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Master

Affiliation (Master)

  • School of Fisheries Sciences　Training Ship “Oshoro-maru”

Affiliation (Master)

  • School of Fisheries Sciences　Training Ship “Oshoro-maru”

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Profile and Settings

Degree

  • fishery（Hokkaido Univ.）

Profile and Settings

  • Name (Japanese)

    IMAI

  • Name (Kana)

    KEIRI

  • Name

    201501008336606354

Achievement

Research Areas

  • Environmental science/Agricultural science / Environmental dynamics

Awards

  • 2023/03 日本水産学会 水産学技術賞
     我が国沖合海域における海洋プラスチックごみ調査の規準化およびデータベース整備 
    受賞者: 内田圭一、萩田隆一、向井徹、今井圭理、清水健一、八木光晴、山中有一、三橋廷央、磯辺篤彦、黒田真央

Published Papers

  • Mao Kuroda, Atsuhiko Isobe, Keiichi Uchida, Tadashi Tokai, Toshihide Kitakado, Miho Yoshitake, Yoshinori Miyamoto, Tohru Mukai, Keiri Imai, Kenichi Shimizu, Mitsuharu Yagi, Takahisa Mituhasi, Akimasa Habano
    Science of The Total Environment 925 171421 - 171421 0048-9697 2024/05
  • Haruka Nakano, María Belén Alfonso, Suppakan Jandang, Keiri Imai, Hisayuki Arakawa
    Regional Studies in Marine Science 69 2024/01 
    Researchers have made a significant effort to reveal plastic pollution's presence and consequences worldwide; however, there are no standard methods to evaluate the level of microplastic pollution. In this context, pollution index assessment in the marine environments contributes to showing a criteria of pollutants. Therefore, the pollution load index (PLI), the polymer risk index (H), and the pollution risk index (PRI) were estimated based on the MPs abundance and polymer-type information in situ for five areas around Japan's sea. According to H values, the contamination due to MPs presented low to moderate pollution levels in each area. Whereas the PRI classified the pollution levels from low (offshore) to very high (coast). Based on MPs' predicted concentrations for 2030 and 2060, the PRI is expected to increase, reaching a very high-pollution level even for offshore areas. The results of this study emphasize the importance of action to prevent additional plastic pollution and the importance of continued monitoring of the MPs in the environment.
  • Keiichi Uchida, Ryuichi Hagita, Tohru Mukai, Keiri Imai, Kenichi Shimizu, Mitsuharu Yagi, Yuichi Yamanaka, Takahisa Mituhasi, Atsuhiko Isobe, Mao Kuroda
    Nippon Suisan Gakkaishi (Japanese Edition) 89 (5) 394 - 397 0021-5392 2023
  • Keiri Imai, Yutaka W. Watanabe
    JOURNAL OF RADIOANALYTICAL AND NUCLEAR CHEMISTRY 0236-5731 2022/07 
    We started monitoring the radiocesium released from Fukushima in the North Pacific surface water from May 2011 after the accident soon to June 2016, using the cruise of the Oshoro-maru of Hokkaido University. We found that the reducing rate from the ocean surface of Cs-137 regardless of the distance from Fukushima was almost constant at 0.0033 +/- 0.0005 day(-1). This finding indicated that 96% of Cs-137 in the surface water over the North Pacific were removed by 1000 days after the accident from the surface water to the ocean interior excluding the decrease by radioactive disintegration.
  • Atsuhiko Isobe, Takafumi Azuma, Muhammad Reza Cordova, Andrés Cózar, Francois Galgani, Ryuichi Hagita, La Daana Kanhai, Keiri Imai, Shinsuke Iwasaki, Shin’ichro Kako, Nikolai Kozlovskii, Amy L. Lusher, Sherri A. Mason, Yutaka Michida, Takahisa Mituhasi, Yasuhiro Morii, Tohru Mukai, Anna Popova, Kenichi Shimizu, Tadashi Tokai, Keiichi Uchida, Mitsuharu Yagi, Weiwei Zhang
    Microplastics and Nanoplastics 1 (1) 2021/12 
    Abstract A total of 8218 pelagic microplastic samples from the world’s oceans were synthesized to create a dataset composed of raw, calibrated, processed, and gridded data which are made available to the public. The raw microplastic abundance data were obtained by different research projects using surface net tows or continuous seawater intake. Fibrous microplastics were removed from the calibrated dataset. Microplastic abundance which fluctuates due to vertical mixing under different oceanic conditions was standardized. An optimum interpolation method was used to create the gridded data; in total, there were 24.4 trillion pieces (8.2 × 104 ~ 57.8 × 104 tons) of microplastics in the world’s upper oceans.
  • Mao Kuroda, Keiichi Uchida, Tadashi Tokai, Yoshinori Miyamoto, Tohru Mukai, Keiri Imai, Kenichi Shimizu, Mitsuharu Yagi, Yuichi Yamanaka, Takahisa Mituhashi
    Marine Pollution Bulletin 161 (Pt A) 111670 - 111670 0025-326X 2020/12 [Refereed][Not invited]
     
    © 2020 The Authors Marine debris on the seafloor has not been thoroughly investigated, and there is little information compared to other types of marine debris. We conducted bottom trawl surveys to determine the present situation of marine debris on the seafloor in offshore areas around Japan. The survey was conducted in three sea areas with different characteristics. As a result, it was found that the amount of marine debris in submarine canyons (2926.1 items/km2) was higher than on the continental shelf. It was revealed that most marine debris on the seafloor is comprised of plastic products, and that debris on the seafloor retains its condition for a long time (over 30 years) without deterioration. In addition, the type of marine debris is affected by the industries operating in each area. Continuing to investigate marine debris on the seafloor in more areas will contribute to solving the problem of marine debris.
  • Xiaorong Fang, Yutaka Isoda, Isao Kudo, Takafumi Aramaki, Keiri Imai, Naoto Ebuchi
    Water (Switzerland) 12 (8) 2020/08 [Refereed][Not invited]
     
    © 2020 by the authors. To better understand the behavior of water mass beneath the Tsushima Warm Current (TWC), we use the vertical cross-sections of potential temperature, salinity and dissolved oxygen in the Japan Sea obtained by the T/V Oshoro Maru of the Hokkaido University during 8-29 June in 2011 to analyze its origins and variations. The results show that the potential temperature and salinity beneath the TWC varies little, but the dissolved oxygen varies largely with the geographical location. There are two deep water masses with different dissolved oxygen content below the TWC. One is on the coastal side with the low dissolved oxygen, and the other is on the offshore side with the high dissolved oxygen. It is inferred that the former one is relatively old water and the latter is the new water. By using the phosphate (PO4) and the apparent oxygen utilization (AOU) relationship, we calculate the PO40 (preformed PO4) as a water mass tracer. These results suggest that the water masses beneath the TWC with high and low dissolved oxygen originate from the same surface water mass in the central Japan Sea.
  • Yutaka W. Watanabe, Bofeng F. Li, Royce Yamasaki, Shun Yunoki, Keiri Imai, Shigeki Hosoda, Yoshiyuku Nakano
    JOURNAL OF OCEANOGRAPHY 76 (2) 155 - 167 0916-8370 2020/04 [Refereed][Not invited]
     
    We constructed parameterizations for the estimation of dissolved inorganic carbon (DIC) and pH in the western North Pacific, including Japanese coastal regions. Parameterizations, determined as a function of potential temperature (theta) and dissolved oxygen (DO), provided strong correlations with direct measurements for DIC [the coefficient of determination (R-2) = 0.99; the root mean square error (RMSE) = 8.49 mu mol kg(-1)] and pH (R-2 = 0.98, RMSE = 0.030). Predicted DIC and pH values were consistent with previous independent hydrographic observations, generally within 3 mu mol kg(-1) for DIC and 0.010 for pH. By applying these parameterizations to climatological seasonal theta and DO data and time-series theta and DO data from autonomous profiling floats (Argo) in the western North Pacific, large spatiotemporal variations in DIC and pH were demonstrated. Results indicated that the rate of increase in anthropogenic DIC above a depth of 400 m along 30 degrees N in the western North Pacific was + 0.86 +/- 0.11 mu mol kg(-1) year(-1) between 1994 and 2004. This decreased by + 0.58 +/- 0.05 mu mol kg(-1) year(-1) after 2004 partly due to increasing anthropogenic nitrogen input, suggesting that the decrease has induced the slowdown of ocean acidification in the western North Pacific. The declining trend in anthropogenically induced ocean pH was found to be 0.0021 +/- 0.0016 during 1994-2013.
  • Hiromichi Ueno, Mizuki Komatsu, Zhaoqianyi Ji, Ryo Dobashi, Miaki Muramatsu, Hiroto Abe, Keiri Imai, Atushi Ooki, Toru Hirawake
    Deep-Sea Research Part II: Topical Studies in Oceanography 181-182 0967-0645 2020 [Refereed][Not invited]
     
    © 2020 Elsevier Ltd We investigated spatial and interannual variation in the physical environment in the northern Bering Sea focusing on stratification, which is one factor affecting biological production in Arctic/subarctic regions. In particular, we analyzed in situ data obtained onboard the training ship Oshoro Maru in early summer in 2017 and 2018. We found that stratification in the areas just north of St. Lawrence Island (around 64.5°N and west of 168.5°W) and south/southwest of St. Lawrence Island was significantly weaker in 2018 than in 2017. These results are consistent with the extremely low sea-ice extent present in the winter of 2017/2018, which would have resulted in less freshwater being supplied to the surface layers and a warmer and less saline bottom water. Conversely, stratification was as strong in 2018 as in 2017 in the area close to the Alaska mainland, including the Bering Strait area, suggesting that the Alaskan Coastal Water dominates stratification in this area in early summer. Moreover, we found that the weakly stratified water column in the Bering Strait area stratified quickly shortly after the occurrence of strong northerly winds, likely because of the Ekman transport of warm and low-salinity Alaskan Coastal Water from the east.
  • Ryohei Sasayama, Nanako Hioki, Yuichiroh Morita, Yutaka Isoda, Keiri Imai, Atsushi Ooki, Kenshi Kuma
    Journal of Oceanography 74 (4) 367 - 379 0916-8370 2018/08/01 [Refereed][Not invited]
     
    © 2018, The Oceanographic Society of Japan and Springer Japan KK, part of Springer Nature. We studied the behavior of chemical substances in the upper 300 m of the water column across the continental shelf–slope interface in the East China Sea off the Okinawa Trough. The behaviors of iron, inorganic nutrients, and humic-like fluorescent dissolved organic matter were strongly influenced by the extensive water exchange between the East China Sea and the Kuroshio Current across the shelf break and slope via upwelling and frontal processes. We attributed the high humic-like fluorescent intensity at the subsurface of the shelf break and slope regions to the lateral supply of humic-like fluorescent dissolved organic matter from the shelf sediments to the outer shelf region due to the intrusion of shelf water into Kuroshio subsurface water. We found that the behavior of iron at the continental shelf–slope was remarkably different from the conservative mixing of inorganic nutrients and humic-like fluorescent dissolved organic matter. In deep and bottom waters at the shelf–slope, high total iron concentrations, which were closely related to water transmittance, possibly resulted from the swept transport of iron-rich resuspended sediments over the shelf floor from the slope by the invading Kuroshio Intermediate Water close to the bottom.
  • Yuichiro Kumamoto, Michio Aoyama, Yasunori Hamajima, Hisao Nagai, Takeyasu Yamagata, Yoshimi Kawai, Eitarou Oka, Atsushi Yamaguchi, Keiri Imai, Akihiko Murata
    Journal of Radioanalytical and Nuclear Chemistry 311 (2) 1209 - 1217 0236-5731 2017/02/01 [Refereed][Not invited]
     
    © 2016, The Author(s). In 2014, we measured activity concentration of radiocesium in the western North Pacific Ocean. In the north of Kuroshio Front high activity concentration of Fukushima-derived radiocesium in surface mixed layer in 2012 had been transported eastward by 2014. In the south of the front we found a radiocesium subsurface maximum in 200–600 m depth, which was similar to that observed in 2012. The subsurface maximum spread southward from 18°N to 15°N between 2012 and 2014, which suggests spreading of Fukushima-derived radiocesium into the whole western subtropical area by 2014 due to formation and subduction of the subtropical mode water.
  • Imai Keiri, Watanabe Yutaka W
    北海道大学水産科学研究彙報 北海道大学大学院水産科学研究科 65 (2) 111 - 116 1346-1842 2015/08 [Not refereed][Not invited]
     
    2011年3月11日，東北地方太平洋沖地震とそれに伴った大津波の影響で東京電力・福島第一原子力発電所において原子力事故が発生し，原子炉の崩壊および汚染水の意図的な放水によって，大量の放射性物質が大気および海洋へ放出された。この原子力発電所から直接海洋に流入したと推定される放射性セシウム量は，事故後から2ヶ月以内の間に，原子力発電所内部から高放射能汚染水として発電所専用港湾を経由して海洋に流出した部分が大半を占めると報告されている(Tsumune et al.，2013; Kanda，2013; 神田，2013)。また，多くの研究機関によって放射性物質による汚染状況を把握するために，海洋，大気および陸域の調査が行われ，放射性セシウムの測定がなされた(Honda et al.，2012; Aoyama et al.，2013a，b)。同時に数値シミュレーションを用いて，福島第一原子力発電所から大気および海洋へ放出された放射性物質量や，その広がりが予測され(Takemura et al.，2011; Tsumune et al.，2013)。
  • Yamauchi Yasutaka, Kuma Kenshi, Tateno Manami, Imai Keiri, Owada Maki, Shoji Kenya, Isoda Yutaka, Arita Shun, Kawano Kohei, Fujiwara Shohei, Fang Xiaorong, Asahi Keijiro, Ida Satoki, Yasutaka Yamauchi, Kenya Shoji, Yutaka Isoda, Shun Arita, Kohei Kawano, Shohei Fujiwara, Xiaorong Fang, Keijiro Asahi, Satoki Ida, Kenshi Kuma, Manami Tateno, Keiri Imai, Maki Owada, Graduate School of Env. Earth Science Hokkaido University, Graduate School of Fisheries Science Hokkaido University, Graduate School of Fisheries Science Hokkaido University, Graduate School of Fisheries Science Hokkaido University, Graduate School of Fisheries Science Hokkaido University, Graduate School of Fisheries Science Hokkaido University, Faculty of Fisheries Hokkaido University, Faculty of Fisheries Hokkaido University, Faculty of Fisheries Hokkaido University
    Oceanography in Japan 日本海洋学会 24 (4) 147 - 169 0916-8362 2015 [Refereed][Not invited]
     

    Because the vertical homogeneous Bottom Water (BW) with N ～0 (N is the buoyancy frequency) is formed in the Japan Sea, the abyssal water always has a depth of N ～f (f is the inertial frequency) above the BW. In the present study, we propose the theoretical existence of buoyancy/inertial oscillation under the weak stratification with N ～f in the non-hydrostatic dynamics and term it "fN-Oscillation". Based on the dispersion relation of internal inertial waves in the limit of N→f, it is concluded that fN-Oscillation with lateral maximum group velocity fulfills the requirement of a finite spatial scale for a ratio of horizontal/vertical wavenumber k/m＝1/√ 3. In the idealized stratification of N=f, in particular, this oscillation is confined to the generation area because it contains zero group velocity. Testing of numerical model experiments, including a localized front under the stratification with N~f, suggests that some of the fN-Oscillation energy trapped in this front escapes as internal waves. We observed that the benthic front existed stably between the Yamato Basin BW (YBBW) and the Japan Basin BW (JBBW) and was the plausible origin of energetic upward-propagating internal waves. The benthic front has a notably sharp structure with a horizontal scale of several hundreds meters. In the vicinity of this front, the periodic variations of temperature and dissolved oxygen were confirmed according to the internal waves, with the wavenumber satisfying approximately k/m~1/√3. Such disturbances represent indirect evidence for the trapping fN-Oscillation.

  • 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. © 2014 The Oceanographic Society of Japan and Springer Japan.
  • Tsurushima Nobuo, Imai Keiri
    Journal of Advanced Marine Science and Technology Society 海洋理工学会 20 (1) 29 - 36 1341-2752 2014 [Refereed][Not invited]
     
    Dissolution of silica from glass bottle sometimes influences accuracy of precision analysis of sea water, e.g. density and pH. We assessed and developed inside-coated glass vials to avoid dissolution of silica from glass at least < 1 mM for seawater sample storage. Seawater storage experiments using specially-treated vials available commercially (sulfur treatment, coating with fluoloresin or inorganic-organic hybrid material or silicone) suggests that dissolution of silica decreases by up to 60–70% related to that in uncoated glass vial at 25–40°C. As a result of storage experiments using test product of thick fluororesin coated vials, we found that dissolution of silica from glass can be avoided by repeating coating process (application and calcination) at least 10 times. We made trial products of thick fluororesin coated vials by this coating process. The results of seawater storage experiments showed that this newly developed vials can avoid dissolution of silica from glass surface at least 163 days at 25–40°C.
  • 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 2009/12/15 [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. © 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 2009/12/15 [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 m) 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. © 2009 Elsevier Ltd. All rights reserved.
  • Takafumi Aramaki, Yukihiro Nojiri, Keiri Imai
    Deep-Sea Research Part II: Topical Studies in Oceanography 56 (26) 2875 - 2888 0967-0645 2009/12/15 [Refereed][Not invited]
     
    During two mesoscale iron-enrichment studies in the northwestern subarctic Pacific (SEEDS in 2001 summer and SEEDS II in 2004 summer), particulate materials from the iron-induced phytoplankton bloom in the upper water column were monitored to analyze the export processes beneath the upper mixed layer, mainly with drifting sediment traps. We could not observe the total downward export process of the high accumulation of particulate organic carbon from the mixed layer induced by the large diatom bloom of SEEDS [e.g., Tsuda, A., Takeda, S., Saito, H., Nishioka, J., Nojiri, Y., Kudo, I., Kiyosawa, H., Shiomoto, A., Imai, K., Ono, T., Shimamoto, A., Tsumune, D., Yoshimura, T., Aono, T., Hinuma, A., Kinugasa, M., Suzuki, K., Sohrin, Y., Noiri, Y., Tani, H., Deguchi, Y., Tsurushima, N., Ogawa, H., Fukami, K., Kuma, K., Saino, T., 2003. A mesoscale iron enrichment in the western subarctic Pacific induces large centric diatom bloom. Science 300, 958-961] because the 2-week observation period was too short to examine the decline phase of the bloom. In contrast, in SEEDS II, the particulate organic carbon and particulate organic nitrogen were accumulated 123 and 23 mmol m-2, respectively, in the mixed layer until day-15 (days from iron-enrichment), and then ca. 90% were removed from the mixed layer by day-25. The sediment traps at 40 m depth between day-15 and day-25 accounted for at least more than 35% of these particles. There was no large variation in chemical composition in settling particles above 100 m depth throughout the experimental periods both in SEEDS and SEEDS II. The content of biogenic opal remained more than 50% of all settling particles during SEEDS, while the content of biogenic calcium carbonate was relatively high, with a low biogenic opal content of consistently less than 30% during SEEDS II. These results suggest that high standing stock of seed population of diatoms before the iron fertilization, indicated by low C/Si ratio of particulate matter, is an important factor to induce the large diatom bloom in SEEDS. © 2009 Elsevier Ltd. All rights reserved.
  • 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 Nakanishi, 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 [Refereed][Not invited]
     
    A 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 (∼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 Fv/ Fm 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 μm) became dominant in the chlorophyll-a size distribution after the bloom. We observed a nitrate drawdown of 3.8 μM 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. © The Oceanographic Society of Japan/TERRAPUB/Springer 2007.
  • Chika Komuro, Hisashi Narita, Keiri Imai, Yukihiro Nojiri, Richard W. Jordan
    Deep-Sea Research Part II: Topical Studies in Oceanography 52 (16-18) 2206 - 2217 0967-0645 2005/08 [Refereed][Not invited]
     
    The microplankton communities on filter samples from three shallow (0-300 m) hydrocasts, conducted in August 1999, January 2000, and May 2000 at Station KNOT in the Western Subarctic Gyre of the NW Pacific, have been analyzed by scanning electron microscopy (SEM). The results clearly show that small (<10 μm) phytoplankton, such as coccolithophorids, Parmales, and the pennate diatom Fragilariopsis pseudonana, play important roles in seasonal productivity. The Parmales were the most dominant mineralized phytoplankton group in May and August, and their year-round presence in scaly form suggests they are not cysts as previously thought. As the Parmales are rare or absent in the underlying sediment traps or surface sediments, their siliceous scales are undoubtably being efficiently recycled in the euphotic zone where silica levels are never depleted. The phytoplankton community structure observed by SEM is different to those reported previously, in which more traditional light microscopical techniques were used, with the consequence of missing the smaller phytoplankton. This study, regardless of its biogeographic location, highlights the potential problem of enumerating marine phytoplankton assemblages purely by light microscopy. An inability to determine the contribution made by these small phytoplankton can lead to an overestimation of the importance of large phytoplankton, and an underestimation of total productivity. © 2005 Elsevier Ltd. All rights reserved.
  • Neelam Ramaiah, Shigenobu Takeda, Ken Furuya, Takeshi Yoshimura, Jun Nishioka, Tatsuo Aono, Yukihiro Nojiri, Keiri Imai, Isao Kudo, Hiroaki Saito, Atsushi 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 μg L-1 and 40-60 μg XG equiv. L-1, respectively. Inside the patch, Chl a increased drastically from day 7 reaching the peak of 19.2 μg L-1 on day 13, which coincided with the TEP peak of 189 μg 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.
  • Tatsuo Aono, Masatoshi Yamada, Isao Kudo, Keiri Imai, Yukihiro Nojiri, Atsushi Tsuda
    Progress in Oceanography 64 (2-4) 263 - 282 0079-6611 2005 [Refereed][Not invited]
     
    The Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study (SEEDS) was carried out in the western subarctic gyre of the North Pacific to verify the iron-limitation hypothesis. Activities of dissolved 234Th in the upper 200 m of the water column and of particulate 234Th collected in a drifting sediment trap were measured during SEEDS 2001. Dissolved 234Th activities decreased and suspended particulate 234Th increased in the upper 20 m of the water column with the formation of an iron-induced phytoplankton bloom. The calculated residence times of the dissolved and particulate 234Th in the upper 20 m of the water column ranged from 2 to 13 d and 13 to 29 d, respectively. The ratios of particulate organic carbon (POC)/234Thp (μmol dpm-1) in the settling particles at 200 m depth varied between 1.94 and 4.02, and these levels were 10% those for the suspended particles. No large increase of the POC export flux was observed for the drifting sediment trap deployments during the observation period. In addition, the POC export fluxes derived from 234Th with a non-steady-state model varied from 10.1 to 38.3 mmol m-2 d-1. The ratio of the estimated POC export flux to primary production, ThE, was nearly constant (0.23-0.24) from July 20 to 27 (D2 and D9 after iron-enrichment) and decreased to 0.18 on July 31 (D13 after iron-enrichment). Regardless of whether the POC export fluxes increased, there was no change in ThE. The increase of the POC export fluxes estimated from 234Th was much smaller than the increase of primary production. Thus, no large POC export fluxes are concluded to have occurred from the iron-enrichment patch, though the removal flux of 234Th in settling particles increased with particulate biogenic silica fluxes as the phytoplankton bloom progressed. © 2005 Elsevier Ltd. All rights reserved.
  • Atsushi Tsuda, Hiroshi Kiyosawa, Akira Kuwata, Mamiko Mochizuki, Naonobu Shiga, Hiroaki Saito, Sanae Chiba, Keiri Imai, Jun Nishioka, Tsuneo 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.
  • Isao Kudo, Yoshifumi Noiri, Keiri Imai, Yukihiro Nojiri, Jun Nishioka, Atsushi 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 13C and 15N 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 μM, 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 μM 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 μM d-1 was observed on Day 9. Total nitrate uptake at 5 m between Days 4 and 9 was 10.8 μM, which was comparable to the net decrease of nitrate at 8.5 μM at this depth. Uptake rate of ammonium was 0.2 μM 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 m decreased from 400 to <100 μmol 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. Odate, K. Imai
    Journal of Plankton Research 25 (12) 1497 - 1505 0142-7873 2003/12 [Refereed][Not invited]
     
    Chlorophyll-specific growth (μ) and grazing mortality rates (g) due to microzooplankton (grazers of <200 μm) were estimated, based on the dilution method, in surface waters collected at four stations in Funka Bay, Japan, from December 1995 to March 1997. From May to October, the mean μ and g were 0.66 ± 0.44 day-1 (n = 21) and 0.60 ± 0.31 day-1 (n = 21), respectively. The estimated chlorophyll variation was 0.02 ± 0.31 μg l-1 day-1 (n = 21), which is consistent with field observations in this bay. On the other hand, μ was 0. 51 ± 0.37 day-1 (n = 4) during the spring bloom (February and March). There was no significant difference between the mean μ in summer and in the spring bloom (P > 0.05). However, g during the bloom (0.15 ± 0.09 day-1) (n = 4) was significantly lower than in summer (P < 0.01). These results suggest that the low g was one of the factors contributing to the development of the spring bloom. On a seasonal basis, it was estimated that 83 and 39% of phytoplankton production were grazed by microzooplankton in summer and winter-spring seasons, respectively. The seasonal variation in the microzooplankton gracing pressure seems to result from the dominant size class of the phytoplankton community of this bay, i.e. picophytoplankton (<2 μm) predominate in summer, while phytoplankton >10 μm prevail in winter-spring.
  • Nojiri Y, Imai K, Saito H, Tsuda A, Whitney F
    Geochimica Et Cosmochimica Acta 67 (18) A342  0016-7037 2003/09 [Refereed][Not invited]
  • Atsushi Tsuda, Shigenobu Takeda, Hiroaki Saito, Jun Nishioka, Yukihiro Nojiri, Isao Kudo, Hiroshi Kiyosawa, Akihiro Shiomoto, Keiri Imai, Tsuneo Ono, Akifumi Shimamoto, Daisuke Tsumune, Takeshi Yoshimura, Tatsuo Aono, Akira Hinuma, Masatoshi Kinugasa, Koji Suzuki, Yoshiki Sohrin, Yoshifumi Noiri, Heihachiro Tani, Yuji Deguchi, Nobuo Tsurushima, Hiroshi Ogawa, Kimio Fukami, Kenshi Kuma, Toshiro Saino
    Science 300 (5621) 958 - 961 0036-8075 2003/05/09 [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.
  • KNOT: Ocean time-series program in western North Pacific completes first phase
    Tsurushima N, Imai K, Nojiri Y, Murphy P.P
    U. S. JGOFS NEWS 11 (4) 11 - 13 2002 [Not refereed][Not invited]
  • K Mizobata, SI Saitoh, A Shiomoto, T Miyamura, N Shiga, K Imai, M Toratani, Y Kajiwara, K Sasaoka
    PROGRESS IN OCEANOGRAPHY 55 (1-2) 65 - 75 0079-6611 2002 [Refereed][Not invited]
     
    Using satellite altimeter and ship data, Bering Sea cyclonic and anticyclonic eddies were observed in summer 2000 and 2001 to examine their biological, chemical and physical structures. Results from the ship transect revealed the interactions between the physical and biological conditions of Bering Sea eddies. At the center of a cyclonic (anticlockwise) eddy, upwelling was transporting nutrient (NO3 + NO2) rich water (>25 muM) to the surface, which resulted in relatively high chlorophyll a concentrations (>1.0 mg m(-3)) developing under the pycnocline. In contrast, in the center of an anticyclonic (clockwise) eddy there was downwelling. This downwelling of surface warm water was destroying a cold layer (at about 150 m depth) caused by winter convection. However, around the periphery of the anticyclonic eddy the isopycnals were tilted up and nutrient-rich water was being transported along with them up into the euphotic zone, so that high chlorophyll a concentrations were being developed above the pycnocline inside the anticyclonic eddy. (C) 2002 Elsevier Science Ltd. All rights reserved.
  • A Shiomoto, S Saitoh, K Imai, M Toratani, Y Ishida, K Sasaoka
    PROGRESS IN OCEANOGRAPHY 55 (1-2) 147 - 163 0079-6611 2002 [Refereed][Not invited]
     
    Surface chlorophyll a concentrations were measured at seven stations located at 1degrees latitude intervals between 52degrees30' N and 58degrees30' N along longitude 179degrees30' W, in late June and early July from 1991 through 1999. Surface chlorophyll a concentrations at the same locations were estimated from Sea-viewing Wide Field-of-view Sensor (SeaWiFS) data from mid June through mid July 2000. Shipboard data collected from 1993-95 showed that the surface chlorophyll a concentration was correlated with chlorophyll a standing stock integrated in the upper 100 m of the water column. Based on the 8-day time series of mean chlorophyll a concentrations at stations on 179degrees30' W in 1998, 1999, and 2000 from SeaWiFS data, high concentration and great variability were observed in chlorophyll a in June. When examining interannual differences in phytoplankton biomass, it is preferable to use a month when high chlorophyll a concentrations are high, and show great variability than one when its concentrations are low with little variability. Thus a comparison of surface data from shipboard and satellite observations in June, was considered best to represent the interannual variation in phytoplankton biomass in the Bering Sea basin in the 1990s. There were no significant differences in chlorophyll a concentrations among years (1991-2000) or among stations, though high chlorophyll a concentrations were observed occasionally around the shelf edge. Chlorophyll a concentrations rarely exceeded 2 mug 1(-1) in the Bering Sea basin and the values were usually < 1 mug 1(-1). The results suggest that phytoplankton biomass in the basin is relatively stable and rather low; in other words, the Bering Sea basin may be considered to be a high nitrate, low chlorophyll (HNLC) region. (C) 2002 Elsevier Science Ltd. All rights reserved.
  • K Sasaoka, S Saitoh, Asanuma, I, K Imai, M Honda, Y Nojiri, T Saino
    DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 49 (24-25) 5557 - 5576 0967-0645 2002 [Refereed][Not invited]
     
    The objectives of this study are to describe and understand the processes controlling the temporal and spatial variability of chlorophyll-a (chl-a) in the subarctic northwestern Pacific Ocean from 1997 to 1999. Remotely sensed data from multi sensors, including ocean color (OCTS and SeaWiFS), sea-surface temperature (SST, AVHRR), and wind (SSM/I) datasets were utilized for the purpose of this study. Ocean-color imagery clearly showed seasonal and interannual variability in the spatial abundance and distribution of chl-a in the study area. Chl-a concentrations were generally low (0.53 +/- 0.24 mg m(-3)) for most part of the year, except for a few peaks (1.0-2.0 mg m(-3)) observed in the spring and fall bloom seasons (May, June, September, and October). Chl-a concentrations (> 10 mg m(-3)) were consistently high along the Kuril Islands and in the coastal waters around the Kamchatka Peninsula, and in 1998 they were clearly higher than in 1999. The Western Subarctic Gyre (WSG) was characterized by positive sea-surface temperature anomalies (SSTA) during the summer to fall of 1998. These anomalies appear to be a high-latitude response to 1997/1998 ENSO event. High concentrations of chl-a appeared in WSG in September-November only in 1998. We suggest that the high chl-a around the WSG from summer to fall in 1998 was facilitated by (1) negative SSTA in winter, in spite of the negative wind anomaly, which provided larger amount of nutrients to the sea surface, (2) positive wind anomaly from April to June, which might cause light limitation of the phytoplankton growth, due to deepening of the surface-mixed layer in summer, and (3) positive SSTA, even with the slightly positive wind anomaly, which was accompanied with the water-column stabilization and hence recovered light limitation of the phytoplankton. Warmer SST also might have enhanced the phytoplankton growth. During summer to fall in 1999, the phytoplankton biomass between 42degreesN and 43degreesN along 165degreesE was greater than in other years. The enhanced chl-a concentration coincided with a distinct frontal temperature gradient located between 40degreesN and 45degreesN in September 1998 and 1999. The peak chl-a concentration in September 1999 (> 2.0 mg m(-3)) within the temperature front was much larger than in L998 (about 0.7 mg m(-3)). The position of this front in September 1999 (41degreesN) was south of its position in 1998 (43degreesN). Convergence of chl-a is linked to the temperature gradient and southward expansion of well-mixed subarctic water that is cold, low in salinity, and nutrient-rich. Our results showed significant interannual biological variability, and illustrate how remote sensing with multi sensors can aid in monitoring relationships between local ecosystems and global climate change. (C) 2002 Elsevier Science Ltd. All rights reserved.
  • MC Honda, K Imai, Y Nojiri, F Hoshi, T Sugawara, M Kusakabe
    DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 49 (24-25) 5595 - 5625 0967-0645 2002 [Refereed][Not invited]
     
    To characterize the export flux of biogenic and lithogenic materials to the ocean interior and evaluate the ability of the biological pump to take up atmospheric CO2 in the northwestern North Pacific, sediment trap experiments were conducted at three stations (station KNOT: 44degreesN, 155degreesE; 50N: 50degreesN, 165degreesE; station 40N: 40degreesN, 165degreesE). The export fluxes showed seasonal variability with a high flux period in summer at station KNOT and in spring and autumn at stations 50N and 40N. The settling particles were mostly biogenic opal, comprising 50% of the total mass flux. The annual average export flux in 1998 was smaller than that in 1999 at the three stations, perhaps a result of the calm 1997/ 1998 winter associated with an El Ni (n) over tildeo event. Station KNOT had the largest total mass, organic carbon, and opal fluxes and the largest mole ratios of opal to carbonate (opal/CaCO3(mole)) and organic carbon to inorganic carbon (Co/Ci) compared with those at other two stations. These fluxes and ratios in the western, central and eastern parts of the northern North Pacific tended to decrease eastward. It is likely that the intensity of winter mixing or the supply of macro- and micronutrients influences the biogeochemistry of the northern North Pacific. Compared to the export flux of biogenic materials previously observed in the world ocean, the northwestern North Pacific, including the Bering Sea, has high opal/CaCO3(mole) and Co/Ci ratios. Moreover, at station KNOT, the ratio of the organic carbon flux in the deep sea to surface primary productivity (transfer efficiency: TE) was estimated to be approximately 5%, 3%, and 2% at 1000, 3000, and 5000 m, respectively. These values were significantly higher than TE values reported previously in the other oceanic regions. The high Co/Ci ratio and TE lead us to conclude that the biological pump in the northwestern North Pacific works more efficiently to decrease pCO(2) in the surface seawater and, consequently, to enhance the uptake of atmospheric CO2 by the ocean. (C) 2002 Elsevier Science Ltd. All rights reserved.
  • M Mochizuki, N Shiga, M Saito, K Imai, Y Nojiri
    DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 49 (24-25) 5421 - 5439 0967-0645 2002 [Refereed][Not invited]
     
    The standing stock and, species composition of phytoplankton (> 10 mum) were studied using monthly water samples collected at Stn KNOT (Kyodo North Pacific Ocean Time Series; 44degreesN, 155degreesE) in the western subarctic gyre in the Pacific Ocean through June 1998 to June 1999 (except for January-April 1999) and January-February 2000. One-liter water samples were preserved in 1% neutrally buffered formalin. Identification and enumeration of phytoplankton were made with an inverted microscope. Nutrients did not appear to be depleted for phytoplakton growth during any season. The vertical distribution of phytoplankton was primarily restricted by the pycnocline, and the bulk of phytoplankton assemblage existed within the surface mixed layer. In July, however, some senescent cells were observed at 200-m. Phytoplankton abundance clearly showed a spring maximum (i.e. spring bloom) in May. The seasonal change in cell numbers, however, did not coincide closely with the change in chlorophyll a concentration. Centric diatoms, which were composed of Thalassiosira, Chaetoceros, and Coscinodiscus, dominated all year round, and showed temporal succession. Pennate diatoms (mostly Neodenticula seminae and Fragilariopsis) increased only during the spring bloom. Dinoflagellates (mostly Gymnodinium and Prorocentrum) were low in abundance, although they increased in summer when the phytoplankton standing stock was low. Silicotlagellate abundance was extremely low. Comparing the annual species composition of phytoplankton between Stn KNOT and Stu P (50degreesN, 145degreesW) in the Alaskan Gyre, there was a remarkable difference between the two sites. The phytoplankton assemblage at Stu P is characterized by a high abundance of Rhizosolenia alata and low abundance of Thalassiosira. In contrast, Thalassiosira dominates at Stu KNOT during most seasons. (C) 2002 Elsevier Science Ltd. All rights reserved.
  • HB Liu, K Imai, K Suzuki, Y Nojiri, N Tsurushima, T Saino
    DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 49 (24-25) 5409 - 5420 0967-0645 2002 [Refereed][Not invited]
     
    Picoplankton community structure in the upper 200 m at Station KNOT in the western subarctic North Pacific was studied by flow cytometric analysis of the time-series samples collected from June 1998 through June 2000. Phototrophic picoplankton, Synechococcus and picoeukaryotes, displayed a clear seasonal cycle in population abundance during the 2-yr period, where chlorophyll a concentration remained relatively low and constant throughout the year, except in spring when the diatom bloom occurred. High picoplankton abundance occurred after the spring diatom bloom when the water column became more stratified. The maximum abundance of Synechococcus occurred in late June to early August, whereas the peak in picoeukaryotes abundance appeared to take place earlier in May and remained high through the summer. Heterotrophic bacteria abundance also displayed a seasonal variation pattern that was closely related to the picoplankton biomass, but not total chlorophyll biomass. (C) 2002 Elsevier Science Ltd. All rights reserved.
  • Nobuo Tsurushima, Yukihiro Nojiri, Keiri Imai, Shuichi Watanabe
    Deep-Sea Research Part II: Topical Studies in Oceanography 49 (24-25) 5377 - 5394 0967-0645 2002 [Refereed][Not invited]
     
    Dissolved inorganic carbon (DIC) and related chemical species have been measured since June 1998 at the western subarctic time-series station KNOT (44°N, 155°E). Seasonal changes in hydrography, concentrations of nutrients and DIC, and total alkalinity (TAlk) were observed above the pycnocline, which exists at 150-300m. DIC in the surface mixed layer showed a maximum in February 2000 (2093 μmol kg-1) and minima in August 1998 and 1999 (1980 and 1987 μmol kg-1, respectively). The seasonal amplitude of DIC was 107 μmol kg-1 in the 1999-2000 time series, which is larger than those at other pelagic ocean time-series sites. This large variation is due mainly to biological production in spring to fall and strong vertical mixing in winter. Surface fugacity of CO2 (fCO2) ranged from 300 to 390 μatm in the 1999-2000 time series, generally decreasing in spring and summer and increasing in fall and winter. The seasonal change in fCO2 was controlled mainly by the large variation in surface DIC. The invasion of atmospheric CO2 into the surface seawater continues from June to January, showing a maximum in October. The decrease in carbon in summer was explained by biological uptake on the basis of Redfield stoichiometry and air-sea CO2 exchange. Net community production in the surface mixed layer estimated from the change in nutrients ranged from 250 to 600 mg C m-2 day-1 and was higher in spring. © 2002 Elsevier Science Ltd. All rights reserved.
  • Keiri Imai, Yukihiro Nojiri, Nobuo Tsurushima, Toshiro Saino
    Deep-Sea Research Part II: Topical Studies in Oceanography 49 (24-25) 5395 - 5408 0967-0645 2002 [Refereed][Not invited]
     
    Phytoplankton biomass and primary productivity were measured as a part of the station KNOT (44°N, 155°E) time-series observation program in the western North Pacific, which began in June 1998 and continued on a nearly monthly basis until February 2000. Primary productivity was measured by the 13C uptake method using a clean sampling technique. The surface temperature changed from 2°C to 15°C, and the minimum and maximum temperatures were in February and August, respectively. The minimum and maximum concentrations of surface-layer nutrients were 2.2 and 19.6 μM for nitrate + nitrite, 0.32 and 1.64 μM for phosphate, and 3.3 and 33.3 μM for silicate. The averaged depth of euphotic zone was about 50 m, which did not show clear seasonality. Integrated chlorophyll a in the euphotic zone ranged between 20 and 40 mg Chl a m-2, except for the high values (51-64 mg Chl a m-2) in early June and middle October 1998 and May 1999, and the low value (17 mg Chl a m-2) in late June 1998. The integrated primary productivities in the upper part of the euphotic zone for spring (May), summer (June-August), autumn (October-November), and winter (December-February) were 517-535 (av. 526), 153-304 (av. 227), 101-307 (av. 173), and 25-121 (av. 48) mg C m-2 d-1, respectively. The annual productivity was estimated to be 90 g C m-2 yr-1. The results showed that primary productivity had a distinct seasonal variation, with a 10-fold range during winter-spring, which is far larger than that at Ocean Station Papa (OSP) in the eastern North Pacific. However, the annual productivity at station KNOT was lower than the at OSP. It was found that the light utilization index was constant during all seasons, and the annual average (± 1 SD) was 0.30±0.10 g C (g Chl a)-1 m2 (mol quanta)-1. © 2002 Elsevier Science Ltd. All rights reserved.
  • A possible explanation for the dominance of chlorophyll in pico and nano-size fractions in the waters around the south Shetland Islands
    Kawaguchi S, Shiomoto A, Imai K, Tsuruga Y, Yamaguchi H, Noiri Y, Iguch N, Kameda T
    Ocean and Polar Research 23 (4) 379 - 388 2001 [Refereed][Not invited]
  • ODATE Tsuneo, SAKAOKA Keiitiro, KAJIWARA Yoshiyuki, IMAI Keiri, KOBAYASHI Naoto, MEGURO Toshimi, FUKUTI Mitsuo
    Bulletin of the Plankton Society of Japan 日本プランクトン学会 46 (2) 134 - 142 0387-8961 1999/08/25 [Refereed][Not invited]
  • IINUMA Tomoaki, IMAI Keiri, ODATE Tsuneo, MAITA Yoshiaki
    水産海洋研究 62 (4) 406 - 407 0388-9149 1998/11/01
  • Iinuma Tomoaki, Imai Keiri, Odate Tsuneo, Maita Yoshiaki
    MEMOIRS OF THE FACULTY OF FISHERIES HOKKAIDO UNIVERSITY 北海道大学 45 (1) 30 - 35 0018-3466 1998/09 [Not refereed][Not invited]
     
    Ⅰ. Physical, Chemical Environment, Primary Production, Zooplankton and Their Coupling Model Studies
  • IMAI Keiri, ODATE Tsuneo, YANADA Mitsuru, MAITA Yoshiaki
    Bulletin of the Japanese Society of Fisheries Oceanography 水産海洋学会 62 (2) 89 - 96 0388-9149 1998/05/01 [Refereed][Not invited]
  • A Shiomoto, S Kawaguchi, K Imai, Y Tsuruga
    POLAR BIOLOGY 19 (5) 361 - 364 0722-4060 1998/05 [Refereed][Not invited]
     
    Size-fractionated chlorophyll a (Chla)-specific productivity (mu gC mu gChla(-1) h(-1)) was measured at 11 stations off the northern coast of the South Shetland Islands during summer. The Chla-specific productivity of the 2- to 10 or 10- to 330-mu m fraction was highest at 100% and 23% light depths. The Chla-specific productivity of the 2- to 10-mu m fraction was generally highest, and that of the < 2 or 10- to 330-mu m fraction was sometimes highest at 12% and 1% light depths. Temperature was less than 3 degrees C within the euphotic zone at all stations. The hypothesis of Shiomoto et al., according to which Chin-specific productivity of picophytoplankton (< 2 mu m) is not significantly higher than that of larger phytoplankton (> 2 mu m) in water colder than 10 degrees C, was supported on condition that light is not limited for larger phytoplankton.
  • Imai Keiri, Yanada Mitsuru, Maita Yoshiaki
    Bulletin of the Faculty of Fisheries,Hokkaido University 北海道大学水産学部 49 (1) 41 - 49 0018-3458 1998/03 [Not refereed][Not invited]
  • ODATE Tsuneo, IMAI Keiri, FUKUCHI Mitsuo
    Bulletin of the Plankton Society of Japan 日本プランクトン学会 45 (1) 1 - 8 0387-8961 1998/02/28 [Refereed][Not invited]

MISC

Association Memberships

  • THE OCEANOGRAPHIC SOCIETY OF JAPAN   

Research Projects

  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (C)
    Date (from‐to) : 2021/04 -2024/03 
    Author : 芳村 毅, 工藤 勲, 今井 圭理
     
    海洋によるCO2の吸収・固定量は植物プランクトンによる炭素と栄養素の利用比率によって変化します。その比率としてレッドフィールド比が良く知られており，気候変化を予測する生態系モデルにおいて固定値として使われていますが，実測データの蓄積は十分ではありません。また，プランクトン群集はサイズの異なる多くの種で形成されますが，それらをひとまとめにした比率しか議論されてきませんでした。海洋環境の変化によりプランクトン群集組成が変化すると予測されていますが，その変化が生態系に与える影響を評価できません。そこで，本研究は有機物粒子のサイズ分画手法を確立したうえで，各サイズ画分の主要4元素組成比（炭素：窒素：リン：ケイ素）に関するデータを取得します。 研究一年目の2021年度は海水中の有機物粒子をサイズ毎に分画するための手法を検討しました。撹拌式セルを用いた限外ろ過システムを応用した「限外ろ過方式」と，孔径の異なるフィルターに連続的に通過させる「逐次ろ過方式」を並行して検討しました。その結果，限外ろ過方式はフィルター上に粒子が捕捉されてしまい，有機物粒子量を過小評価することがわかりました。有機物粒子のサイズ分画手法としては逐次ろ過方式が適していることがわかりました。逐次ろ過方式はリンおよびケイ素の分析試料を作成するのに適していますが，炭素および窒素を分析するためにはフィルター上に集めた有機物粒子を液体に懸濁させて回収する必要があります。フィルターに付着する有機物粒子を完全に回収する手法を検討した結果，超音波洗浄を利用することで達成できることがわかりました。これらの検討により，孔径の異なるメンブレンフィルターを用いた逐次ろ過方式によって有機物粒子を三つにサイズ分画し，超音波洗浄を活用して回収した有機物粒子の主要4元素を分析することで，各サイズ画分の元素組成比を分析するための道筋が完成しました。
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research
    Date (from‐to) : 2019/04 -2022/03 
    Author : Imai Keiri
     
    We aimed to develop a parameterization of pH (pH = f (salinity, oxygen, chlorophyll-a, etc.)) that can be applied to the Japanese coastal regions to predict the temporal and spatial changes and magnitude of ocean acidification in the coastal waters of Japan. Since a large amount of pH data is required to understand the dynamics of ocean acidification in coastal waters with large environmental variability, we improved the surface seawater monitoring system equipped with the Oshoro Maru of Hokkaido University, which makes several cruises per year to the Japanese coastal regions (off the coast of Hokkaido and Tohoku), to enable continuous multi-item data acquisition. The system has been improved to enable continuous data acquisition of multiple items.
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research on Innovative Areas (Research in a proposed research area)
    Date (from‐to) : 2012/06 -2017/03 
    Author : Kanda Jota, MORIYA Shigeharu, KOBAYASHI Takuya, YOSHIMURA Takeshi, ISHIMARU Takashi, IGARASHI Satoshi, SUGA Jiro, YAMAKAWA Hiroshi, OTSU Hideaki, ITO Yukari, TAKAZAWA Nobue, UCHIYAMA Kaori, KUBO Atshushi
     
    Radioactive material derived from the Fukushima Daiichi Nuclear Power Plant accident was investigated in marine ecosystem. 1) For radioactive cesium in marine organisms and environment in the coastal area of Fukushima, we obtained a continuous time-series data set by ship observations twice a year. Time-series data set for radioactive cesium in zooplankton was also obtained in several open ocean areas. 2) Examinations of transfer routes of radioactive cesium in coastal rocky seafloor revealed that transition of radioactive cesium from suspended or descending particles, and from organic fraction of sediments was small. 3) Using the on-site observation data, the transition of radioactive cesium in the ecosystem was reproduced by a numerical model, and the model was applied to prediction of the future trend of the radioactivity level of marine organisms.

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