in high-nitrate low-chlorophyll oceanic waters(1-5). However, the fate of the carbon fixed by these blooms, and how efficiently it is exported into the ocean's interior, remains largely unknown(1-5). Here we report on the decline and fate of an iron-stimulated diatom bloom in the Gulf of Alaska. The bloom terminated on day 18, following the depletion of iron and then silicic acid, after which mixed-layer particulate organic carbon (POC) concentrations declined over six days. Increased particulate silica export via sinking diatoms was recorded in sediment traps at depths between 50 and 125 m from day 21, yet increased POC export was not evident until day 24. Only a small proportion of the mixed-layer POC was intercepted by the traps, with more than half of the mixed-layer POC deficit attributable to bacterial remineralization and mesozooplankton grazing. The depletion of silicic acid and the inefficient transfer of iron-increased POC below the permanent thermocline have major implications both for the biogeochemical interpretation of times of greater iron supply in the geological past(6,7), and also for proposed geo-engineering schemes to increase oceanic carbon sequestration(3,8).
  • W Thimdee, G Deein, C Sangrungruang, J Nishioka, K Matsunaga
    WETLANDS 23 (4) 729 - 738 0277-5212 2003/12 [Refereed][Not invited]
     
    Stable carbon isotope ratios (delta(13)C) and C/N atomic ratio {(C/N)(a)} were used to investigate the sources and fate of organic matter in Khung Krabaen Bay, Thailand (KKB) and to assess the effect of shrimp pond effluent on the KKB system. Five major sources (mangroves, seagrass, algae, plankton, and shrimp feeds) contributing organic matter into waters and sediments were identified. Statistically significant differences between the VC values and (C/N)(a) of mangrove leaves (delta(13)C similar to -29parts per thousand (C/N)(a) similar to 40-105), seagrass (delta(13)C similar to -11parts per thousand (C/N)(a) similar to 19), and algae (delta(13)C similar to -16parts per thousand (C/N)(a) similar to 18) were found, while plankton (delta(13)C similar to -21parts per thousand (C/N)(a) similar to 5-6) and shrimp feeds (delta(13)C similar to -23parts per thousand (C/N)(a) similar to 7) had similar values. The most negative delta(13)C values of particulate organic matter (POM) and sediments in the mangrove area indicated that mangroves were the major sources of organic matter in this area. The delta(13)C levels in POM and in sediments found in the canals indicated that the organic matter was predominantly of mangrove origin rather than from shrimp-pond effluent. High nitrogen content in mangrove leaves and in sediments, and the low nutrient concentrations in the bay waters, suggested that the KKB system was not affected by shrimp-pond effluent during the sampling period. These results showed that mangroves were probably effective in trapping nutrients from shrimp-pond effluent. In the inner bay area, sources of POM were a mixture of seagrass, algae, and plankton, while sediments contained a strong signature from seagrass and algae. Plankton represent a major source contributing organic matter to POM and sediment in the offshore area. Our results showed that the delta(13)C value was a good tracer in identifying sources and fate of organic matter. On the other hand, the (C/N)(a) was less reliable in identifying organic matter sources because the original sources in POM and, particularly, sediment may be lost or masked by biochemical alteration. However, the combined analysis of delta(13)C and (C/N)(a) provided substantially more information regarding the source and fate of organic matter in the KKB system than could be achieved by using one tracer alone.
  • H Tani, J Nishioka, K Kuma, H Takata, Y Yamashita, E Tanoue, T Midorikawa
    DEEP-SEA RESEARCH PART I-OCEANOGRAPHIC RESEARCH PAPERS 50 (9) 1063 - 1078 0967-0637 2003/09 [Refereed][Not invited]
     
    Vertical distributions of Fe(III) hydroxide solubility were studied in the Okhotsk Sea and the northwestern North Pacific Ocean during May and June 2000. Fe(III) solubility minima (0.35-0.45 nM) were present in a narrow depth range (80-100 m) below the surface mixed layer at all stations. In general, the Fe(III) solubility levels in intermediate and deep waters are characterized by mid-depth maxima (0.76-0.86 nM) at 800-1250 m depth and, below that, a slight decrease to 0.4-0.6 nM with depth in association with increase in nutrient, apparent oxygen utilization (AOU) and humic-type fluorescence intensity. The most significant correlation between the Fe(III) solubility and humic-type fluorescence in intermediate and deep waters suggests that the distribution of humic-type fluorescent organic matter may control the distribution of Fe(111) solubility in deep ocean waters. The solubility profiles reveal that dissolved Fe concentrations in deep ocean waters may be controlled primarily by Fe(III) complexation with natural organic ligands, such as marine dissolved humic substances released through the oxidative decomposition and transformation of biogenic organic matter in intermediate and deep waters. In addition, high Fe(III) hydroxide solubility values (1.0-1.6 nM) were observed in the surface mixed layer at a station in the northwestern North Pacific Ocean where a phytoplankton bloom was observed. The higher Fe(III) solubility in the surface waters was probably due to a higher concentration or stronger affinity of natural organic Fe(III) chelators, which may be released by dominant phytoplankton and/or bacteria during the spring bloom and probably have a different chemical composition from those found in intermediate and deep waters. (C) 2003 Elsevier Ltd. All rights reserved.
  • Jun Nishioka, Shigenobu Takeda, Isao Kudo, Daisuke Tsumune, Takeshi Yoshimura, Kenshi Kuma, Atsushi Tsuda
    GEOPHYSICAL RESEARCH LETTERS 30 (14) 0094-8276 2003/07 [Refereed][Not invited]
     
    Comparison of vertical profiles of size-fractionated iron between the western and eastern subarctic North Pacific clearly showed higher labile particulate iron concentrations towards the west and this result strongly supports the higher iron supply in the western region. Additionally, the results of the SEEDS experiment, the first meso-scale iron enrichment experiment in the subarctic North Pacific, clearly showed that artificially enriched iron in the dissolved fraction (mainly in colloidal fraction) was rapidly transformed to suspended labile particulate iron during phytoplankton growth and was retained in the surface mixed layer. Probably, this same rapid transformation process occurs naturally after sporadic atmospheric iron supply and the labile particulate iron is retained in the western region. Furthermore, this transformation process reduces dissolved concentration of iron and its bioavailability. Therefore, the transformation process is important for understanding how phytoplankton became iron limited and the biogeochemical iron cycle in the western subarctic North Pacific.
  • A Tsuda, S Takeda, H Saito, J Nishioka, Y Nojiri, Kudo, I, H Kiyosawa, A Shiomoto, K Imai, T Ono, A Shimamoto, D Tsumune, T Yoshimura, T Aono, A Hinuma, M Kinugasa, K Suzuki, Y Sohrin, Y Noiri, H Tani, Y Deguchi, N Tsurushima, H Ogawa, K Fukami, K Kuma, T Saino
    SCIENCE 300 (5621) 958 - 961 0036-8075 2003/05 [Refereed][Not invited]
     
    We have performed an in situ test of the iron limitation hypothesis in the subarctic North Pacific Ocean. A single enrichment of dissolved iron caused a large increase in phytoplankton standing stock and decreases in macronutrients and dissolved carbon dioxide. The dominant phytoplankton species shifted after the iron addition from pennate diatoms to a centric diatom, Chaetoceros debilis, that showed a very high growth rate, 2.6 doublings per day. We conclude that the bioavailability of iron regulates the magnitude of the phytoplankton biomass and the key phytoplankton species that determine the biogeochemical sensitivity to iron supply of high-nitrate, low-chlorophyll waters.
  • K Suzuki, A Tsuda, H Kiyosawa, S Takeda, J Nishioka, T Saino, M Takahashi, CS Wong
    JOURNAL OF EXPERIMENTAL MARINE BIOLOGY AND ECOLOGY 271 (1) 99 - 120 0022-0981 2002/05 [Refereed][Not invited]
     
    To investigate the impact of microzooplankton grazing on phytoplankton bloom in coastal waters, an enclosure experiment was conducted in Saanich Inlet, Canada during the summer of 1996. Daily changes in the microzooplankton grazing rate on each phytoplankton group were investigated with the growth rates of each phytoplankton group from the beginning toward the end of bloom using the dilution technique with high-performance liquid chromatography (HPLC). On Day I when nitrate and iron were artificially added, chlorophyll a concentration was relatively low (4.3 mug l(-1)) and 19'-hexanoyloxyfucoxanthin-containing prymnesiophytes were predominant in the chlorophyll biomass. However, both the synthetic rates and concentrations of 19'-hexanoyloxyfucoxanthin declined before bloom, suggesting that 19'-hexanoyloxyfucoxanthin-containing prymnesiophytes weakened. Chlorophyll a concentration peaked at 23 mug l(-1) on Day 4 and the bloom consisted of the small chain-forming diatoms Chaetoceros spp. (4 mum in cell diameter). Diatoms were secondary constituents in the chlorophyll biomass at the beginning of the experiment, and the growth rates of diatoms (fucoxanthin) were consistently high (>0.5 d(-1)) until Day 3. Microzooplankton grazing rates on each phytoplankton group remarkably increased except on alloxanthin-containing cryptophytes after the nutrient enrichments, and peaked with >0.6 d(-1) on Day 3. indicating that >45% of the standing stock of each phytoplankton group was removed per day. Both the growth and mortality rates of alloxanthin-containing cryptophytes were relatively high (>1 and >0.5 d(-1), respectively) until the bloom, suggesting that a homeostatic mechanism might exist between predators and their prey. Overall, microzooplankton grazing showed a rapid response to the increase in phytoplankton abundance after the nutrient enrichments, and affected the magnitude of the bloom significantly. High grazing activity of microzooplankton contributed to an increase in the abundance of heterotrophic dinoflagellates with 7-24 mum in cell size, the fraction of large-sized (>10 mum) chlorophyll a, and stimulated the growth of larger-sized ciliates after the bloom. (C) 2002 Elsevier Science B.V. All rights reserved.
  • T Ono, K Tadokoro, T Midorikawa, J Nishioka, T Saino
    GEOPHYSICAL RESEARCH LETTERS 29 (8) 10.1029/2001 GL014332  0094-8276 2002/04 [Refereed][Not invited]
     
    [1] Temporal variation of phosphate concentration in the mixed layer of western subarctic North Pacific from 1968 to 1998 was investigated with quasi-monthly resolution. February and July phosphate time series showed a linear decreasing trend of -0.008 +/- 0.003 and -0.019 +/- 0.005 mumol/l/y, respectively, while no multi-decadal trend was observed in April/May phosphate time series. With this result, net biological phosphate consumption between February and April linearly decreased at the rate of -0.43 +/- 0.11 mmolP/m(2)/y, while that between May and July increased at the rate of +0.16 +/- 0.05 mmolP/m(2)/y. Net biological phosphate consumption from February through August was linearly diminished at the rate of -0.40 +/- 0.07 mmolP/m(2)/y, corresponding to a -0.51 +/- 0.09 gC/m(2)/y decrease of net community production under assumption of the constant C/P ratio of 106. Chl.a inventory in the April mixed layer also showed a decreasing trend of -2.35 +/- 1.22 mg/m(2)/y, which correlated with the observed net production decrease between February and April.
  • J Nishioka, S Takeda, CS Wong, WK Johnson
    MARINE CHEMISTRY 74 (2-3) 157 - 179 0304-4203 2001/05 [Refereed][Not invited]
     
    The spatial and temporal changes in vertical distribution of soluble Fe(< 200 kDa or < 0.03 mum) and small colloidal Fe (200 kDa-0.2 mum, 200 kDa-0.1 mum, 0.03-0.2 mum or 0.03-0.1 mum) were studied during September 1997, June and September 1998, and February 1999 along a transect extending from southern Vancouver Island to Ocean Station Papa (OSP). At OSP, vertical profiles of soluble Fe exhibited nutrient-like distributions. In June and September, concentrations of soluble Fe were low in the surface mixed layer (0.05-0.07 nM) and increased below 300 m depth with temporal variation (0.17-0.42 nM, 300-600 m depth). The concentrations of small colloidal Fe were generally low in the surface mixed layer (0.01-0.06 nM) with higher concentrations below 200 m depth. From 200 to 600 m depth, small colloidal Fe represented 13-50% of dissolved Fe (<0.2 <mu>m) and it has temporal variation. Concentrations of soluble Fe (0.20-0.57 nM) were higher in February 1999 than those in June and September 1997-1998. Concentrations of small colloidal Fe from 200 to 1000 m depth were also higher in February 1999 (0.17-0.37 nM) than those in June and September 1997-1998. These results suggest that, in OSP, a substantial portion of small colloidal Fe exists in the dissolved Fe fraction (< 0.2 <mu>m) and that there are temporal changes in soluble and small colloidal Fe concentration in OSP seawater. Meanwhile, vertical profiles of Fe at stations near Vancouver Island did not show nutrient-like distributions. High concentrations of small colloidal Fe and large labile particulate Fe (> 0.2 mum, labile at pH 3.2) were observed in deep water at coastal stations both in September 1998 and February 1999. These results suggest that soluble and small colloidal Fe vary spatially and temporally. Particularly, in our observation, the soluble Fe has temporal changes in open ocean seawater while the small colloidal and large labile particulate Fe has spatial changes at coastal area. Thus, we need to consider the existence of small colloidal Fe in the dissolved Fe fraction (< 0.2 <mu>m) in seawater to better understand the Fe dynamics in the ocean. (C) 2001 Elsevier Science B.V. All rights reserved.
  • J Nishioka, S Takeda, CS Wong
    JOURNAL OF EXPERIMENTAL MARINE BIOLOGY AND ECOLOGY 258 (2) 237 - 255 0022-0981 2001/04 [Refereed][Not invited]
     
    To observe micronutrient dynamics in the plankton ecosystem, controlled ecosystem enclosure (CEE) experiments were conducted in Saanich Inlet, B.C., Canada. Two CEEs (2.5 m in diameter, 16 m in length, one for Fe studies and the other for biological studies) were launched for the period 22 July to 5 August 1996 and enriched with 10 muM nitrate and 5.2 nM Fe (13% of total Fe) on day 1. Sampling from three integrated depths, intervals 0-4, 4-8 and 8-12 m, was performed on days 0, 1, 2, 3, 4, 5, 7, 9 and 11. Iron concentrations were measured for five size fractions: > 25 mum particles, 2-25 mum particles, 0.2-2 mum particles, 0.2 mum-200 kDa small colloidal particles and < 200 kDa soluble species. The sediment in the Fe enclosure was also collected on every sampling day after day 2 and its Fe was determined. Size-fractionated particulate organic carbon and total chlorophyll-a were also analyzed. The Fe in small colloidal particles (200 kDa-0.2 <mu>m) comprised 78% of the traditionally defined dissolved phase (< 0.2 <mu>m) on day 1. Of all the size fractions of Fe, the small colloidal particulate fraction decreased most significantly during the phytoplankton bloom. In the dissolved fraction (< 0.2 <mu>m), the small colloidal particle fraction comprised 79% of the decrease. The decrease in concentration of Fe in small colloidal particles was larger than that of total Fr from day 1 to day 4. In contrast, the > 25 mum Fe particles increased over the same period. These results suggest that Fe in small colloidal particles changed to > 25 mum Fe particles during phytoplankton growth. A large amount of Fe was kept in the surface layer with the phytoplankton, and transported to the deep layer by phytoplankton sedimentation, at the end of the bloom. From these results, the small colloidal particulate Fe seems to be the most dynamic size fraction and a high percentage of Fe in small colloidal particles changed to large particles due to chemical/physical aggregation and/or physical adsorption to suspended particles such as phytoplankton cells. (C) 2001 Elsevier Science B.V. All rights reserved.
  • IKEYA Tohru, NAKAYA Seiji, FUKABORI Yoshio, NISHIOKA Jun, TAKEDA Shigenobu, KAWANOBE Kyoko, TAKAHASH Masayuki Mac
    Deep Ocean Water Research 海洋深層水利用学会 2 (1) 73 - 86 1345-8477 2001 [Refereed][Not invited]
     
    Responses of phytoplankton community in subtropical surface waters by the addition of deep ocean water (DOW) were examined under natural sunlight. Two culture experiments were carried out in September and October, 1999; DOW collected from ca 800 m was mixed into the surface water at high concentrations (13 and 27% (v/v)) in 9 l polycarbonate bottles and incubated in an outdoor incubator (EXP 1), and DOW from ca 400 m was daily added to ca 800 l of surface water at low concentrations (0.6 and 1.2%) in polyethylene bags moored in a harbor (EXP 2). Both experiments showed that the DOW enrichment, equivalent to more than 0.15 μmol nitrate l-1 day-1, increased concentrations of chlorophyll a (Chl a) after a lag period of 5 to 7 days and reached maximum concentrations at 10 to 12 days after the start of the experiments. The maximum levels of Chl a were proportional to the total addition of nitrate; 1 mol nitrate = around 1.4 g Chl a. The specific rates of increase of Chl a concentration were also in proportion to the amount of added DOW. Low Chl a share percentage of>2 μm fraction, such as 15% of total Chl a, in the initial surface water was changed to more than 70%. These results indicated that the DOW enrichment effectively enhanced the growth of nano- and microphytoplankton. Within those phytoplankton, some diatom species, being extremely less abundant in the subtropical surface water, increased rapidly. Results indicate primary production even in subtropical ocean can be raised effectively to enhance effective phytoplankton community for fish production by the DOW enrichment. Since the enhancement of the phytoplankton growth by the DOW enrichment was observed after 5 to 7 days, DOW should be added to keep at least 0.1 μmol nitrate l-1 day-1 during the lag period for initialing the fertilization effects in the subtropical waters.
  • J Nishioka, S Takeda
    MARINE BIOLOGY 137 (2) 231 - 238 0025-3162 2000/09 [Refereed][Not invited]
     
    Laboratory culture experiments were performed to study the changes in size-fractionated Fe concentrations during the growth of the oceanic diatom Chaetoceros sp. Fe concentration was estimated for three size fractions: large labile particles (>0.2 mu m), small colloidal particles (0.2 mu m to 200 kDa) and soluble species (<200 kDa). The size-fractionated Fe concentration in the nutrient-enriched filtered seawater medium without diatom cells became stable within 4 d after the spike of FeCl3 solution. Light irradiation by white fluorescent tubes with a 14 h light:10 h dark cycle did not significantly alter concentrations of the size-fractionated Fe. For the phytoplankton culture experiment, Fe-starved diatom cells were inoculated into the nutrient-enriched media aged for 19 d after the addition of FeCl3. With the growth of diatom cells, total acid-labile Fe concentrations decreased from 0.60 to 0.46 nM during 7 d of incubation. However, only the concentration of the small colloidal particles showed a significant decrease; the concentration of the other size fractions remained relatively constant. Although the media still contained sufficient amounts Fe as large labile particles and soluble species, diatom cells appeared to be Fe-limited once Fe as small colloidal particles had been used up. These results suggest that Fe in the small colloidal particle fraction was the most dynamic size fraction during the growth of the diatom Chaetoceros sp. In addition, to better understand Fe dynamics in the ocean, we must consider the influence of phytoplankton growth on small colloidal Fe [which is typically included in the dissolved Fe fraction (<0.2 mu m)].
  • K Matsunaga, J Nishioka, K Kuma, K Toya, Y Suzuki
    WATER RESEARCH 32 (11) 3436 - 3442 0043-1354 1998/11 [Refereed][Not invited]
     
    The effects of riverain iron and nutrient inputs on phytoplankton growth in Kesennuma Bay were studied. The effects of iron and Fulvic acid-iron complex additions on phytoplankton growth were studied in iron-enriched and -limited culture experiments of coastal marine diatom Chaetoceros sp. (the dominant species inside and outside of the bay) using media prepared from bay and outer waters. Bay water is not iron-limited. The addition of Fe(III) to bay water or autoclaved bay water gave no increase in cell yield. However, when bay water was autoclaved after UV-irradiation, there was little growth. This suggests that the UV irradiation destroyed organic compounds that affected iron bioavailability. Outer water is iron-limited. The addition of Fe(III) to outer water increased cell yield and iron-enriched outer water prepared by autoclaving after adding fulvic acid-Fe increased also cell yield. When outer water after adding Fe(III) was autoclaved, there was little growth. This suggests that fulvic acid made the iron bioavailable. The riverain inputs of organically bound iron, such as fulvic acid-Fe, and nutrients probably play an important role for supporting phytoplankton growth in the bay. (C) 1998 Elsevier Science Ltd. All rights reserved.
  • K Kuma, A Katsumoto, J Nishioka, K Matsunaga
    ESTUARINE COASTAL AND SHELF SCIENCE 47 (3) 275 - 283 0272-7714 1998/09 [Refereed][Not invited]
     
    The distribution of iron in two size fractions (<0.025 and < 0.45 mu m) was studied in various coastal waters from northern Japan and in oceanic water, and it was found that estuarine waters and surface waters in Funka Bay (Hokkaido) had the highest colloidal iron concentrations (0.025-0.45-mu m fraction, 7-19 nM), with a large portion (85-93%) in the < 0.45 mu m fraction. The high colloidal iron concentrations in estuarine and bay surface waters were probably due to the rapid formation or contribution of colloidal iron through riverine and atmospheric inputs, respectively. Dissolved organic substances, such as humic substances and other organic Fe(III) chelators, possibly contributed to the formation and stabilization of the colloidal iron fraction in seawater. In addition, ultraviolet (UV)-irradiation treatment of coastal and estuarine waters reduced the iron concentrations in all size fractions (< 0.025 mu m, 0.025-0.45 mu m, < 0.45 mu m) and the Fe(III) hydroxide solubilities, resulting from the decomposition of natural organic iron chelators. These results suggest that natural organic Fe(III) chelators control the colloidal and dissolved iron concentrations in seawater. (C) 1998 Academic Press.
  • K Kuma, J Nishioka, K Matsunaga
    LIMNOLOGY AND OCEANOGRAPHY 41 (3) 396 - 407 0024-3590 1996/05 [Refereed][Not invited]
     
    Iron solubilities of Fe(III) hydroxide in coastal and oceanic waters and in the ultraviolet (UV)-irradiated seawaters over a pH range of 5.7-8.2 at 20 degrees C were determined by a simple filtration (0.025 mu m) involving gamma-activity measurement of Fe-59. At pH ranges of 5.7-7.2 (coastal water) and of 5.7-7.6 (oceanic water), only the Fe(OH)(2)(+) species is significant. The calculated solubility products, log*K-SO, for coastal and oceanic waters were 4.8-5.0 and 4.4-4.6, respectively. The solubilities within the pH range of 7.8-8.2 are relatively independent of pH and aging time. Solubility in the oceanic water was about one order of magnitude lower than that in the coastal water, and UV irradiation reduced solubility to less than or equal to 0.1 nM. The vertical profiles of ambient Fe(III) solubility (pH 8.0-8.2) in oceanic waters have the following features in common: solubility in the surface mixed layer is high and variable (0.3-0.6 nM), generally corresponding with the depth of high chlorophyll a concentrations; solubility minima (0.15-0.2 nM) occur at a depth of 50-200 m. These results suggest that natural organic Fe(III) chelators exist in significant concentrations and control the dissolved iron concentration in seawaters.
  • Determination of the freshwater origin of Coastal Oyashio Water using humic-like fluorescence in dissolved organic matter
    Yu Mizuno, Jun Nishioka, Takahiro Tanaka, Yuya Tada, Koji Suzuki, Yuta Tsuzuki, Atsuko Sugimoto, Youhei Yamashita
    Journal of Oceanography 74 (5) 509 - 521 [Refereed][Not invited]

MISC

Association Memberships

  • 日本地球化学会   日本海洋学会   American Society of Limnoligy and Oceanography   American Geophysical Union   PICES IFEP Advisory Panel   

Research Projects

  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research
    Date (from‐to) : 2024/04 -2029/03 
    Author : 平譯 享, 西岡 純, 高尾 信太郎, 吉田 和広, 真壁 竜介
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Transformative Research Areas (A)
    Date (from‐to) : 2022/06 -2027/03 
    Author : 羽角 博康, 伊藤 幸彦, 杉本 亮, 黒田 寛, 松村 義正, 西岡 純, 郭 新宇, 纐纈 慎也, 山崎 大, 安田 一郎, 小畑 元, 田中 潔, 平譯 享, 中田 聡史, 西川 悠
  • Material exchange between coastal area and the subarctic North Pacific and biological production
    Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Transformative Research Areas (A)
    Date (from‐to) : 2022/06 -2027/03 
    Author : 西岡 純, 小畑 元, 平譯 享, 安田 一郎, 山下 洋平
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research
    Date (from‐to) : 2022/04 -2026/03 
    Author : 小畑 元, 則末 和宏, 岡 顕, 西岡 純
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (S)
    Date (from‐to) : 2021/07 -2026/03 
    Author : 西岡 純
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (S)
    Date (from‐to) : 2020/08 -2025/03 
    Author : 大島 慶一郎, 深町 康, 西岡 純, 二橋 創平, 三谷 曜子, 木村 詞明
     
    高海氷生産域であるフラジルアイス域を検知できるマイクロ波放射計AMSRのアルゴリズムを開発し、南大洋全域で初めてフラジルアイス域のマッピングを行った。フラジルアイス域を検知できたことで、今まで過小評価されていた沿岸ポリニヤでの海氷生産量をより高精度で見積もることができ、従来のアルゴリズムより、20－50％高い海氷生産量が示された。30年あまり継続してあるマイクロ波放射計SSM/I-SSMISデータに対し、薄氷タイプを識別することでより高精度の薄氷厚が推定できるアルゴリズムを開発した。南大洋をベースに開発したこのアルゴリズムは北極海でも適用可能であることも示され、全球に汎用性のあるアルゴリズムであることが示された。この開発により、長期間での全球での沿岸ポリニヤの海氷生産量の見積もりに道が開けた。プロファイリングフロートの溶存酸素量の変動から、初めてオホーツク海の広範囲で正味の生物生産量（純群集生産量）を見積もることに成功し、春の植物プランクトンの大増殖（春季ブルーム）は海氷融解によってもたらされていることを定量的指標をもって明らかにした。さらに、海氷融解時に放出される物質（鉄分）がブルームを誘起していることを示唆し、海氷による物質輸送の重要性を示した。観測としては、2021年3月に小型砕氷船ガリンコ号を傭船し、海氷サンプリングや氷縁での観測を実施し、海氷による物質輸送の観測研究に着手した。この他、CTDタグを購入し、データ通信用のアルゴスシステムに登録するなど、アザラシ、トドによるバイオロギング観測を行う準備を整えた。
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Challenging Research (Exploratory)
    Date (from‐to) : 2020/07 -2022/03 
    Author : Nishioka Jun
     
    The results obtained from this study clearly show that the Sub-Antarctic intermediate water becomes an area where Fe and Silicic acid are insufficient to support the growth of diatoms in the surface layer, while the North Pacific intermediate water is a Si-rich water mass and the supply of Fe is relatively high in its western part (NPIE-W), which also supports the production of diatoms in the surface layer. The results of the study indicated the need for a quantitative evaluation of the entire material circulation system, including the marginal seas in the North Pacific.
  • 科学研究費：基盤研究(A)
    Date (from‐to) : 2017/04 -2021/03 
    Author : 西岡 純
  • 科研費補助金：基盤研究(A)
    Date (from‐to) : 2017/04 -2020/03 
    Author : 大島慶一郎
  • カムチャツカ半島の淡水供給が制御する環オホーツク陸海結合システム
    科研費補助金：基盤研究(A)
    Date (from‐to) : 2017/04 -2020/03 
    Author : 三寺史夫
  • 科研費補助金：新学術領域研究(研究領域提案型)
    Date (from‐to) : 2015/06 -2020/03 
    Author : 安田一郎
  • 科研費補助金：新学術領域研究(研究領域提案型)
    Date (from‐to) : 2015/06 -2020/03 
    Author : 西岡 純
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research
    Date (from‐to) : 2016/04 -2019/03 
    Author : Nakanowatari Takuya, Mitsudera Humio, Nakamura Tomoyuki, Uchimoto Keisuke, Nishioka Jun, Watanabe Eiji
     
    By using an ice-ocean coupled model with an biogeochemical model, the transport process of dissolved iron and its variability in the subarctic North Pacific including the Sea of Okhotsk were investigated. Seasonal to interannual variability of dissolved iron and macro nutrient concentration in the Oyashio and northwestern subarctic North Pacific is controlled by the wind-driven transport from the Okhotsk Sea and the Bering Sea as well as the local transport from the subsurface layer due to winter convection. Such lateral transport process has an essential role on the determination of primary production in the subarctic North Pacific. On the other hand, the impact of sea ice formation and melting process on the primary production is limited to the Sea of Okhotsk.
  • 表層と中層を繋ぐ北太平洋の子午面循環：その三次元構造と変動メカニズムの新たな描像
    科研費補助金：基盤研究(A)
    Date (from‐to) : 2014/04 -2017/03 
    Author : 三寺 史夫
  • 北太平洋HNLC海域形成過程の解明
    科研費補助金：基盤研究(B)
    Date (from‐to) : 2014/04 -2017/03 
    Author : 西岡 純
  • オホーツク海太平洋交換流・混合と太平洋深層構造観測研究
    科研費補助金：基盤研究(A)
    Date (from‐to) : 2013/04 -2016/03 
    Author : 安田一郎
  • 海氷融解過程を考慮した極域海洋像の構築
    科研費補助金：新学術領域研究(公募研究)
    Date (from‐to) : 2013/04 -2015/03 
    Author : 西岡 純
  • 凍る海の豊な生態系を生み出す機構の解明
    キャノン財団：理想の追求
    Date (from‐to) : 2012/04 -2015/03 
    Author : 西岡 純
  • オホーツク海と北太平洋亜寒帯域をつなぐ熱塩／物質循環システムの実態解明
    科研費補助金：基盤研究(S)
    Date (from‐to) : 2010/04 -2015/03 
    Author : 若土 正曉
  • 海氷に含まれる微量元素分析法の開発
    科研費補助金：挑戦的萌芽研究
    Date (from‐to) : 2011/04 -2013/03 
    Author : 西岡 純
  • 海洋の気体交換フィードバックシステムと生物生産応答に関する研究
    科研費補助金：基盤研究(A)
    Date (from‐to) : 2010/04 -2013/03 
    Author : 渡辺 豊
  • 海氷の融解過程を含めた「環オホーツク海域」の鉄循環像の構築
    科研費補助金：基盤研究(B)
    Date (from‐to) : 2010/04 -2013/03 
    Author : 西岡 純
  • 潮汐混合の直接観測と潮汐１８．６年振動に関わる海洋・気候変動の解明
    科研費補助金：基盤研究(S)
    Date (from‐to) : 2008/04 -2013/03 
    Author : 安田一郎
  • 海洋表層マッピングによる親潮域の鉄供給過程の評価
    科研費補助金：特定領域研究 （公募研究）
    Date (from‐to) : 2009/04 -2011/03 
    Author : 西岡 純
  • 環オホーツク海域における化学的変質過程を含めた鉄移送量の定量的評価
    科研費補助金：若手研究(A)
    Date (from‐to) : 2007/04 -2010/03 
    Author : 西岡 純
  • 縁辺海(ベーリング海、オホーツク海、日本海)の基礎生産を支える鉄の挙動とその起源
    科研費補助金：基盤研究(A)
    Date (from‐to) : 2006/04 -2010/03 
    Author : 久万健志
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research
    Date (from‐to) : 2007 -2010 
    Author : GAMO Toshitaka, OBATA Hajime, AMAKAWA Hiroshi, SOHRIN Yoshiki, HASUMI Hiroyasu, OKAMURA Kei, ZHANG Jing, NAKAGUCHI Yuzuru, YAMADA Masatoshi, SHIRAIWA Yoshihiro, NORIKI Shinichiro, MINAMI Hideki, NISHIOKA Jun, SHITASHIMA Kiminori, KATO Yoshihisa, MINAGAWA Masao, MARUO Masahiro, OTOSAKA Shigeyoshi, NAKAYAMA Noriko, NAGAI Hisao, MURAYAMA Masafumi
     
    研究成果の概要(英文):As part of the international GEOTRACES (Global marine biogeochemical cycles of trace elements and their isotopes) program, we successfully conducted meridional research cruises in the Indian Ocean (from Arabian Sea to the Antarctic Ocean) and in the Japan Sea using the research vessel Hakuho Maru. We took clean seawater samples (total ～300 samples) for the measurement of GEOTRACES key paremeters(Fe,Al,Zn,Mn,Cd,Cu,δ^<15>N,δ^<13>C,^<230>Th,^<231>Pa, Pb isotopes,Nd isotopes), and their analyses are now ongoing in shorebased laboratories.
  • 西部北太平洋亜寒帯域(親潮域)表層の鉄濃度の変動と大気ダスト供給量との関係
    科研費補助金：特定領域研究 （公募研究）
    Date (from‐to) : 2007/04 -2009/03 
    Author : 西岡 純
  • 環オホーツク海域における中層循環と物質輸送のモデリング
    科研費補助金：基盤研究(A)
    Date (from‐to) : 2006/04 -2009/03 
    Author : 江淵直人
  • 海洋表層における微量栄養物質(鉄)の広域水平分布観測技術の開発
    科研費補助金：若手研究(B)
    Date (from‐to) : 2005/04 -2007/03 
    Author : 西岡 純
  • 北太平洋亜寒帯域の鉄濃度が生物生産と二酸化炭素収支に及ぼす影響に関する観測研究
    科研費補助金：基盤研究(B)
    Date (from‐to) : 2004/04 -2007/03 
    Author : 小埜恒夫

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