研究者データベース

山下 洋平(ヤマシタ ヨウヘイ)
地球環境科学研究院 地球圏科学部門 環境変動解析学分野
准教授

基本情報

所属

  • 地球環境科学研究院 地球圏科学部門 環境変動解析学分野

職名

  • 准教授

学位

  • 博士(理学)(名古屋大学)
  • 修士(理学)(名古屋大学)

ホームページURL

科研費研究者番号

  • 50432224

ORCID ID

J-Global ID

研究キーワード

  • 溶存有機物   水圏   生物地球化学   Dissolved organic Matter   Aquatic Environments   Biogeochemistry   

研究分野

  • 環境・農学 / 環境動態解析
  • 自然科学一般 / 宇宙惑星科学

職歴

  • 2009年12月 - 現在 北海道大学 大学院地球環境科学研究院 准教授
  • 2007年 - 2009年 Southeast Environmental Research Center, Florida International University 研究員
  • 2007年 - 2009年 Researcher,Southeast Environmental Research Center, Florida International University
  • 2006年 - 2007年 名古屋大学 大学院環境学研究科 研究員
  • 2006年 - 2007年 Researcher,Graduate School of Environmental Studies, Nagoya University
  • 2004年 - 2006年 名古屋大学 大学院環境学研究科 日本学術振興会特別研究員
  • 2004年 - 2006年 Postdoctoral Fellowships of Japan Society for the Promotion of Science,Graduate School of Environmental Studies, Nagoya University

学歴

  •         - 2004年   名古屋大学   環境学研究科   地球環境科学専攻
  •         - 2002年   名古屋大学   理学研究科   地球惑星理学専攻
  •         - 2000年   明治大学   農学部   農芸化学科

所属学協会

  • 日本有機地球化学会   日本地球化学会   日本海洋学会   The Japanese Association of Organic Geochemist   The Geochemical Society of Japan   The Oceanographic Society of Japan   

研究活動情報

論文

  • 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年05月
  • 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 2021年11月
  • Toru Hirawake, Joji Oida, Youhei Yamashita, Hisatomo Waga, Hiroto Abe, Jun Nishioka, Daiki Nomura, Hiromichi Ueno, Atsushi Ooki
    Progress in Oceanography 197 102641 - 102641 2021年09月
  • Toru Hirawake, Joji Oida, Youhei Yamashita, Hisatomo Waga, Hiroto Abe, Jun Nishioka, Daiki Nomura, Hiromichi Ueno, Atsushi Ooki
    Progress in Oceanography 197 2021年09月01日 
    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.
  • Jun Nishioka, Hajime Obata, Toru Hirawake, Yoshiko Kondo, Youhei Yamashita, Kazuhiro Misumi, Ichiro Yasuda
    Journal of Oceanography 77 4 561 - 587 2021年08月13日 [査読有り][通常論文]
     
    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.
  • Daiki Nomura, Hiroto Abe, Toru Hirawake, Atsushi Ooki, Youhei Yamashita, Aiko Murayama, Kazuya Ono, Jun Nishioka
    PROGRESS IN OCEANOGRAPHY 196 2021年08月 [査読有り][通常論文]
     
    We analyzed temperature and salinity data and oxygen isotope ratios (818O) 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 818O displayed a strong linear relationship in samples upstream (southwest) of the Gulf of Anadyr. However, in bottom water (>26.5 00) 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 818O 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 818O. 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.
  • Shinya Takano, Youhei Yamashita, Shunsuke Tei, Maochang Liang, Ryo Shingubara, Tomoki Morozumi, Trofim C. Maximov, Atsuko Sugimoto
    FRONTIERS IN EARTH SCIENCE 9 2021年07月 [査読有り][招待有り]
     
    Arctic tundra wetlands may be an important source of dissolved organic carbon (DOC) in Arctic rivers and the Arctic Ocean under global warming. We investigated stable water isotopes and DOC concentration in wetlands, tributaries, and the mainstream at the lower reaches of the Indigirka River in northeastern Siberia during the summers of 2010-2014 to assess the complex hydrology and role of wetlands as sources of riverine DOC. The wetlands had higher values of delta O-18 and DOC concentration than the tributaries and mainstream of the Indigirka River. A relationship between the two parameters was observed in the wetlands, tributaries, and mainstream, suggesting the wetlands can be a source of DOC for the mainstream through the tributaries. The combined temporal variations in riverine delta O-18 and DOC concentration indicate the mainstream water flowed into the tributaries during relatively high river-level periods in summer, whereas high DOC water in the downstream wetlands could be discharged to the mainstream through the tributaries during the low river-level periods. A minor fraction (7-13%) of riverine and wetland DOC was degraded during 40 days of dark incubation. Overall, the downstream wetlands potentially provide relatively less biodegradable DOC to the Arctic river and costal ecosystem during the low river-level periods-from late summer to autumn.
  • Johanna Sjostedt, Jean-Francois Lapierre, Youhei Yamashita, Federico Baltar
    FRONTIERS IN MICROBIOLOGY 12 2021年05月 [査読有り][通常論文]
  • Youhei Yamashita, Daiki Kojima, Natsumi Yoshida, Hideaki Shibata
    Chemosphere 271 129824 - 129824 2021年05月 [査読有り]
     
    Black carbon (BC) is a pyrolyzed product derived from incomplete combustion. A major fraction of BC produced by landscape fires is initially deposited onto onsite soils. Atmospheric deposition of soot is known to be an important source of soil BC, especially in watersheds that are not affected by landscape fires. The transport of the dissolved fraction of oxidized BC in soil, defined as dissolved black carbon (DBC), to streams is considered one of the important loss pathways of BC in soil, but the mechanism is not well documented. We measured the quantity and quality of DBC, determined by a benzenepolycarboxylic acid method, and the quantitative and qualitative parameters of bulk dissolved organic matter (DOM) in streams in Hokkaido, northern Japan, whose catchments were not affected by landscape fire for at least 110 years. DBC with relatively low polycondensed signatures occurred in the streams, irrespective of differences in watershed characteristics and seasons, suggesting that atmospheric deposition of soot into the catchment is probably a major source of stream DBC. The DBC concentration was linearly related to the dissolved organic carbon (DOC) concentration, irrespective of the differences in watershed characteristics and seasons. Furthermore, the polycondensation degree of DBC was observed to correlate with the qualitative parameters of bulk DOM. Such quantitative and qualitative relationships between DBC and bulk DOM imply that the transfer mechanism from soils to streams of soot-derived polycondensed DBC is linked with that of higher plant-derived, high-molecular-weight aromatic DOM.
  • Yutaro Mori, Jun Nishioka, Shinzou Fujio, Youhei Yamashita
    PROGRESS IN OCEANOGRAPHY 193 2021年04月 [査読有り]
     
    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.
  • 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 27 2021年03月 [査読有り][通常論文]
     
    Arctic ecosystems are altered profoundly by climate changes. However, the responses of Arctic marine and terrestrial ecosystems as well as their biodiversity to global warming remain largely unknown. This article provides comprehensive insights into the results and major findings from the Arctic Challenge for Sustainability (ArCS) Project ? an Arctic region research program initiated in Japan, which aims to address and advance our understanding of these uncertainties. Marine ecosystem studies have identified several biogeochemical processes that are associated with sea ice decline and northward transport and shift of marine species across multiple trophic levels over the Bering and Chukchi Sea shelves. Studies of the terrestrial ecosystem have identified factors that are important for the understanding of terrestrial biodiversity and ecosystems, including Arctic lakes, under the presence of global warming. Novel fungal species from the Arctic terrestrial ecosystem have also been isolated and described. Overall, these results could contribute to the conservation and sustainable management of the Arctic ecosystem services.
  • 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 2021年02月 [査読有り][通常論文]
     
    Visible wavelength fluorescent dissolved organic matter (FDOM), usually defined as humic-like FDOM, plays important roles in marine carbon and iron cycles as biorefractory dissolved organic matter and organic ligands, respectively. The major fractions of FDOM in the open ocean have been considered to be of marine origin (autochthonous) according to linear relationships between FDOM and apparent oxygen utilization (AOU) in the mesopelagic (200-1000 m) and bathypelagic (>1000 m) layers. Recently, Yamashita et al. (2020) quantified allochthonous FDOM from the positive deviation of the general FDOM-AOU relationship in the bathypelagic layer and found that allochthonous FDOM derived from the Sea of Okhotsk was conservatively transported to the western North Pacific through the circulation of intermediate water, including North Pacific Intermediate Water (NPIW). However, the contribution of allochthonous FDOM from the Bering Sea, the other source region of NPIW, has not been evaluated. Here, we determined the distributions of allochthonous FDOM in the entire North Pacific, including the Sea of Okhotsk and the Bering Sea, and the spatial distribution of allochthonous FDOM in the density range of NPIW for the entire North Pacific. We found that the major source region of allochthonous FDOM in NPIW was the Sea of Okhotsk but not the Bering Sea, although the water mass contributing to the lower part of NPIW is known to be mainly derived from the Bering Sea. Such different contributions of allochthonous FDOM from the marginal seas are likely due to a lack of expansion of the dense shelf water, which forms at coastal polynya and interacts with shelf sediments with a strong tidal current, to the basin region of the Bering Sea. We also demonstrated that allochthonous FDOM derived from the shelf sediments of the Sea of Okhotsk was conservatively distributed to the wide area of the North Pacific through the circulation of intermediate water, particularly the upper part of NPIW. Furthermore, the negative deviations of the FDOM-AOU relationship in the bathypelagic layer were evident for other water masses, such as Antarctic Intermediate Water (AAIW), suggesting that FDOM can help determine the spatiotemporal distribution of NPIW and its mixing with other water masses, such as AAIW.
  • Shuji Goto, Yuya Tada, Koji Suzuki, Youhei Yamashita
    FRONTIERS IN MICROBIOLOGY 11 2020年10月 [査読有り][招待有り]
     
    A large part of marine dissolved organic matter (DOM) is considered to be recalcitrant DOM (RDOM) produced by marine bacteria. However, it is still unclear whether differences in bacterial species and/or physiology control the efficiency of RDOM production. Here, batch culture experiments with glucose as the sole carbon source were carried out using three model marine bacterial strains, namely, Alteromonas macleodii (Alt), Vibrio splendidus (Vib), and Phaeobacter gallaeciensis (Pha). Dissolved organic carbon (DOC) concentrations drastically decreased during the exponential growth phases of these bacteria due to the consumption of glucose. The efficiency of bacterial DOC production at the end of incubation was largely different among the strains and was higher for Vib (20%) than for the other two strains (Alt, 4%; Pha, 6%). All strains produced fluorescent DOM (FDOM), including humic-like FDOM which is considered as recalcitrant component in the ocean, even though the composition of bacterial FDOM was also different among the strains. The efficiency of humic-like FDOM production during the exponential growth phase was different among the bacterial strains; that is, Pha produced humic-like FDOM efficiently compared with the other two species. The efficiency of humic-like FDOM production with mineralization of organic matter was lower during the exponential growth phase than during the stationary phase of Alt and Pha. Four processes for the production of bacterially derived recalcitrant humic-like FDOM are suggested from this study: (1) production during active growing (in all strains), (2) production with the reutilization of bacterial DOM (Alt), (3) production with the consumption of cellular materials (Pha), and (4) release from lysis (Vib). Our results suggest that bacterial species and physiology can regulate RDOM production and accumulation in the ocean.
  • Yuzo Miyazaki, Koji Suzuki, Eri Tachibana, Youhei Yamashita, Astrid Mueller, Kaori Kawana, Jun Nishioka
    SCIENTIFIC REPORTS 10 1 2020年10月 [査読有り][通常論文]
     
    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. 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.
  • 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 202000658  2020年05月27日 [査読有り][通常論文]
     
    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.
  • Youhei Yamashita, Jun Nishioka, Hajime Obata, Hiroshi Ogawa
    SCIENTIFIC REPORTS 10 1 2020年03月 [査読有り][通常論文]
     
    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.
  • Stanford B. Hooker, Atsushi Matsuoka, Raphael M. Kudela, Youhei Yamashita, Koji Suzuki, Henry F. Houskeeper
    Biogeosciences 17 2 475 - 497 2020年01月30日 [査読有り][通常論文]
     
    Abstract. This study establishes an optical inversion scheme for deriving the absorption coefficient of colored (or chromophoric, depending on the literature) dissolved organic material (CDOM) at the 440 nm wavelength, which can be applied to global water masses with near-equal efficacy. The approach uses a ratio of diffuse attenuation coefficient spectral end-members, i.e., a short- and long-wavelength pair. The global perspective is established by sampling “extremely” clear water plus a generalized extent in turbidity and optical properties that each span 3 decades of dynamic range. A unique data set was collected in oceanic, coastal, and inland waters (as shallow as 0.6 m) from the North Pacific Ocean, the Arctic Ocean, Hawaii, Japan, Puerto Rico, and the western coast of the United States. The data were partitioned using subjective categorizations to define a validation quality subset of conservative water masses (i.e., the inflow and outflow of properties constrain the range in the gradient of a constituent) plus 15 subcategories of more complex water masses that were not necessarily evolving conservatively. The dependence on optical complexity was confirmed with an objective methodology based on a cluster analysis technique. The latter defined five distinct classes with validation quality data present in all classes, but which also decreased in percent composition as a function of increasing class number and optical complexity. Four algorithms based on different validation quality end-members were validated with accuracies of 1.2 %–6.2 %, wherein pairs with the largest spectral span were most accurate. Although algorithm accuracy decreased with the inclusion of more subcategories containing nonconservative water masses, changes to the algorithm fit were small when a preponderance of subcategories were included. The high accuracy for all end-member algorithms was the result of data acquisition and data processing improvements, e.g., increased vertical sampling resolution to less than 1 mm (with pressure transducer precision of 0.03–0.08 mm) and a boundary constraint to mitigate wave-focusing effects, respectively. An independent evaluation with a historical database confirmed the consistency of the algorithmic approach and its application to quality assurance, e.g., to flag data outside expected ranges, identify suspect spectra, and objectively determine the in-water extrapolation interval by converging agreement for all applicable end-member algorithms. The legacy data exhibit degraded performance (as 44 % uncertainty) due to a lack of high-quality near-surface observations, especially for clear waters wherein wave-focusing effects are problematic. The novel optical approach allows the in situ estimation of an in-water constituent in keeping with the accuracy obtained in the laboratory.
  • Yamashita Youhei, Yagi Yuki, Ueno Hiromichi, Ooki Atsushi, Hirawake Toru
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 124 11 7545 - 7556 2019年11月11日 [査読有り][通常論文]
     
    ©2019. The Authors. Pacific water is an important nutrient source for sustaining biological production in the Chukchi Sea, western Arctic Ocean, which is one of the productive regions in the world. Therefore, to understand the impacts of future environmental changes on biological production in the sea, it is crucial to understand the origins, modification processes, and spatiotemporal variations of the water masses from the Bering Sea with changes in nutrient concentrations. To improve water mass analysis in the shelf regions of the Bering and Chukchi Seas, we observed levels of humic-like fluorescent organic matter (FOMH) by using an in situ fluorometer directly connected to a temperature-salinity sensor during a cruise in the early summer of 2013 and evaluated the potential of FOMH as a third parameter of water mass analysis. The levels of FOMH were different among specific water masses in the region, and FOMH seemed to behave semiconservatively in the shelf regions of the Bering and Chukchi Seas during the early summer of 2013. The distributional pattern of FOMH implies that FOMH estimated by the in situ fluorometer has the potential to (1) separate warm water into riverine-affected Alaskan Coastal Water and historically photobleached summer Bering Basin Water; (2) distinguish the Anadyr Water, which has low FOMH levels and high nutrient concentrations, from the Bering Shelf Water; and (3) determine different formation/modification processes of dense shelf water that contains high nutrient concentrations.
  • Lei Fujiyoshi, Atsuko Sugimoto, Youhei Yamashita, Xiaoyang Li
    Ecological Indicators 101 1086 - 1093 2019年06月01日 [査読有り][通常論文]
     
    © 2018 Elsevier Ltd Nitrogen isotope ratios (δ 15 N) in plants and soil are widely known as indicators of the N cycle in terrestrial ecosystem. Recent studies have proposed that the difference between plant and soil δ 15 N (Δδ 15 N) is a better indicator of the N cycle than plant δ 15 N or soil δ 15 N alone. However, the processes of the N cycle indicated by Δδ 15 N are not well understood. The present study compared Δδ 15 N variations between different ecosystems of northern Mongolia and northern Japan (Hokkaido) to associate the Δδ 15 N characteristics with soil N availability. Needles of Siberian larch (Larix sibirica Ledeb.) in Mongolia, Todo-fir (Abies sachalinensis (F.Schmidt) Mast.) in Hokkaido, and mineral soils from both regions were acquired for determination of Δδ 15 N values. Δδ 15 N showed similar large variations (8‰) in the two regions with no significant correlations to climate factors. On the other hand, the relationship between Δδ 15 N and soil δ 15 N was opposite between the two regions with a positive correlation in Mongolia (r s = 0.504) and a negative correlation in Hokkaido (r s = −0.600). Moreover, total inorganic N (total amount of NH 4+ and NO 3− ) contents were up to 20 times higher in Hokkaido than in Mongolia. Δδ 15 N showed significant correlation with the fraction of NO 3− relative to total inorganic N in the 0–10 cm soil layer in Hokkaido. These results indicate that Δδ 15 N variation in Hokkaido can be explained by progression of nitrification in soil, which is different in Mongolia where Δδ 15 N variation is explained by microbial N immobilization. Our findings suggest that soil N availability affects Δδ 15 N indicator owing to changes in the N cycle process, which are reflected in the relationships of foliage δ 15 N or soil δ 15 N with Δδ 15 N.
  • Mitsuhiro Toratani, Kazunori Ogata, Koji Suzuki, Joji Ishizaka, Toru Hirawake, Takafumi Hirata, Tomonori Isada, Hiroto Higa, Victor S. Kuwahara, Stanford B. Hooker, Yoko Kiyomoto, Hiroshi Murakami, Yukio Kurihara, Masahiro Hori, Hisatomo Waga, Youhei Yamashita, Akihiko Tanaka
    2019 IEEE INTERNATIONAL GEOSCIENCE AND REMOTE SENSING SYMPOSIUM (IGARSS 2019) 4741 - 4744 2019年 
    GCOM-C/SGLI is a multi-wavelength optical radiometer launched on December 23, 2017. The data provision has started from December 20, 2018. In this research, we briefly introduce standard Level 2 products and their validation results based on in situ data. Mean absolute percentage differences are 16.3 - 69.6% for NLW between 380 - 670 nm, 39 and 64.5% for AOT at 670 nm and 865 nm, 27.9% for CHL and 44.2% for aCDOM. Although the number of in situ validation data are still scare for a few ocean color products, the accuracy of GCOM-C/SGLI data will be improved by our future efforts of calibration/validation activities.
  • Miyazaki Yuzo, Yamashita Youhei, Kawana Kaori, Tachibana Eri, Kagami Sara, Mochida Michihiro, Suzuki Koji, Nishioka Jun
    SCIENTIFIC REPORTS 8 2018年10月05日 [査読有り][通常論文]
  • Yu Mizuno, Jun Nishioka, Takahiro Tanaka, Yuya Tada, Koji Suzuki, Yuta Tsuzuki, Atsuko Sugimoto, Youhei Yamashita
    Journal of Oceanography 74 5 1 - 13 2018年05月09日 [査読有り][通常論文]
     
    Coastal Oyashio Water (COW), defined as a water mass with a temperature lower than 2 °C and a salinity lower than 33.0, is distributed in the North Pacific Ocean off southeastern Hokkaido, Japan, from winter to spring. COW is rich in macronutrients and dissolved iron and is thus considered to affect the spring phytoplankton blooms in the Oyashio region. Although river water and sea-ice melt water have been considered freshwater end-members of COW, the contributions of these freshwater sources to COW have not been well described. In this study, the humic-like components in dissolved organic matter were first applied as a parameter to evaluate the freshwater end-members of COW in March 2015. Linear regressions with negative slopes were determined between the humic-like components and the salinity of COW. The intercepts of the regressions against the humic-like components were within the ranges of those observed for the local rivers of Hokkaido but were very different from those of sea ice. These findings suggest that river water contributed to the COW observed here as a freshwater end-member, although the contribution of sea-ice melt water to COW could not be evaluated. This novel approach also highlighted two different less-saline water masses in COW. The first was characterized by a lower temperature and relatively high levels of humic-like components, while the second was higher in temperature and had higher levels of humic-like components. It is suggested that these different characteristics are due to the contributions of water from different rivers and/or different effects of sea-ice melt water.
  • Plancq J, McColl J.L, Bendle J.A, Seki O, Couto J.M, Henderson A.C.G, Yamashita Y, Kawamura K, Toney J.L
    Organic Geochemistry 125 189 - 195 2018年 [査読有り][通常論文]
     
    Identifying the lacustrine haptophyte species that produce long-chain alkenones (LCAs) is essential prior to down-core temperature reconstructions. Here, we investigated the identity of LCA-producing species from Lake Toyoni, Japan using 18S ribosomal DNA (rDNA) and organic geochemical analyses. The rDNA analyses identified eighteen operational taxonomic units (OTUs) of which only one fell within the haptophyte phylotype. This haptophyte belongs to the Group I phylotype, as supported by the LCA distribution found in surface and down-core sediments, and is closely related to a haptophyte found in Lake BrayaSo (Greenland). The inferred temperature using the Greenland calibration is very close to the Lake Toyoni surface temperature recorded during the spring/early summer season, when the LCA-producing haptophyte is likely to bloom. We therefore suggest that the temperature calibration from the Lake BrayaSo, Greenland is a suitable calibration for down-core temperature reconstructions at Lake Toyoni. (C) 2018 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license.
  • Youhei Yamashita, Fuminori Hashihama, Hiroaki Saito, Hideki Fukuda, Hiroshi Ogawa
    Limnology and Oceanography 62 6 2360 - 2374 2017年11月01日 [査読有り][通常論文]
     
    Dissolved organic matter (DOM) in the epipelagic ocean is produced by marine organisms and consumed by microbes. Thus, the distributional patterns of DOM quantity and quality in surface waters are possibly related to marine ecological provinces. In this study, surface waters collected throughout the Pacific Ocean were used to investigate the geographical distributions of fluorescent DOM (FDOM) quantity and quality. An excitation-emission matrix and parallel factor analysis revealed two humic-like and two protein-like components. The levels of humic-like components showed clear meridional trends with some zonal variability and were highest and lowest in the northern high-latitude and the subtropical provinces, respectively. The photochemical reactivity, determined by the ratio of two humic-like components, was found to be lowest in the subtropical provinces, implying that the major factor controlling the geographical distribution of humic-like components is the degree of photobleaching. The distributional patterns differed between levels of two protein-like components, i.e., tryptophan-like and tyrosine-like. The ratio of tyrosine-like to tryptophan-like components was established as a possible indicator of microbial degradability, and the highest ratio occurred in subtropical provinces. A negative correlation was found between this ratio and the chlorophyll a concentration. Such geographical distributions of protein-like components imply that relatively recalcitrant protein-like components are distributed uniformly throughout the surface waters, but substantial contributions of reactive fractions occur in regions characterized by high biological production. Cluster analysis with the FDOM composition clarified that the diagenetic states of DOM were similar and variable in the northern high-latitude and the subtropical provinces, respectively.
  • Yuya Tada, Rie Nakaya, Shuji Goto, Youhei Yamashita, Koji Suzuki
    JOURNAL OF EXPERIMENTAL MARINE BIOLOGY AND ECOLOGY 495 119 - 128 2017年10月 [査読有り][通常論文]
     
    Dissolved organic matter derived from phytoplankton (DOMP) can affect the bacterial growth and community composition, with concomitant changes in DOM characteristics, of aquatic ecosystems. Here, we examined the chemical and fluorescent characteristics of DOMPs derived from the diatom Thalassiosira weissflogii and the dinoflagellate Heterocapsa triquetra, and the community and diversity responses of free-living bacteria to these DOMPs. The DOMP quality was evaluated based on amino acid composition and excitation -emission matrix (EEM) analyses. In addition, EEM analysis was used to examine compositional changes in DOMPs. The total hydrolysable and free amino acid (THAA and FAA, respectively) compositions of each DOMP differed between phytoplankton strains. In addition, THAA and FAA compositions in diatom DOMPs were slightly different between the exponential and stationary growth phases. Terminal restriction fragment -length polymorphism (TRFLP) analysis with 16S rRNA genes revealed that the community shifts varied significantly with different DOMP additions. Furthermore, the specific amino acids in the diatom and dinoflagellate DOMPs potentially affected the bacterial community shifts. Similarity percentage analysis with 16S rRNA gene deep -sequencing revealed that distinct DOMP additions caused the community and diversity shifts with the growth of some specific bacterial lineages. Alteromonas (class Gammaproteobacteria) and Bacteroidetes lineages were strongly associated with the diatom DOMPs, whereas the proportion of Rhodobacteraceae sequences (class Alphaproteobacteria) among the total sequences increased in response to the addition of the dinoflagellate DOMP. EEM analysis revealed that the fluorescent DOM (FDOM) compositions of the diatom and dinoflagellate DOMPs changed in association with shifts in the bacterial community. Especially, a decrease in the fluorescence intensities of DOM was observed from the dinoflagellate treatment with the growth of Rhodobacteraceae lineage. These results indicate that there was a strong linkage between FDOM dynamics and Rhodobacteraceae lineage in the coastal water. This study suggests that DOMPs from different phytoplankton constitute a primary factor that alters the dominant bacterial groups with compositional changes in FDOM in coastal environments.
  • Motohiro Nakane, Taku Ajioka, Youhei Yamashita
    Frontiers in Earth Science 5 2017年05月16日 [査読有り][招待有り]
     
    Pyrogenic carbon, also called black carbon (BC), is an important component in the global carbon cycle. BC produced by biomass burning or fossil fuel combustion is transported to oceans by the atmosphere or rivers. However, environmental dynamics (i.e., major sources and sinks) of BC in marine environments have not been well-documented. In this study, dissolved BC (DBC) collected from surface waters of the Chukchi Sea, the Bering Sea, and the subarctic and subtropical North Pacific were analyzed using the benzene polycarboxylic acid (BPCA) method. The DBC concentration and the ratio of B5CA and B6CA to all BPCAs (an index of the DBC condensation degree) ranged from 4.8 to 15.5μg-C L−1 and from 0.20 to 0.43, respectively, in surface waters of the Chukchi/Bering Seas and the North Pacific Ocean. The concentration and condensation degree of DBC in the Chukchi/Bering Seas were higher and more variable than those in the subarctic and subtropical North Pacific, which implies that the major factors controlling DBC distribution were different in these marine provinces. In the Chukchi/Bering Seas, the DBC concentration was negatively correlated to salinity but positively correlated to chromophoric dissolved organic matter (CDOM) quantity and total dissolved lignin phenol concentration estimated by CDOM parameters. These correlations indicated that the possible major source of DBC in the Chukchi/Bering Seas was Arctic rivers. However, in the North Pacific, where riverine inputs are negligible for most sampling sites, DBC was possibly derived from the atmosphere. Although spectral slopes of CDOM at 275-295 nm (an index of the photodegradation degree of CDOM) differed widely between the subarctic and subtropical North Pacific, the concentration and condensation degrees of DBC were similar between the subarctic and subtropical North Pacific, which suggests that photodegradation was not the only major factor controlling DBC distribution. Therefore, DBC distributions of the North PacificOceanwere considered to be mainly controlled by atmospheric deposition of BC and subsequent losses by photodegradation and adsorption onto sinking particles. This study implies that the main influence on DBC distribution in the open ocean and the coastal ocean are atmospheric deposition and fluvial inputs, respectively.
  • Yuichi Nosaka, Youhei Yamashita, Koji Suzuki
    Frontiers in Marine Science 4 MAR 2017年03月30日 [査読有り][招待有り]
     
    The seasonal biological drawdown of the partial pressure of CO2 (pCO2) in the surface waters of the Oyashio region of the western subarctic Pacific is one of the greatest among the world's oceans. This is attributable to spring diatom blooms. Transparent exopolymer particles (TEPs) are known to affect efficiency of the biological carbon pump, and higher TEP levels are frequently associated with massive diatom blooms. However, TEP dynamics in the Oyashio region remain unclear. We investigated the TEP distribution from three cruises during the spring diatom bloom periods in 2010 and 2011. TEP concentrations varied from < 15 to 196 ± 71 μg xanthan gum equiv. L-1 above 300 m and generally declined with depth. Vertical TEP concentrations were significantly related not only to chlorophyll a concentrations but also to bacterial abundance. Average TEP concentrations within the mixed layer (> 30 m) were significantly higher during the bloom (155 ± 12 μg xanthan gum equiv. L-1) than in the post-bloom phase (90 ± 32 μg xanthan gum equiv. L-1). In contrast, bacteria abundance within the mixed layer changed little during the bloom to post-bloom phases. These results suggest that the abundance of phytoplankton greatly contributed to dynamics of the TEP distribution. To evaluate the ability of the phytoplankton to produce TEP, an axenic strain of the diatom Thalassiosira nordenskioeldii, which is a representative species of Oyashio blooms, was examined within a batch culture system. Cell abundance-normalized TEP and dissolved organic carbon (DOC) production rates changed simultaneously with growth of the strain. Although these production rates were significantly higher in the stationary phase than in the exponential growth period, values of the TEP/DOC ratio changed little throughout incubation. These findings suggest that TEP production in the Oyashio region may be enhanced by an increase in DOC production from spring diatoms.
  • Shuji Goto, Yuya Tada, Koji Suzuki, Youhei Yamashita
    FRONTIERS IN MICROBIOLOGY 8 2017年03月 [査読有り][招待有り]
     
    The recalcitrant fraction of marine dissolved organic matter (DOM) plays an important role in carbon storage on the earth's surface. Bacterial production of recalcitrant DOM (RDOM) has been proposed as a carbon sequestration process. It is still unclear whether bacterial physiology can affect RDOM production. In this study, we conducted a batch culture using the marine bacterial isolate Alteromonas macleodii, a ubiquitous gammaproteobacterium, to evaluate the linkage between bacterial growth and DOM production. Glucose (1 mmol C L-1) was used as the sole carbon source, and the bacterial number, the DOM concentration in terms of carbon, and the excitationemission matrices (EEMs) of DOM were monitored during the 168-h incubation. The incubation period was partitioned into the exponential growth (0-24 h) and stationary phases (24-168 h) based on the growth curve. Although the DOM concentration decreased during the exponential growth phase due to glucose consumption, it remained stable during the stationary phase, corresponding to approximately 4% of the initial glucose in terms of carbon. Distinct fluorophores were not evident in the EEMs at the beginning of the incubation, but DOM produced by the strain exhibited five fluorescent peaks during exponential growth. Two fluorescent peaks were similar to protein-like fluorophores, while the others could be categorized as humiclike fluorophores. All fluorophores increased during the exponential growth phase. The tryptophan-like fluorophore decreased during the stationary phase, suggesting that the strain reused the large exopolymer. The tyrosine-like fluorophore seemed to be stable during the stationary phase, implying that the production of tyrosine containing small peptides through the degradation of exopolymers was correlated with the reutilization of the tyrosine-like fluorophore. Two humic-like fluorophores that showed emission maxima at the longer wavelength (525 nm) increased during the stationary phase, while the other humic-like fluorophore, which had a shorter emission wavelength (400 nm) and was categorized as recalcitrant, was stable. These humic-like fluorophore behaviors during incubation indicated that the composition of bacterial humic-like fluorophores, which were unavailable to the strain, differed between growth phases. Our results suggest that bacterial physiology can affect RDOM production and accumulation in the ocean interior.
  • Kazuki Tanaka, Nobuyuki Takesue, Jun Nishioka, Yoshiko Kondo, Atsushi Ooki, Kenshi Kuma, Toru Hirawake, Youhei Yamashita
    SCIENTIFIC REPORTS 6 2016年09月 [査読有り][通常論文]
     
    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.
  • Chia-Jung Lu, Ronald Benner, Cédric G. Fichot, Hideki Fukuda, Youhei Yamashita, Hiroshi Ogawa
    Frontiers in Marine Science 3 JUN 2016年 [査読有り][通常論文]
     
    Dissolved lignin phenols and optical properties of dissolved organic matter (DOM) were measured to investigate the sources and transformations of terrigenous DOM (tDOM) in Otsuchi Bay, Japan. Three rivers discharge into the bay, and relatively high values of syringyl:vanillyl phenols (0.73 ± 0.07) and cinnamyl:vanillyl phenols (0.33 ± 0.10) indicated large contributions of non-woody angiosperm tissues to lignin and tDOM. The physical mixing of river and seawater played an important role in controlling the concentrations and distributions of lignin phenols and chromophoric DOM (CDOM) optical properties in the bay. Lignin phenol concentrations and the CDOM absorption coefficient at 350 nm, a(350), were strongly correlated in river and bay waters. Measurements of lignin phenols and CDOM in bay waters indicated a variety of photochemical and biological transformations of tDOM, including oxidation reactions, photobleaching and a decrease in molecular weight. Photodegradation and biodegradation of lignin and CDOM were investigated in decomposition experiments with river water and native microbial assemblages exposed to natural sunlight or kept in the dark. There was a rapid and substantial removal of lignin phenols and CDOM during the first few days in the light treatment, indicating transformations of tDOM and CDOM can occur soon after discharge of buoyant river water into the bay. The removal of lignin phenols was slightly greater in the dark (34%) than in the light (30%) during the remaining 59 days of the incubation. Comparison of the light and dark treatments indicated biodegradation was responsible for 67% of total lignin phenols removal during the 62-day incubation exposed to natural sunlight, indicating biodegradation is a dominant removal process in Otsuchi Bay.
  • Y. Yamashita, C. -J. Lu, H. Ogawa, J. Nishioka, H. Obata, H. Saito
    MARINE CHEMISTRY 177 298 - 305 2015年12月 [査読有り][招待有り]
     
    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.
  • Youhei Yamashita, S. Leigh McCallister, Boris P. Koch, Michael Gonsior, Rudolf Jaffe
    ESTUARINE COASTAL AND SHELF SCIENCE 159 37 - 49 2015年06月 [査読有り][通常論文]
     
    Annually, rivers and inland water systems deliver a significant amount of terrestrial organic matter (OM) to the adjacent coastal ocean in both particulate and dissolved forms; however, the metabolic and biogeochemical transformations of OM during its seaward transport remains one of the least understood components of the global carbon cycle. This transfer of terrestrial carbon to marine ecosystems is crucial in maintaining trophic dynamics in coastal areas and critical in global carbon cycling. Although coastal regions have been proposed as important sinks for exported terrestrial materials, most of the global carbon cycling data, have not included fjords in their budgets. Here we present distributional patterns on the quantity and quality of dissolved OM in Fiordland National Park, New Zealand. Specifically, we describe carbon dynamics under diverse environmental settings based on dissolved organic carbon (DOC) depth profiles, oxygen concentrations, optical properties (fluorescence) and stable carbon isotopes. We illustrate a distinct change in the character of DOC in deep waters compared to surface and mid-depth waters. Our results suggest that, both, microbial reworking of terrestrially derived plant detritus and subsequent desorption of DOC from its particulate counterpart (as verified in a desorption experiment) are the main sources of the humic-like enriched DOC in the deep basins of the studied fjords. While it has been suggested that short transit times and protection of OM by mineral sorption may ultimately result in significant terrestrial carbon burial and preservation in fjords, our data suggests the existence of an additional source of terrestrial OM in the form of DOC generated in deep, fjord water. (C) 2015 Elsevier Ltd. All rights reserved.
  • Pingqing Fu, Kimitaka Kawamura, Jing Chen, Mingyue Qin, Lujie Ren, Yele Sun, Zifa Wang, Leonard A. Barrie, Eri Tachibana, Aijun Ding, Youhei Yamashita
    Scientific Reports 5 2015年04月 [査読有り][通常論文]
     
    Organic aerosols are ubiquitous in the earth's atmosphere. They have been extensively studied in urban, rural and marine environments. However, little is known about the fluorescence properties of water-soluble organic carbon (WSOC) or their transport to and distribution in the polar regions. Here, we present evidence that fluorescent WSOC is a substantial component of High Arctic aerosols. The ratios of fluorescence intensity of protein-like peak to humic-like peak generally increased from dark winter to early summer, indicating an enhanced contribution of protein-like organics from the ocean to Arctic aerosols after the polar sunrise. Such a seasonal pattern is in agreement with an increase of stable carbon isotope ratios of total carbon (delta C-13(TC)) from -26.8 parts per thousand to -22.5 parts per thousand. Our results suggest that Arctic aerosols are derived from a combination of the long-range transport of terrestrial organics and local sea-to-air emission of marine organics, with an estimated contribution from the latter of 8.7-77% (mean 45%).
  • YueHan Lu, Jennifer W. Edmonds, Youhei Yamashita, Bin Zhou, Andrea Jaegge, Matthew Baxley
    OCEAN DYNAMICS 65 1 17 - 32 2015年01月 [査読有り][通常論文]
     
    Combining stable carbon isotopic signatures (delta C-13-DOC) and optical properties of dissolved organic matter (DOM), we examined spatial variability in the sources and reactivity of DOM from Oregon-Washington coastal waters, with a particular focus on evaluating whether these measurements may reliably trace terrigenous DOM in coastal oceans. We sampled four stations on the continental shelf and four stations on the continental slope near the mouth of the Columbia River, with sampling depths ranging from 0 to 1,678 m. Nitrate and phosphate concentrations were largely controlled by organic matter (OM) regeneration although the river plume may have led to excess nitrates in relation to phosphates near the river mouth and/or the surface. Four fluorescence components (C1 to C4) were identified by using excitation emission matrices-parallel factor analysis. C1 and C2 were assigned as humic-like components which represented degraded DOM rather than OM of allochthonous or autochthonous origin. C3 and C4 were both labile, protein-like components representing autochthonous contributions, while C4 was more sensitive to diagenesis. In the shallow water layer (salinity a parts per thousand currency sign32.5 and depth a parts per thousand currency sign50 m), the variation in absorption properties (SUVA(254) and E >(280)), fluorescence index, freshness index (beta/alpha), percent fluorescence of C3, and delta C-13-DOC revealed that the Columbia River plume exported DOM that was characterized by greater aromaticity, higher molecular weight, and being more decomposed than marine, autochthonous DOM. However, these signatures of terrigenous DOM disappeared rapidly with increasing depth and offshore distance. In the intermediate and deep water layers (salinity > 32.5), the DOM indices were most driven by diagenesis, with changes in percent fluorescence components indicating increasing accumulation of humic DOM relative to protein-like DOM with depth. Principal component analysis that collectively assessed the DOM indices suggests that diagenesis was the primary factor driving the spatial variability of DOM properties in the study region, underlining challenges in tracing allochthonous DOM in coastal waters.
  • 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 2014年10月 [査読有り][通常論文]
     
    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.
  • Nagamitsu Maie, Satoshi Sekiguchi, Akira Watanabe, Kiyoshi Tsutsuki, Youhei Yamashita, Lulie Melling, Kaelin M. Cawley, Eikichi Shima, Rudolf Jaffe
    JOURNAL OF SEA RESEARCH 91 58 - 69 2014年08月 [査読有り][通常論文]
     
    Wetlands are key components in the global carbon cycle and export significant amounts of terrestrial carbon to the coastal oceans in the form of dissolved organic carbon (DOC). Conservative behavior along the salinity gradient of DOC and chromophoric dissolved organic matter (CDOM) has often been observed in estuaries from their freshwater end-member (salinity = 0) to the ocean (salinity = 35). While the oligo/meso-haline (salinity <10) tidal zone of upper estuaries has been suggested to be more complex and locally influenced by geomorphological and hydrological features, the environmental dynamics of dissolved organic matter (DOM) and the environmental drivers controlling its source, transport, and fate have scarcely been evaluated. Here, we investigated the distribution patterns of DOC and CDOM optical properties determined by UV absorbance at 254 nm (A(254)) and excitation emission matrix (EEM) fluorescence coupled with parallel factor analysis (PARAFAC) along the lower salinity range (salinity < 10) of the oligo/meso-haline zone for three distinct wetland-influenced rivers; namely the Bekanbeushi River, a cool-temperate river with estuarine lake in Hokkaido, Japan, the Harney River, a subtropical river with tidally-submerged mangrove fringe in Florida, USA, and the Judan River, a small, acidic, tropical rainforest river in Borneo, Malaysia. For the first two rivers, a clear decoupling between DOC and A(254) was observed, while these parameters showed similar conservative behavior for the third. Three distinct EEM-PARAFAC models established for each of the rivers provided similar spectroscopic characteristics except for some unique fluorescence features observed for the Judan River. The distribution patterns of PARAFAC components suggested that the inputs from plankton and/or submerged aquatic vegetation can be important in the Bekanbeushi River. Further, DOM photo-products formed in the estuarine lake were also found to be transported upstream. In the Harney River, whereas upriver-derived terrestrial humic-like components were mostly distributed conservatively, some of these components were also derived from mangrove inputs in the oligo/meso-haline zone. Interestingly, fluorescence intensities of some terrestrial humic-like components increased with salinity for the Judan River possibly due to changes in the dissociation state of acidic functional groups and/or increase in the fluorescence quantum yield along the salinity gradient The protein-like and microbial humic-like components were distributed differently between three wetland rivers, implying that interplay between loss to microbial degradation and inputs from diverse sources are different for the three wetland-influenced rivers. The results presented here indicate that upper estuarine oligo/meso-haline regions of coastal wetland rivers are highly dynamic with regard to the biogeochemical behavior of DOM. (C) 2014 Elsevier B.V. All rights reserved.
  • Kazuki Tanaka, Kenshi Kuma, Koji Hamasaki, Youhei Yamashita
    SCIENTIFIC REPORTS 4 2014年07月 [査読有り][通常論文]
     
    Major fraction of marine dissolved organic matter (DOM) is biologically recalcitrant, however, the accumulation mechanism of recalcitrant DOM has not been fully understood. Here, we examine the distributions of humic-like fluorescent DOM, factions of recalcitrant DOM, and the level of apparent oxygen utilization in the Japan Sea. We find linear relationships between these parameters for the deep water (>200 m) of the Japan Sea, suggesting that fluorescent DOM is produced in situ in the Japan Sea. Furthermore, we find that the amount of fluorescent DOM at a given apparent oxygen utilization is greater in the deep water of the Japan Sea than it is in the North Pacific, where the highest level of fluorescent DOM in the open ocean was previously observed. We conclude that the repeated renewal of the deep water contributes to the accumulation of fluorescent DOM in the interior of the Japan Sea.
  • Yue Han Lu, James E. Bauer, Elizabeth A. Canuel, R. M. Chambers, Youhei Yamashita, Rudolf Jaffe, Amy Barrett
    BIOGEOCHEMISTRY 119 1-3 275 - 292 2014年06月 [査読有り][通常論文]
     
    The amounts, sources and relative ages of inorganic and organic carbon pools were assessed in eight headwater streams draining watersheds dominated by either forest, pasture, cropland or urban development in the lower Chesapeake Bay region (Virginia, USA). Streams were sampled at baseflow conditions six different times over 1 year. The sources and ages of the carbon pools were characterized by isotopic (delta C-13 and a dagger C-14) analyses and excitation emission matrix fluorescence with parallel factor analysis (EEM-PARAFAC). The findings from this study showed that human land use may alter aquatic carbon cycling in three primary ways. First, human land use affects the sources and ages of DIC by controlling different rates of weathering and erosion. Relative to dissolved inorganic carbon (DIC) in forested streams which originated primarily from respiration of young, C-14-enriched organic matter (OM; delta C-13 = -22.2 +/- A 3 aEuro degrees; a dagger C-14 = 69 +/- A 14 aEuro degrees), DIC in urbanized streams was influenced more by sedimentary carbonate weathering (delta C-13 = -12.4 +/- A 1 aEuro degrees; a dagger C-14 = -270 +/- A 37 aEuro degrees) and one of pasture streams showed a greater influence from young soil carbonates (delta C-13 = -5.7 +/- A 2.5 aEuro degrees; a dagger C-14 = 69 aEuro degrees). Second, human land use alters the proportions of terrestrial versus autochthonous/microbial sources of stream water OM. Fluorescence properties of dissolved OM (DOM) and the C:N of particulate OM (POM) suggested that streams draining human-altered watersheds contained greater relative contributions of DOM and POM from autochthonous/microbial sources than forested streams. Third, human land uses can mobilize geologically aged inorganic carbon and enable its participation in contemporary carbon cycling. Aged DOM (a dagger C-14 = -248 to -202 aEuro degrees, equivalent(14)C ages of 1,811-2,284 years BP) and POM (a dagger C-14 = -90 to -88 aEuro degrees, C-14 ages of 669-887 years BP) were observed exclusively in urbanized streams, presumably a result of autotrophic fixation of aged DIC (-297 to -244 aEuro degrees, C-14 age = 2,251-2,833 years BP) from sedimentary shell dissolution and perhaps also watershed export of fossil fuel carbon. This study demonstrates that human land use may have significant impacts on the amounts, sources, ages and cycling of carbon in headwater streams and their associated watersheds.
  • Cristina Romera-Castillo, Meilian Chen, Youhei Yamashita, Rudolf Jaffe
    WATER RESEARCH 55 40 - 51 2014年05月 [査読有り][通常論文]
     
    Surface freshwater samples from Everglades National Park, Florida, were used to investigate the size distributions of natural dissolved organic matter (DOM) and associated fluorescence characteristics along the molecular weight continuum. Samples were fractionated using size exclusion chromatography (SEC) and characterized by spectroscopic means, in particular Excitation-Emission Matrix fluorescence modeled with parallel factor analysis (EEM-PARAFAC). Most of the eight components obtained from PARAFAC modeling were broadly distributed across the DOM molecular weight range, and the optical properties of the eight size fractions for all samples studied were quite consistent among each other. Humic-like components presented a similar distribution in all the samples, with enrichment in the middle molecular weight range. Some variability in the relative distribution of the different humic-like components was observed among the different size fractions and among samples. The protein like fluorescence, although also generally present in all fractions, was more variable but generally enriched in the highest and lowest molecular weight fractions. These observations are in agreement with the hypothesis of a supramolecular structure for DOM, and suggest that DOM fluorescence characteristics may be controlled by molecular assemblies with similar optical properties, distributed along the molecular weight continuum. This study highlights the importance of studying the molecular structure of DOM on a molecular size distribution perspective, which may have important implications in understanding the environmental dynamics such materials. (C) 2014 Elsevier Ltd. All rights reserved.
  • Kaelin M. Cawley, Youhei Yamashita, Nagamitsu Maie, Rudolf Jaffe
    ESTUARIES AND COASTS 37 2 399 - 410 2014年03月 [査読有り][通常論文]
     
    Dissolved organic carbon (DOC) concentration and dissolved organic matter (DOM) optical properties were analyzed along two estuarine river transects during the wet and dry seasons to better understand DOM dynamics and quantify mangrove inputs. A tidal study was performed to assess the impacts of tidal pumping on DOM transport. DOM in the estuaries showed non-conservative mixing indicative of mangrove-derived inputs. Similarly, fluorescence data suggest that some terrestrial humic-like components showed non-conservative behavior. An Everglades freshwater-derived fluorescent component, which is associated with soil inputs from the Northern Everglades, behaved conservatively. During the dry season, a protein-like component behaved conservatively until the mid-salinity range when non-conservative behavior due to degradation and/or loss was observed. The tidal study data suggests mangrove porewater inputs to the rivers following low tide. The differences in quantity of DOM exported by the Shark and Harney Rivers imply that geomorphology and tidal hydrology may be a dominant factor controlling the amount of DOM exported from the mangrove ecotone, where up to 21 % of the DOC is mangrove-derived. Additionally, nutrient concentrations and other temporal factors may control DOM export from the mangroves, particularly for the microbially derived fluorescent components, contributing to the seasonal differences. The wet and dry season fluxes of mangrove DOM from the Shark River is estimated as 0.27 x 10(9) mg C d(-1) and 0.075 x 10(9) mg C d(-1), respectively, and the Harney River is estimated as 1.9 x 10(9) mg C d(-1) and 0.20 x 10(9) mg C d(-1).
  • 近藤 能子, 小畑 元, 大木 淳之, 山下 洋平, 西岡 純, 久万 健志
    日本地球化学会年会要旨集 61 199 - 199 日本地球化学会 2014年 
    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。
  • Rudolf Jaffe, Kaelin M. Cawley, Youhei Yamashita
    ADVANCES IN THE PHYSICOCHEMICAL CHARACTERIZATION OF DISSOLVED ORGANIC MATTER: IMPACT ON NATURAL AND ENGINEERED SYSTEMS 1160 27 - + 2014年 [査読有り][招待有り]
     
    Excitation emission matrix fluorescence combined with parallel factor analysis (EEM-PARAFAC) has emerged over the past ten years as a powerful and popular technique applied in the characterization of natural dissolved organic matter (DOM) in aquatic ecosystems. The exponential production of peer reviewed manuscripts including EEM-PARAFAC in recent years merits a review, intended to assist researchers interested in applying this methodology in ecosystem studies of aquatic environments. In this paper the authors list most of the existing EEM-PARAFAC applications in studies related to DOM dynamics in headwater streams, rivers, lakes, wetlands, estuaries, coastal areas and the open ocean, and provide specific examples of such applications as illustrative case studies. While the inclusions of technical details of the technique are beyond the scope of this paper, pros and cons are briefly discussed and general approaches to its applicability are suggested.
  • Youhei Yamashita, Joseph N. Boyer, Rudolf Jaffé
    Continental Shelf Research 66 136 - 144 2013年09月01日 [査読有り][通常論文]
     
    The coastal zone of the Florida Keys features the only living coral reef in the continental United States and as such represents a unique regional environmental resource. Anthropogenic pressures combined with climate disturbances such as hurricanes can affect the biogeochemistry of the region and threaten the health of this unique ecosystem. As such, water quality monitoring has historically been implemented in the Florida Keys, and six spatially distinct zones have been identified. In these studies however, dissolved organic matter (DOM) has only been studied as a quantitative parameter, and DOM composition can be a valuable biogeochemical parameter in assessing environmental change in coastal regions. Here we report the first data of its kind on the application of optical properties of DOM, in particular excitation emission matrix fluorescence with parallel factor analysis (EEM-PARAFAC), throughout these six Florida Keys regions in an attempt to assess spatial differences in DOM sources. Our data suggests that while DOM in the Florida Keys can be influenced by distant terrestrial environments such as the Everglades, spatial differences in DOM distribution were also controlled in part by local surface runoff/fringe mangroves, contributions from seasgrass communities, as well as the reefs and waters from the Florida Current. Application of principal component analysis (PCA) of the relative abundance of EEM-PARAFAC components allowed for a clear distinction between the sources of DOM (allochthonous vs. autochthonous), between different autochthonous sources and/or the diagenetic status of DOM, and further clarified contribution of terrestrial DOM in zones where levels of DOM were low in abundance. The combination between EEM-PARAFAC and PCA proved to be ideally suited to discern DOM composition and source differences in coastal zones with complex hydrology and multiple DOM sources. © 2013 Elsevier Ltd.
  • Yuehan Lu, James E. Bauer, Elizabeth A. Canuel, Youhei Yamashita, R. M. Chambers, Rudolf Jaffe
    JOURNAL OF GEOPHYSICAL RESEARCH-BIOGEOSCIENCES 118 2 566 - 580 2013年06月 [査読有り][通常論文]
     
    Photochemical and microbial transformations of DOM were evaluated in headwater streams draining forested and human-modified lands (pasture, cropland, and urban development) by laboratory incubations. Changes in DOC concentrations, DOC isotopic signatures, and DOM fluorescence properties were measured to assess the amounts, sources, ages, and properties of reactive and refractory DOM under the influence of photochemistry and/or bacteria. DOC in streams draining forest-dominated watersheds was more photoreactive than in streams draining mostly human-modified watersheds, possibly due to greater contributions of terrestrial plant-derived DOC and lower amounts of prior light exposure in forested streams. Overall, the percentage of photoreactive DOC in stream waters was best predicted by the relative content of terrestrial fluorophores. The bioreactivity of DOC was similar in forested and human-modified streams, but variations were correlated with temperature and may be further controlled by the diagenetic status of organic matter. Alterations to DOC isotopes and DOM fluorescence properties during photochemical and microbial incubations were similar between forested and human-modified streams and included (1) negligible effects of microbial alteration on DOC isotopes and DOM fluorescence properties, (2) selective removal of C-13-depleted and C-14-enriched DOC under the combined influence of photochemical and microbial processes, and (3) photochemical alteration of DOM resulting in a preferential loss of terrestrial humic fluorescence components relative to microbial fluorescence components. This study provides a unique comparison of DOC reactivity in a regional group of streams draining forested and human-modified watersheds and indicates the importance of land use on the photoreactivity of DOC exported from upstream watersheds.
  • Yan Ding, Youhei Yamashita, Walter K. Dodds, Rudolf Jaffe
    CHEMOSPHERE 90 10 2557 - 2562 2013年03月 [査読有り][通常論文]
     
    While the existence of black carbon as part of dissolved organic matter (DOM) has been confirmed, quantitative determinations of dissolved black carbon (DBC) in freshwater ecosystem and information on factors controlling its concentration are scarce. In this study, stream surface water samples from a series of watersheds subject to different burn frequencies in Konza Prairie (Kansas, USA) were collected in order to determine if recent fire history has a noticeable effect on DBC concentration. The DBC levels detected ranged from 0.04 to 0.11 mg L-1, accounting for ca. 3.32 +/- 0.51% of dissolved organic carbon (DOC). No correlation was found between DBC concentration and neither fire frequency nor time since last burn. We suggest that limited DBC flux is related to high burning efficiency, possibly greater export during periods of high discharge and/or the continuous export of DBC over long time scales. A linear correlation between DOC and DBC concentrations was observed, suggesting the export mechanisms determining DOC and DBC concentrations are likely coupled. The potential influence of fire history was less than the influence of other factors controlling the DOC and DBC dynamics in this ecosystem. Assuming similar conditions and processes apply in grasslands elsewhere, extrapolation to a global scale would suggest a global grasslands flux of DBC on the order of 0.14 Mt carbon year(-1). (C) 2012 Elsevier Ltd. All rights reserved.
  • 神田 穣太, 石井 雅男, 小川 浩史, 小埜 恒夫, 小畑 元, 川合 美千代, 鈴村 昌弘, 本多 牧生, 山下 洋平, 渡邉 豊
    海の研究 22 6 219 - 251 日本海洋学会 2013年 [査読有り][通常論文]
     
    化学海洋学を中心とする視点から,海洋学の過去10年程度の研究の進展を総括するとともに,今後10年程度の期間でわが国として取り組むべき研究の方向性について論じた。物質循環は依然として海洋学の主たる研究対象である。海洋における物質の定量は化学系の海洋観測の基本であるが,センサーを搭載した各種の能動型のプラットフォームにより海洋物理系の観測並みに高い時空間解像度を目指す方向が大きな柱になりつつある。一方で,このような高密度観測を補完するプロセス研究の重要性も指摘された。この両者の相互連携の仲立ちとしてモデルの役割が位置づけられ,また両者の連携による研究進展における新技術の導入および技術開発へのフィードバックの必要性が議論された。物質動態の可視化に不可欠なセンサーおよびプラットフォームの現状と展望,プロセス研究の対象となる未解明部分の例示,両者を統合した今後の物質循環研究のあり方とモデル海域の順で,研究基盤との関わりも含めて展望した。Progress in oceanographic research in the past decade is reviewed and future direction in the Japanese oceanographic research in the next decade is discussed, mainly from the chemical oceanographic perspective. While biogeochemical cycle (or material cycle) remains to be a major theme in oceanography, innovative chemical sensors on various maneuverable platforms will be able to depict dynamics of chemical constituents with very fine resolution, equivalent of those utilized in physical oceanography; a high-definition or "high-vision" image of material cycle will be available to researchers in near future. On the other hand, process studies that unveil the mechanisms behind such dynamics should be implemented concurrently. Numerical modeling may effectively connect these two approaches, and interactions with technological innovations will facilitate progress of research. Current status and expectations of sensor/platform development, possible targets of the process studies, integration of the two research approaches, and some target oceanic regions for the integrated study are discussed, with respect to necessary infrastructures of research.
  • Y. Yamashita, Y. Nosaka, K. Suzuki, H. Ogawa, K. Takahashi, H. Saito
    BIOGEOSCIENCES 10 11 7207 - 7217 2013年 [査読有り][通常論文]
     
    Chromophoric dissolved organic matter (CDOM) ubiquitously occurs in marine environments and plays a significant role in the marine biogeochemical cycles. Basin scale distributions of CDOM have recently been surveyed in the global ocean and indicate that quantity and quality of oceanic CDOM are mainly controlled by in situ production and photobleaching. However, factors controlling the spectral parameters of CDOM in the UV region, i.e., spectral slope of CDOM determined at 275-295 nm (S275-295) and the ratio of two spectral slope parameters (S-R); the ratio of S275-295 to S350-400, have not been well documented. To evaluate the factor controlling the spectral characteristics of CDOM in the UV region in the open ocean, we determined the quantitative and qualitative characteristics of CDOM in the subarctic and subtropical surface waters (5-300 m) of the western North Pacific. Absorption coefficients at 320 nm in the subarctic region were higher than those in the subtropical region throughout surface waters, suggesting that magnitudes of photobleaching were different between the two regions. The values of S275-295 and SR were also higher in the subtropical region than the subarctic region. The dark microbial incubation showed biodegradation of DOM little affected S275-295, but slightly decreased SR. On the other hand, increases in S275-295 and relative stableness of SR were observed during photo-irradiation incubations respectively. These experimental results indicated that photobleaching of CDOM mainly induced qualitative differences in CDOM at UV region between the subarctic and subtropical surface waters. The results of this study imply that S275-295 can be used as a tracer of photochemical history of CDOM in the open ocean.
  • R. Jaffe, Y. Yamashita, N. Maie, W. T. Cooper, T. Dittmar, W. K. Dodds, J. B. Jones, T. Myoshi, J. R. Ortiz-Zayas, D. C. Podgorski, A. Watanabe
    GEOCHIMICA ET COSMOCHIMICA ACTA 94 95 - 108 2012年10月 [査読有り][通常論文]
     
    Dissolved organic matter (DOM) represents the largest organic matter pool in freshwater systems, but much of it remains molecularly uncharacterized. Although freshwater systems cover only a small area of the earth's surface, inland waters are an important component of the global carbon cycle. The traditional idea that rivers are simply conduits for refractory carbon delivery to coastal areas is inconsistent with carbon flux estimates, and streams have been shown to serve as reactors for DOM cycling. The overall quality of DOM, and its associated reactivity, can be related to its chemical composition and molecular structure. However, the variability of DOM composition in freshwater ecosystems, particularly in headwater streams, is poorly characterized. Detailed molecular studies of DOM from small streams across climatic regions, which could provide critical information regarding carbon dynamics on a more global scale, have not been performed. To address these issues, this study applies a multi-method analytical approach in an attempt to assess molecular characteristics of DOM and ultrafiltered DOM (UDOM) in headwater streams from different climatic regions in North America. In general terms the chemical and molecular characteristics of UDOM from six different biomes were determined in unsurpassed detail to feature some clear general similarities but also specific differences. While the degree of similarity is remarkable, and suggests similar source strengths, such as soil-derived organic matter and/or similar diagenetic degradation processes for DOM from vastly different environments, each sample was clearly unique in its overall composition, featuring some distinct molecular patterns for at least one or more of the analytical determinations. Molecular and compositional differences of DOM from headwater streams should result from variations in DOM sources and localized environmental conditions, and consequently feature different photo- and bio-reactivity and associated re-mineralization potentials during fluvial transport. Such knowledge could assist in predicting the consequences of global change and its relationship to global carbon cycling. (C) 2012 Elsevier Ltd. All rights reserved.
  • N. Maie, Y. Yamashita, R. M. Cory, J. N. Boyer, R. Jaffe
    APPLIED GEOCHEMISTRY 27 4 917 - 929 2012年04月 [査読有り][通常論文]
     
    The environmental dynamics of dissolved organic matter (DOM) were characterized for a shallow, subtropical, seagrass-dominated estuarine bay, namely Florida Bay, USA. Large spatial and seasonal variations in DOM quantity and quality were assessed using dissolved organic C (DOC) measurements and spectrophotometric properties including excitation emission matrix (EEM) fluorescence with parallel factor analysis (PARAFAC). Surface water samples were collected monthly for 2 years across the bay. DOM characteristics were statistically different across the bay, and the bay was spatially characterized into four basins based on chemical characteristics of DOM as determined by EEM-PARAFAC. Differences between zones were explained based on hydrology, geomorphology, and primary productivity of the local seagrass community. In addition, potential disturbance effects from a very active hurricane season were identified. Although the overall seasonal patterns of DOM variations were not significantly affected on a bay-wide scale by this disturbance, enhanced freshwater delivery and associated P and DOM inputs (both quantity and quality) were suggested as potential drivers for the appearance of algal blooms in high impact areas. The application of EEM-PARAFAC proved to be ideally suited for studies requiring high sample throughput methods to assess spatial and temporal ecological drivers and to determine disturbance-induced impacts in aquatic ecosystems. (C) 2012 Elsevier Ltd. All rights reserved.
  • 太平洋における溶存腐植様蛍光物質の分布 –北太平洋中層水による陸起源腐植物質輸送の可能性-
    山下洋平
    月刊海洋 44 486 - 491 2012年 [査読無し][通常論文]
  • Youhei Yamashita, Brian D. Kloeppel, Jennifer Knoepp, Gregory L. Zausen, Rudolf Jaffe
    ECOSYSTEMS 14 7 1110 - 1122 2011年11月 [査読有り][通常論文]
     
    Dissolved organic matter (DOM) is recognized as a major component in the global carbon cycle and is an important driver in aquatic ecosystem function. Climate, land use, and forest cover changes all impact stream DOM and alter biogeochemical cycles in terrestrial environments. We determined the temporal variation in DOM quantity and quality in headwater streams at a reference watershed (REF), a watershed clear-cut 30 years ago (CC), and a watershed converted to a white pine plantation 50 years ago (WP) at the US Forest Service, Coweeta Hydrologic Laboratory, in the Nantahala Mountains of western North Carolina, USA. Average stream dissolved organic carbon (DOC) concentrations in CC or WP were 60 and 80% of those in REF, respectively. Stream DOM composition showed that the difference was mainly due to changes in humic-ike components in chromophoric DOM. In addition, excitation-emission matrix fluorescence data with parallel factor analysis indicate that although the concentration of protein-like components did not differ significantly among watersheds, their relative abundance showed an enrichment in CC and WP compared to REF. The ratio of humic acid-type to fulvic acid-type components was highest and lowest at REF and WP, respectively. Our data suggest that forest ecosystem disturbance history affects the DOM quantity and quality in headwater streams over decades as a result of changes in watershed soil organic matter characteristics due to differences in organic matter inputs.
  • Oliva Pisani, Youhei Yamashita, Rudolf Jaffe
    WATER RESEARCH 45 13 3836 - 3844 2011年07月 [査読有り][通常論文]
     
    This study shows that light exposure of flocculent material (floc) from the Florida Coastal Everglades (FCE) results in significant dissolved organic matter (DOM) generation through photo-dissolution processes. Floc was collected at two sites along the Shark River Slough (SRS) and irradiated with artificial sunlight. The DOM generated was characterized using elemental analysis and excitation emission matrix fluorescence coupled with parallel factor analysis. To investigate the seasonal variations of DOM photo-generation from floc, this experiment was performed in typical dry (April) and wet (October) seasons for the FCE. Our results show that the dissolved organic carbon (DOC) for samples incubated under dark conditions displayed a relatively small increase, suggesting that microbial processes and/or leaching might be minor processes in comparison to photo-dissolution for the generation of DOM from floc. On the other hand, DOC increased substantially (as much as 259 mgC gC(-1)) for samples exposed to artificial sunlight, indicating the release of DOM through photo-induced alterations of floc. The fluorescence intensity of both humic-like and protein-like components also increased with light exposure. Terrestrial humic-like components were found to be the main contributors (up to 70%) to the chromophoric DOM (CDOM) pool, while protein-like components comprised a relatively small percentage (up to 16%) of the total CDOM. Simultaneously to the generation of DOC, both total dissolved nitrogen and soluble reactive phosphorus also increased substantially during the photo-incubation period. Thus, the photo-dissolution of floc can be an important source of DOM to the FCE environment, with the potential to influence nutrient dynamics in this system. (C) 2011 Elsevier Ltd. All rights reserved.
  • Yamashita, Y, A. Panton, C. Mahaffey, R. Jaffé
    Ocean Dynamics 61 5 569 - 579 2011年05月 [査読有り][通常論文]
     
    The spatial and diurnal tidal variability of dissolved organic carbon (DOC) concentrations and the composition of dissolved organic matter (DOM), as evaluated by high-temperature catalytic oxidation and excitation-emission matrix combined with parallel factor analysis (EEM-PARAFAC), respectively, were determined in Liverpool Bay. EEM-PARAFAC modeling resulted in six fluorescent components characterized as terrestrial humic-like (two), microbial humic-like (two), and protein-like (two). The spatial distributions of DOC and the four humic-like components were negatively correlated with salinity in the high-salinity waters observed in this study (30.41-33.75), suggesting that terrestrial DOM was conservatively distributed. The spatial patterns of protein-like components were largely different from those of DOC, humic-like components, and chlorophyll a, suggesting that these distributions were the combined result of production and degradation in the bay in addition to river inputs. These findings suggest that the DOM dynamics in Liverpool Bay are strongly controlled by river-dominated allochthonous DOM inputs with some less significant contributions of autochthonous DOM within the bay. In addition, the temporal variations of DOM associated with the diurnal tidal cycles were determined at one inshore (31.34-32.24 salinity) and one offshore (33.64-33.75 salinity) station in the bay. Negative linear relationships between salinity and DOM characteristics, i.e., DOC, humic-like, and protein-like components, were observed at the inshore station. In contrast, no relationship was observed at the offshore station, suggesting that the export of DOM through rivers and possibly tidal flats have a noticeable influence on DOM concentration and composition up to a relatively elevated salinity of around 33 in Liverpool Bay.
  • G. Christopher Shank, Anne Evans, Youhei Yamashita, Rudolf Jaffe
    LIMNOLOGY AND OCEANOGRAPHY 56 2 577 - 588 2011年03月 [査読有り][通常論文]
     
    We investigated the influence of solar radiation on the transfer of organic matter from the particulate to dissolved phase during resuspension of coastal sediments collected from seven sites across Florida Bay (organic carbon values ranged from 2% to 9% by weight). Sediments were resuspended in oligotrophic seawater for 48 h in 1-liter quartz flasks in the dark and under simulated solar radiation (SunTest XLS+) at wet weight concentrations of 100 mg L(-1) and 1 g L(-1) (dry weights ranged from 27 to 630 mg L(-1)). There were little to no dissolved organic carbon (DOC) increases in dark resuspensions, but substantial DOC increases occurred in irradiated resuspensions. DOC levels increased 4 mg C L(-1) in an irradiated 1 g L(-1) suspension (dry weight 400 mg L(-1)) of an organic-rich (7% organic carbon) sediment. At a particle load commonly found in coastal waters (dry weight 40 mg L(-1)), an irradiated suspension of the same organic-rich sediment produced 1 mg C L(-1). DOC increases in irradiated resuspensions were well-correlated with particulate organic carbon (POC) added. Photodissolution of POC ranged from 6% to 15% at high sediment levels and 10% to 33% at low sediment levels. Parallel factor analysis modeling of excitation-emission matrix fluorescence data (EEM PARAFAC) suggested the dissolved organic matter (DOM) produced during photodissolution included primarily humic-like components and a less important input of protein-like components. Principal component analysis (PCA) of EEM data revealed a marked similarity in the humic character of photodissolved DOM from organic-rich sediments and the humic character of Florida Bay waters.
  • 都築 佑太, 時岡 仁志, 山下 洋平, 柴田 英昭, 杉本 敦子
    日本地球化学会年会要旨集 58 333 - 333 日本地球化学会 2011年 
    水循環システムを明らかにすることは様々な方面において重要である。そこで水の安定同位体比を用いることで水の挙動を推定できる。これによって、降水の元となる水蒸気の起源や河川水、地下水の供給源、流下経路などの情報を得ることが出来る。これを応用することで水資源の管理、汚染源の特定や地球化学的研究に役立つ。そこで本研究では、北海道の水循環を解明するため、北海道水同位体マップの作成を目標とし、それに向けた第一歩として道内4地点(札幌、苫小牧、天塩、芽室)で降水と河川水または地下水の定点観測を行った。特に苫小牧の幌内川では、小さい集水域における降水から土壌水、地下水位帯を経て河川へ流下するメカニズムを調べ、一方でそれら小さい集水域を多く含む天塩川では、天塩川全体の水循環を明らかにするため広い範囲でサンプリングを行った。
  • Yamashita, Y, L.J. Scinto, N. Maie, R. Jaffé
    Ecosystems 13 7 1006 - 1019 2010年11月 [査読有り][通常論文]
     
    Wetlands are known to be important sources of dissolved organic matter (DOM) to rivers and coastal environments. However, the environmental dynamics of DOM within wetlands have not been well documented on large spatial scales. To better assess DOM dynamics within large wetlands, we determined high resolution spatial distributions of dissolved organic carbon (DOC) concentrations and DOM quality by excitation-emission matrix spectroscopy combined with parallel factor analysis (EEM-PARAFAC) in a subtropical freshwater wetland, the Everglades, Florida, USA. DOC concentrations decreased from north to south along the general water flow path and were linearly correlated with chloride concentration, a tracer of water derived from the Everglades Agricultural Area (EAA), suggesting that agricultural activities are directly or indirectly a major source of DOM in the Everglades. The optical properties of DOM, however, also changed successively along the water flow path from high molecular weight, peat-soil and highly oxidized agricultural soil-derived DOM to the north, to lower molecular weight, biologically produced DOM to the south. These results suggest that even though DOC concentration seems to be distributed conservatively, DOM sources and diagenetic processing can be dynamic throughout wetland landscapes. As such, EEM-PARAFAC clearly revealed that humic-enriched DOM from the EAA is gradually replaced by microbial- and plant-derived DOM along the general water flow path, while additional humic-like contributions are added from marsh soils. Results presented here indicate that both hydrology and primary productivity are important drivers controlling DOM dynamics in large wetlands. The biogeochemical processes controlling the DOM composition are complex and merit further investigation.
  • Nagata, T, C. Tamburini, J. Arístegui, F. Baltar, A. Bochdansky, S. Fonda-Umani, H. Fukuda, A. Gogou, D. A. Hansell, R. L. Hansman, G. J. Herndl, C. Panagiotopoulos, T. Reinthaler, R. Sohrin, P. Verdugo, N. Yamada, Y. Yamashita, T. Yokokawa, D. H. Bartlet
    Deep-Sea Research Part II 57 16 1519 - 1536 2010年08月 [査読有り][通常論文]
     
    This paper synthesizes recent findings regarding microbial distributions and processes in the bathypelagic ocean (depth > 1000 m). Abundance, production and respiration of prokaryotes reflect supplies of particulate and dissolved organic matter to the bathypelagic zone. Better resolution of carbon fluxes mediated by deep microbes requires further testing on the validity of conversion factors. Archaea, especially marine Crenarchaeota Group I, are abundant in deep waters where they can fix dissolved inorganic carbon. Viruses appear to be important in the microbial loop in deep waters, displaying remarkably high virus to prokaryote abundance ratios in some oceanic regions. Sequencing of 18S rRNA genes revealed a tremendous diversity of small-sized protists in bathypelagic waters. Abundances of heterotrophic nanoflagellates (HNF) and ciliates decrease with depth more steeply than prokaryotes; nonetheless, data indicated that HNF consumed half of prokaryote production in the bathypelagic zone. Aggregates are important habitats for deep-water microbes, which produce more extracellular enzymes (on a per-cell basis) than surface communities. The theory of marine gel formation provides a framework to unravel complex interactions between microbes and organic polymers. Recent data on the effects of hydrostatic pressure on microbial activities indicate that bathypelagic microbial activity is generally higher under in situ pressure conditions than at atmospheric pressures. High-throughput sequencing of 16S rRNA genes revealed a remarkable diversity of Bacteria in the bathypelagic ocean. Metagenomics and comparative genomics of piezophiles reveal not only the high diversity of deep sea microbes but also specific functional attributes of these piezophilic microbes, interpreted as an adaptation to the deep water environment. Taken together, the data compiled on bathypelagic microbes indicate that, despite high-pressure and low-temperature conditions, microbes in the bathypelagic ocean dynamically interact with complex mixtures of organic matter, responding to changes in the ocean's biogeochemical state. (C) 2010 Elsevier Ltd. All rights reserved.
  • 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 2010年08月 [査読有り][通常論文]
     
    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.
  • Meilian Chen, Rene M. Price, Youhei Yamashita, Rudolf Jaffe
    APPLIED GEOCHEMISTRY 25 6 872 - 880 2010年06月 [査読有り][通常論文]
     
    Dissolved organic matter (DOM) in groundwater and surface water samples from the Florida coastal Everglades were studied using excitation-emission matrix fluorescence modeled through parallel factor analysis (EEM-PARAFAC). DOM in both surface and groundwater from the eastern Everglades S332 basin reflected a terrestrial-derived fingerprint through dominantly higher abundances of humic-like PARAFAC components. In contrast, surface water DOM from northeastern Florida Bay featured a microbial-derived DOM signature based on the higher abundance of microbial humic-like and protein-like components consistent with its marine source. Surprisingly, groundwater DOM from northeastern Florida Bay reflected a terrestrial-derived source except for samples from central Florida Bay well, which mirrored a combination of terrestrial and marine end-member origin. Furthermore, surface water and groundwater displayed effects of different degradation pathways such as photodegradation and biodegradation as exemplified by two PARAFAC components seemingly indicative of such degradation processes. Finally, Principal Component Analysis of the EEM-PARAFAC data was able to distinguish and classify most of the samples according to DOM origins and degradation processes experienced, except for a small overlap of S332 surface water and groundwater, implying rather active surface-to-ground water interaction in some sites particularly during the rainy season. This study highlights that EEM-PARAFAC could be used successfully to trace and differentiate DOM from diverse sources across both horizontal and vertical flow profiles, and as such could be a convenient and useful tool for the better understanding of hydrological interactions and carbon biogeochemical cycling. (C) 2010 Elsevier Ltd. All rights reserved.
  • Clayton J. Williams, Youhei Yamashita, Henry F. Wilson, Rudolf Jaffe, Marguerite A. Xenopoulos
    LIMNOLOGY AND OCEANOGRAPHY 55 3 1159 - 1171 2010年05月 [査読有り][通常論文]
     
    Surface water samples were collected from 43 streams distributed throughout watersheds of mixed land use in southern Ontario, Canada. Absorbance and fluorescence spectroscopy with parallel factor analysis (PARAFAC) was used to characterize dissolved organic matter (DOM). DOM characteristics were related to environmental variables, microbial activity indicators (bacterial production and extracellular leucine aminopeptidase activity), and riparian land use to understand better how these factors influence DOM in streams. PARAFAC produced a six-component model (C1 to C6). Temperature correlated with each PARAFAC component, suggesting that water source, drainage area, and light penetration broadly affected DOM characteristics. C1 and C2 represented terrestrial, humic-like DOM fluorophore groups and comprised 41-65% of stream DOM fluorescence. C5, a tryptophan-like component, related negatively to a humification index but positively to leucine-aminopeptidase activity and recently produced DOM, suggesting that C5 consisted of autochthonous, microbially produced DOM. C3, C4, and C6 showed signs of quinone-like, humic-like, and microbial transformable fluorophores. The distribution of these potentially redox-active PARAFAC components indicated that DOM was in a more reduced state in streams with higher bacterial production and agricultural land use than in streams with increased wetlands area, which had greater relative abundance of the oxidized quinone-like component. Anthropogenic land use and microbial activity altered the quantity and quality of DOM exported from human-affected streams from that observed in forest-and wetland-dominated streams. DOM in agriculturally affected streams was likely more labile and accessible to the microbial community than DOM in wetland streams, which supported low rates of microbial activity.
  • Youhei Yamashita, Nagamitsu Maie, Henry Briceno, Rudolf Jaffe
    JOURNAL OF GEOPHYSICAL RESEARCH-BIOGEOSCIENCES 115 G00F10  2010年01月 [査読有り][通常論文]
     
    Tropical rivers are an important source of dissolved organic matter (DOM) to coastal oceans. However, temporal and spatial variability of DOM composition and thus its quality in such rivers, on landscape and basin scales, have not been well documented. In this study, we present data on the spatial distribution of DOM quantity and quality based on source, molecular weight, and composition using optical properties including excitation emission matrix fluorescence with parallel factor analysis. We compared such DOM quantity and quality determinations in main river channels and their tributaries for three river systems of the Guayana Shield, Venezuela. Spatial variabilities of DOM parameters were strongly related to differences in the geological settings of the drainage basins and presumably their associated vegetation cover. Linear relationships between quantitative and qualitative DOM parameters were also evident, suggesting that high DOC concentration correlated with chromophoric dissolved organic matter (CDOM) characteristics of higher molecular weight associated with terrestrial sources, while low DOC concentrations correlated with CDOM characteristics of lower molecular weight associated primarily with microbial sources. Such relationships seem to imply that DOM concentrations and their sources/characteristics may be coupled in the studied fluvial systems. In addition, shifts in DOM compositions between terrestrial and microbial signals were observed with changes in water discharge and in watersheds disturbed by gold mining activities. The observed linkages between, and the changes among DOM quantity and quality, suggest that the biogeochemistry of DOM in tropical rivers may be quite sensitive to climatic and land use change.
  • 海洋における発色団含有溶存有機物の機能と動態に関する研究
    山下洋平
    海の研究 19 41 - 56 2010年 [査読有り][招待有り]
  • C. Santin, Y. Yamashita, X. L. Otero, M. A. Alvarez, R. Jaffe
    BIOGEOCHEMISTRY 96 1-3 131 - 147 2009年12月 [査読有り][通常論文]
     
    The determination of optical properties of organic matter using spectroscopic techniques is a powerful tool for the characterization of humic substances (HS) in soils and sediments because of sensitivity, specificity and sample throughput. However, basic spectroscopic techniques have practical limitations because of the similarity in the optical properties of many HS. To improve resolution, the combination of excitation-emission matrix (EEM) fluorescence and parallel factor analysis (PARAFAC) was applied for characterizing fulvic acid (FA) and humic acid (HA) fractions from soils and sediments of two estuarine environments in Spain. Five fluorescent components were identified by EEM-PARAFAC and were found in both FA and HA fractions, consistent with the new paradigm of HS as supramolecular associations as well as the ubiquity of the HS components in the environment. Their contribution was, however, different between the FA and HA fractions. Two different, humic-like, fluorescent components were representative of FA and HA fractions, respectively. The spectral characteristics of these components were similar to previously reported PARAFAC components in dissolved organic matter (DOM) in a wide range of environments, suggesting their applicability in assessing OM quality and environmental dynamics. A microbial humic-like component was much more abundant in FA than in HA fractions. Furthermore, principal component analysis clarified that the two identified protein-like components, were enriched in sediment HA compared to soil HA, suggesting a larger contribution of refractory algaenan in sediment HA. The results of the present study demonstrate that EEM-PARAFAC is a useful technique for the biogeochemical characterization of soil and sedimentary HS.
  • Youhei Yamashita, Eiichiro Tanoue
    LIMNOLOGY AND OCEANOGRAPHY 54 2 598 - 609 2009年03月 [査読有り][通常論文]
     
    Chromophoric dissolved organic matter (CDOM) was surveyed along the 160 degrees W transect from the equatorial to the subarctic Pacific. CDOM characteristics were evaluated through the measurement of fluorescence intensity at 320 nm excitation and 420 nm emission and of absorption coefficient (a) at 320 nm, both indicative of marine humic-like CDOM. In the surface layer (similar to 200 m), different levels but similar optical characteristics of CDOM were found among the oceanic regions studied. In the mesopelagic (200-1000 m) and abyssal layers (1000 m-bottom), levels of fluorescence intensities were linearly correlated to those of absorption coefficients, with a similar slope between these layers. However, the intercept of the linear relationship between the two optical parameters was significantly lower for the mesopelagic layer than for the abyssal layer. Differences between intercepts were consistent with the transport of optically distinct CDOM to the mesopelagic layer through the formation of the North Pacific intermediate water (NPIW). At wavelengths shorter than 300 nm, the absorption coefficients of CDOM in the surface layer systematically deviated from the natural logarithmic relationships between absorption coefficients and wavelengths in the 320-350 nm range. This class of CDOM, estimated using absorption coefficient at 275 nm, was defined as surface-specific CDOM and may be derived from biocomponents. Surface-specific CDOM was higher in the relatively young mode water than in old NPIW, suggesting that this class of CDOM is semi-labile.
  • Youhei Yamashita, Rudolf Jaffe
    ENVIRONMENTAL SCIENCE & TECHNOLOGY 42 19 7374 - 7379 2008年10月 [査読有り][通常論文]
     
    Natural dissolved organic matter (DOM) is composed of a variety of organic compounds, which can interact with metals in aquatic environments. The interactions between DOM and two metals of environmental concern (Cu(II) and Hg(II)) were studied using fluorescence quenching titrations combined with excitation-emission matrix (EEM) spectra and parallel factor analysis (PARAFAC). This allowed characterizing the specific interactions between eight fluorescent components in DOM and two metals. Triplicate titration experiments showed good reproducibility when assessing the interactions between humic-like components with Cu(II). Our data show clear differences in metal-DOM interaction for samples of different DOM composition and between two different metals. The results demonstrate that the combination of fluorescence quenching titrations with EEM-PARAFAC was reproducible and sensitive to determine the binding properties of humic-like components with trace metals. The enhancement in fluorescence intensity after its initial decrease for the protein-like components with addition of Cu(II) was observed at mangrove-dominated sites, suggesting changes in the molecular environments of protein-like components due to increased Cu(II) interaction. The application of EEM-PARAFAC in fluorescence quenching studies is a useful tool to evaluate intermolecular DOM and DOM-trace metals interactions.
  • Youhei Yamashita, Rudolf Jaffe, Nagamitsu Maie, Eiichiro Tanoue
    LIMNOLOGY AND OCEANOGRAPHY 53 5 1900 - 1908 2008年09月 [査読有り][通常論文]
     
    The distributions of fluorescent components in dissolved organic matter (DOM) from Ise Bay, Japan, were determined by excitation emission matrix (EEM) fluorescence spectroscopy combined with parallel factor analysis (PARAFAC). Three terrestrial humic-like, one marine humic-like, and three non-humic-like fluorescent components were identified by PARAFAC, and the environmental dynamics of individual fluorescent components in the bay area were evaluated. The observed linear relationships between salinity and abundance of two of the three humic-like components in the bay area indicate a terrestrial origin and conservative mixing behavior of these components. On the other hand, nonconservative mixing for the other terrestrial and the marine humic-like components was observed, indicating that the sources of these were other than solely riverine inputs. Thus, in addition to riverine sources, this terrestrial humic-like component may receive inputs from biogeochemical reworking of terrestrial DOM and/or particulate organic matter, while the most likely sources for the marine humic-like component are estuarine biological activity and/or microbial reworking of plankton-derived DOM. From the spatial distributions in the bay area as well as their relationships with salinity, two of the non-humic-like components were suggested to be of autochthonous estuarine origin and likely represent biologically labile components. Microbial degradation processes were suggested to be important factors driving the dynamics of another non-humic-like component. This study exemplifies the potential applicability of EEM-PARAFAC in studies of fluorescent DOM dynamics in estuaries.
  • Youhei Yamashita, Eiichiro Tanoue
    Nature Geoscience 1 9 579 - 582 2008年09月 [査読有り][通常論文]
     
    Dissolved organic matter in the ocean constitutes one of the largest pools of reduced carbon on the Earth' s surface, but its role in the global carbon cycle is poorly understood. A large part of this pool is thought to be of marine origin and to be composed of constituents that are resistant to biological degradation. It is not clear how this refractory fraction of the carbon pool is produced in the ocean. Here, we analyse observations of fluorescence intensity and apparent oxygen utilization in the interior of the Pacific Ocean, and find that the two parameters are linearly related in the mesopelagic and abyssal layers. We conclude that fluorescent dissolved organic matter is produced in situ in the ocean interior as organic matter is oxidized biologically, and that it is resistant to biological degradation on centennial to millennial timescales. The rate of production of bio-refractory fluorescent dissolved organic matter in the ocean interior is larger than the riverine input of terrestrial humic substances, suggesting that its in situ production is one of the key processes in maintaining the oceanic pool of refractory dissolved organic matter. © 2008 Macmillan Publishers Limited.
  • Youhei Yamashita, Ayumi Tsukasaki, Tamihito Nishida, Eiichiro Tanoue
    MARINE CHEMISTRY 106 3-4 498 - 509 2007年08月 [査読有り][通常論文]
     
    The vertical and horizontal distribution of fluorescent dissolved organic matter (FDOM), determined by fluorescence intensity at 320 nm excitation and 420 nm emission, were clarified in nine stations on two transects at the Southern Ocean, including a subtropical, subantarctic, polar frontal and Antarctic zone. All vertical profiles of fluorescence intensity showed that levels were lowest in the surface waters, increased with increasing the depth in mid-depth waters (similar to 2000 m), and then stayed within a relatively narrow range from there to the bottom. Such vertical profiles of FDOM were similar to those of nutrients, but were adverse to dissolved oxygen. In water columns below the temperature-minimum subsurface water (dichothermal waters) in the Antarctic zone and below the winter mixed layer in the other zones, we determined the relationships of fluorescence intensity to concentrations of nutrients and apparent oxygen utilization (AOU) over the entire area of the present study, and found significant linear correlations between the levels of fluorescence intensity and nutrient concentrations (r= 0.70 and 0.71 for phosphate and nitrate + nitrite, respectively) and AOU (r= 0.91). From the strong correlation coefficient between fluorescence intensity and AOU, we concluded that FDOM in the Southern Ocean is formed in situ via the biological oxidation of organic matter. The regeneration of the nutrients/consumption of the oxygen/formation of FDOM was active in mid-depth waters. However, the correlations between fluorescence intensities and nutrients and AOU were different in the mid-depth water masses, Subantarctic Mode Water (SAMW), and Antarctic Intermediate Water (AAIW), indicating that the sources of organic matter responsible for FDOM formation were different. A considerable amount of FDOM in the SAMW is thought to be produced by the remineralization of DOM in addition to sinking particulate organic matter, while DOM is less responsible for FDOM formation in the AAIW. (c) 2007 Elsevier B.V. All rights reserved.
  • Yamashita, Y, E. Tanoue
    Research in Organic Geochemistry 21 13 - 22 日本有機地球化学会 2006年 [査読有り][招待有り]
     
    Vertical profiles of chromophoric dissolved organic matter (CDOM) were determined by absorption coefficient and fluorescence intensity in surface waters (〜300m) at 4 stations of the western North Pacific. Vertical characteristics of CDOM were similar irrespective of differences in oceanic regions, on the one hand, i.e., levels were lower and higher in and below the temporal mixed layer, respectively. CDOM levels, on the other hand, were higher in the subarctic than in the subtropical region throughout the water columns. The ratios of fluorescence intensity to absorption coefficient increased with depth, while, the ratios were within a relatively narrow range in water layers at 4 stations, i.e., the temporal mixed layer (4.11±0.53), beneath the temporal mixed layer to 200m (6.06±1.73) and below (8.33±0.94), irrespective of differences of oceanic regions, indicating that the photochemical characteristics of CDOM in the respective water layers were similar irrespective of differences in oceanic regions. Such distributional characteristics suggested that an integrated effect of the in situ production, and photo- and bio-reactivity of CDOM in the upper water columns might give rise to a different quantity but a similar quality of CDOM among the various oceanic regimes.
  • Y Yamashita, E Tanoue
    GEOPHYSICAL RESEARCH LETTERS 31 14 L14032  2004年07月 [査読有り][通常論文]
     
    The horizontal and vertical distribution of marine humic-like fluorophore, namely, chromophoric dissolved organic matter ( CDOM) fluorescence, was surveyed in Ise Bay, Japan. The distribution patterns of salinity and marine humic-like fluorescence intensity suggested that riverine humic-like fluorophore conservatively distributed along with fresh-sea water mixing in surface water at Ise Bay. However, analysis of mixing behavior of riverine CDOM implied that the 25.1 +/- 10.6% in average of bulk marine humic-like fluorescence intensity of Ise Bay surface waters was derived not from a terrestrial origin but an in situ production. A degradation experiment using natural plankton demonstrated the rapid production of marine humic-like fluorophore within a day. The results consistently suggested that in situ production of marine humic-like fluorophore plays an important role in the dynamics of CDOM in coastal environments.
  • 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 2004年05月 [査読有り][通常論文]
     
    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.
  • Y Yamashita, E Tanoue
    ORGANIC GEOCHEMISTRY 35 6 679 - 692 2004年 [査読有り][通常論文]
     
    Total hydrolyzable amino acid and fluorescence properties, using 3-dimensional excitation and emission matrix (3DEEM) spectroscopy of the bulk and four molecular mass fractions (GF/F-0.1 mum, 0.1 mum-10 kDa, 10 kDa-5 kDa and < 5 kDa) of dissolved organic matter (DOM), were determined in the surface (10 m) and deep (1000 m) waters of Sagami Bay, Japan. Among the molecular mass fractions, we found a successive decrease in degradation index (DI) derived from amino acid composition with decreasing molecular mass in both surface and deep waters, indicating that relatively fresh and large molecules become smaller in size along a degradation pathway. In the 3DEEM fluorescence patterns of high molecular mass DOM, only a tryptophan-like fluorophore was observed in the surface water. In contrast, both tryptophan- and tyrosine-like fluorophores were observed in the deep water, indicating that amino acid-containing DOM in the high molecular mass fractions was comprised of protein molecules or least degraded proteins (polypeptides) in surface water, but in deep water other forms of amino acids were dominant even though the molecular mass fractions were of the same size. We hypothesize that degradation products (peptides) form high molecular mass fraction in deep water DOM. (C) 2004 Elsevier Ltd. All rights reserved.
  • 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 2003年09月 [査読有り][通常論文]
     
    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.
  • Y Yamashita, E Tanoue
    MARINE CHEMISTRY 82 3-4 145 - 160 2003年08月 [査読有り][通常論文]
     
    A systematic survey of the concentrations and composition of total hydrolyzable amino acids (THAA) in bulk dissolved organic matter (DOM) was conducted at 11 stations along the 137 degreesE transect from bay to oceanic areas in the northwestern Pacific. Concentrations of THAA and their contributions to dissolved organic carbon were high in the bay and coastal areas, declined toward the oceanic area and decreased with depth in the water columns. From the distribution patterns of the relative abundances of amino acids along the transect, individual amino acids were divided into four groups. One group included tyrosine, valine, isoleucine, phenylalanine, leucine and tryptophan, and was considered to represent easily degradable THAA, while glycine and alanine belonged to a more biorefractory group of THAA. Principal components analysis (PCA) was conducted to quantitatively differentiate patterns of amino acid composition. Amino acid groups based on PCA agreed with the groups classified by distribution patterns, indicating that first principal component scores reflected the degree of degradation of THAA in DOM, and were defined as a degradation index (DI). Two amino acids, glycine and alanine, increased in relative abundance with increasing DI, while valine, isoleucine, phenylalanine and leucine decreased with decreasing DI. The agreement indicated that the degradation process was the key factor controlling the quantity and quality of THAA in bulk DOM. (C) 2003 Elsevier B.V. All rights reserved.
  • Y Yamashita, E Tanoue
    MARINE CHEMISTRY 82 3-4 255 - 271 2003年08月 [査読有り][通常論文]
     
    A systematic survey of the protein-like fluorescence intensities was conducted at 11 stations along the 137 degreesE transect from bay to oceanic areas in the northwestern Pacific using three-dimensional excitation emission matrix (3DEEM) spectroscopy and the concentrations of aromatic amino acids obtained by high-performance liquid chromatography (HPLC) in dissolved organic matter (DOM). The 3DEEM patterns of tyrosine-like and tryptophan-like peaks in protein-like fluorophores were variable across different marine environments. The tyrosine-like peak was observed at all stations and depths, while the tryptophan-like peak was only distinguishable as a clear peak in bay and coastal surface waters. These distribution patterns were similar to those of actual concentrations of tyrosine and tryptophan in total hydrolyzable amino acids (THAA) in DOM. Consequently, the tyrosine- and tryptophan-like fluorescence intensities were interpreted to be correlated to the concentrations of tyrosine and tryptophan, respectively, indicating that tyrosine and tryptophan were responsible for the protein-like fluorescence intensities of DOM. The protein-like fluorescence intensities were also correlated to the concentrations of THAA, suggesting that the dynamics of aromatic amino acids were similar to those of THAA. In addition, protein-like fluorescence intensities may be useful indicators as to the dynamics of not only aromatic amino acids but of THAA in bulk DOM as well. Comparison of the tyrosine- and tryptophan-like fluorescence intensities with the concentrations of aromatic amino acids and THAA suggests that the dissolved amino acids were components of relatively small peptides and not protein molecules. (C) 2003 Elsevier B.V. All rights reserved.
  • 田中剛毅, 山下洋平, 中林和重
    植物工場学会誌 13 1 21 - 28 JAPANESE SOCIETY OF AGRICULTURAL, BIOLOGICAL AND ENVIRONMENTAL ENGINEERS AND SCIENTISTS 2001年 [査読有り][通常論文]
     
    The concentration of dissolved oxygen in a culture solution is one of the most important environmental factors affecting the growth of the tops and roots of plants under hydroponic culture. In particular, the roots of tomatos readily absorb oxygen in the solution, and are thus sensitive to the concentration of dissolved oxygen. Thus, the effects of dissolved oxygen concentration on the morphology and growth of roots, water uptake rate and nutrient uptake were investigated. In hydroponic culture, the supply of dissolved oxygen is essential. Previously, we reported that supersaturation of dissolved oxygen in culture solution promoted the uptake of phosphate. In the present study, we found that supersaturation of dissolved oxygen in culture solution low in nitrogen and phosphate led to an increase in plant height, fresh weight of stems and leaves and root weight. It also increased the phosphorus concentration in the sap, stems, leaves and fruits of tomato plants.

書籍

  • 海洋生命系のダイナミクス第三巻「海洋生物の連鎖-生命は海でそう連環しているか」
    山下 洋平 (担当:共著範囲:海洋における有機物の代謝-生物体有機物から非生物態有機物へ-)
    東海大学出版会 2005年

その他活動・業績

受賞

  • 2015年03月 北海道大学 研究総長賞(奨励賞)
     
    受賞者: 山下 洋平
  • 2009年04月 日本海洋学会 岡田賞
     
    受賞者: 山下 洋平
  • 2005年07月 日本有機地球化学会 奨励賞(田口賞)
     
    受賞者: 山下 洋平

共同研究・競争的資金等の研究課題

教育活動情報

主要な担当授業

  • 化学海洋学特論
    開講年度 : 2021年
    課程区分 : 修士課程
    開講学部 : 環境科学院
    キーワード : 海洋生物地球化学、化学海洋学、物質循環、データ解析、有機物、化学トレーサー、微量元素 Ocean biogeochemistry, Chemical Oceanography, Material cycle, Data analysis, organic matter, chemical tracers
  • 海洋生物地球化学特論
    開講年度 : 2021年
    課程区分 : 修士課程
    開講学部 : 環境科学院
    キーワード : Ocean biogeochemistry, nutrient cycle, trace metal cycle, primary production, non-living organic materials, chemical tracers
  • 環境と人間
    開講年度 : 2021年
    課程区分 : 学士課程
    開講学部 : 全学教育
    キーワード : 大気、海洋、生態系、気候変化、地球環境問題、地球温暖化、成層圏オゾン層破壊、大気汚染
  • 地球環境学
    開講年度 : 2021年
    課程区分 : 学士課程
    開講学部 : 理学部
    キーワード : 気候変化、炭素循環、古環境、氷期・間氷期、人類環境、生物圏
  • 一般教育演習(フレッシュマンセミナー)
    開講年度 : 2021年
    課程区分 : 学士課程
    開講学部 : 全学教育
    キーワード : 北極域、地球環境、温暖化、気候変動、炭素循環、水循環、人間活動、永久凍土、北極域航路、国際政治
  • 科学・技術の世界
    開講年度 : 2021年
    課程区分 : 学士課程
    開講学部 : 全学教育
    キーワード : 地球環境,気候,古気候,炭素循環,有機物,CO2, 海洋,大気,陸地,植生

大学運営

委員歴

  • 2020年12月 - 現在   Joint PICES/ICES   Working Group on Ocean Negative Carbon Emissions (ONCE)
  • 2018年04月 - 現在   日本学術会議   IMBeR小委員会委員
  • 2015年05月 - 現在   Scientific Reports   Editorial Board Member
  • 2012年10月 - 現在   生物地球化学研究会   将来計画委員
  • 2017年06月 - 2019年05月   Joint PICES/ICES   Working Group on Climate Change and Biologically-driven Ocean Carbon Sequestration
  • 2013年04月 - 2016年12月   Limnology   Associate Editor
  • 2010年08月 - 2015年05月   有機地球化学会   ニュースレター編集委員
  • 2012年07月 - 2013年03月   日本海洋学会   将来構想委員会委員


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