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研究者情報

マスター

アカウント（マスター）

  • 氏名

    芳村　毅(ヨシムラ　タケシ), ヨシムラ　タケシ

所属（マスター）

  • 水産科学研究院　海洋生物資源科学部門　海洋環境科学分野

所属（マスター）

  • 水産科学研究院　海洋生物資源科学部門　海洋環境科学分野

独自項目

syllabus

  • 2021, 海洋生物環境学基礎論, Fundamental Course in Marine Biological Processes, 修士課程, 環境科学院, 海洋学、海洋生物科学 Oceanography、Marine Biological Science
  • 2021, 海洋生物圏環境科学特論Ⅰ, Advanced Course in Marine Biosphere Science I (Marine Environmental Biogeochemistry), 修士課程, 環境科学院, 海洋生物地球化学、微量金属、栄養塩、栄養塩摂取機構、植物プランクトン、基礎生産 Marine Biogeochemistry, Trace Metals, Nutrients, Biological Uptake of Nutrients, Phytoplankton, Primary Production
  • 2021, 海洋学入門, Introduction to Oceanography, 学士課程, 水産学部, 海底地形、海洋大循環、熱/物質循環、プランクトン、海底資源、海洋観測、海洋法、海洋生物資源の保全
  • 2021, 一般教育演習(ﾌﾚｯｼｭﾏﾝｾﾐﾅｰ), Freshman Seminar, 学士課程, 全学教育, 海洋環境、水圏生態、水産資源、海洋物理学、海洋化学、プランクトン、イカ、耳石
  • 2021, 海洋基礎生産学, Marine Primary Production Ecology, 学士課程, 水産学部, 光合成、植物プランクトン、食物網、親生物元素循環、栄養塩
  • 2021, 化学海洋学, Chemical Oceanography, 学士課程, 水産学部, 栄養塩、基礎生産、物質循環、炭素循環、窒素循環、植物プランクトン、微量金属
  • 2021, 海洋保全学, Marine Environment Conservation, 学士課程, 水産学部, SDGs，持続可能な開発目標，最大持続生産，保全生物学，水産資源，資源評価，栄養塩環境，海洋酸性化，バリューチェーン，漁業管理，持続可能性と海洋漁業
  • 2021, 分析化学, Analytical Chemistry, 学士課程, 水産学部, データ処理，環境分析，海水分析，底質分析
  • 2021, 海洋生物科学実験Ⅰ, Marine Science Biology I - Laboratory, 学士課程, 水産学部, 分析化学，環境分析，基礎?産，岩礁海岸の生物相
  • 2021, 教科教育法（水産I）, Teaching Method of School Subjects(Fishery Ⅰ), 学士課程, 教育学部

researchmap

プロフィール情報

学位

  • 博士（水産科学）（北海道大学）

プロフィール情報

  • 姓

    芳村, ヨシムラ

  • 名

    毅, タケシ

  • ID各種

    M-1778-2017, 201701004420982913

対象リソース

業績リスト

研究キーワード

  • 海洋酸性化   生元素   植物プランクトン   栄養塩   リン   海洋   

研究分野

  • 環境・農学 / 環境動態解析

論文

  • Andrew Kalani Carlson, Takeshi Yoshimura, Isao Kudo
    Journal of Phycology 2024年 [査読有り][通常論文]
     
    Macroalgae influence local and global biogeochemical cycles through their production of dissolved organic carbon (DOC). Yet, data remain scarce and annualized estimates are typically based on high growth periods without considering seasonal variability. Although the mechanisms of active exudation and passive leakage need clarifying, ecophysiological stress is known to enhance DOC release. Therefore, DOC leakage from seasonally senescent macroalgae may be overlooked. This study focuses on the annual kelp Saccharina japonica var. religiosa (class Phaeophyceae) from Oshoro Bay, Hokkaido, Japan. Three years (2020–2022) of seasonal data were collected and analyzed, with least squares mean DOC release rates established for kelp (n = 88) across 16 incubation experiments (t ≥ 4 d, DOC samples ≥1 · d−1) under different photosynthetically active radiation (PAR) treatments (200, 400, 1200, or 1500 μmol photons · m−2 · s−1). Differences in PAR, dry weight biomass (g DW), sea surface temperature, or salinity could not explain DOC release-rate variability, which was high between individual kelp. Instead, there were significant intra-annual differences, with mean DOC release rates (mg C · g−1 DW · d−1 ± standard error between n kelp) higher during the autumn “late decay” period (0.71 ± 0.10, n = 27) compared to the winter “early growth” period (0.14 ± 0.025, n = 10) and summer “early decay” period (0.25 ± 0.050, n = 24). This relationship between seasonal senescence and macroalgal DOC release is further evidence that long-term, place-based studies of DOC dynamics are essential and that global extrapolations are premature.
  • Akari Aizawa, Yuka Watanabe, Kaori Hashioka, Aya Kadoya, Satoru Suzuki, Takeshi Yoshimura, Isao Kudo
    Regional Studies in Marine Science 63 2023年10月 [査読有り]
     
    Nitrification, in which ammonium is oxidized to nitrate via nitrite, is a regeneration process of bioavailable nitrogen. Ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) carry out the ammonium oxidation. Here we measured the ammonium oxidation rates (AORs), distinguishing those of AOA (VA) and AOB (VB), for two years in Mutsu Bay, a subarctic semi-enclosed bay. Nitrate and ammonium were depleted in the upper layer in summer, but a high concentration of nitrate was observed at the bottom (below 40 m), of up to 7.8 μM with a strong vertical gradient within several meters of the seafloor. A High AOR of up to 0.17 μM d−1 were observed in the bottom waters. The AOA were responsible for most of the total AOR, but the VB exceeded the VA just above the seafloor (0.3 m) in September. Removal of the suspended solids (SS) did not change the AOA's oxidation activity but eliminated the AOB's. This finding suggested that AOA were free-living whereas AOB were attached to the SS's surface. It is also suggested that the sediment was this SS's source. When ammonia mono-oxygenase, amoA, of AOA and AOB, was quantified, the copy number of AOA-amoA was an order of magnitude higher than that of AOB-amoA throughout the study period. A significant correlation (p < 0.05) was found between the AOB-amoA copy number and VB whereas no correlation was found between the AOA-amoA copy number and VA suggesting the AOA-amoA copy number did not always represent active cells. Ammonia oxidation was occurring near the bottom waters of Mutsu Bay during the summer, and it was accompanied by a dynamic balance reflecting the difference in the AOA's and AOB's lifestyles, free-living and particle-associated, respectively.
  • 坪野 考樹, 津旨 大輔, 三角 和弘, 芳村 毅
    土木学会論文集B3（海洋開発） 78 2 I_625 - I_630 2022年05月 [査読有り]
  • 藤井賢彦, 芳村毅, 小埜恒夫
    月刊海洋 53 6 2021年 [査読無し]
  • 村田昌彦, 青山道夫, 青山道夫, チョン千香子, 三浦勉, 藤井武史, 光田均, 北尾隆, 笹野大輔, 中野俊也, 永井直樹, 児玉武稔, 葛西広海, 清本容子, 瀬藤聡, 小埜恒夫, 横川真一朗, 有井康博, 曽根知実, 石川賀子, 芳村毅, 内田裕, 田中辰弥, 粥川洋平, 脇田昌英
    海の研究 29 5 2020年 [査読有り][通常論文]
  • Takashi Ishimaru, Yutaka Tateda, Daisuke Tsumune, Michio Aoyama, Yasunori Hamajima, Nobue Kasamatsu, Manabu Yamada, Takeshi Yoshimura, Takuji Mizuno, Jota Kanda
    Journal of Environmental Radioactivity 203 200 - 209 2019年07月 [査読有り]
     
    The cesium depuration mechanisms were studied in Japanese rockfish Sebastes cheni off Fukushima, in which the radiocesium level remains higher than in other teleost. Samples were collected approximately 5 km south from the nuclear power plant during 2014-2016, and the 137Cs concentrations in fish, stomach content and prey species were measured. The stable cesium content in fish was also analyzed and compared with fish age which was determined by annual ring analysis in otoliths. The 137Cs concentrations in the dominant prey species, mysids and brown shrimp, were several Bq kg-w.w.-1; indicating that transfer via the food chain was substantial compared to that from seawater during the study period. The 137Cs concentrations in S. cheni decreased from 2014 to 2016 due to the metabolic excretion and the rate of decrease in its diet. Biokinetic model analyses confirmed the slower turnover of stable cesium in S. cheni, represented as a biological half-life (Tb1/2) of 140-215 d, and was associated with stable Cs levels in food of 5-7 ng g-w.w.-1. The 137Cs levels in S. cheni were also simulated, which showed that the 137Cs depuration in fish exposed to the initial contaminated plume in 2011 resulted from slower metabolic excretion, while the 137Cs levels in fish born after 2012 could be regarded as equilibrated with the environmental levels of 137Cs. Furthermore, the simulation results suggest that 137Cs depuration in S. cheni population was also caused by the alternation of generation, which can be substantial by the addition of new year class population hatched after 2012 that were not contaminated by the initial contaminated plume from the 2011 accident.
  • Takeshi Yoshimura
    Ecotoxicology and Environmental Safety 169 640 - 644 2019年03月01日 [査読有り][通常論文]
     
    Chlorination is the most common method to control water qualities, in some case on-site outdoor measurements are required to measure easily-decaying residual chlorine concentration appropriately without delay. In this study sunlight-induced unexpected colour development (UCD) of N, N-diethyl-p-phenylenediamine (DPD) colorimetric measurement was studied under several sun exposure conditions. The colour development level was evaluated with reference to chlorine concentration (mg/L) and relationships between colour development rate (mg/L min) and intensities of solar were investigated. UCD was found to be related to both exposure intensity and time. By means of exposure experiment under specific wavelength of ultraviolet (UV), it was confirmed that both middle and short wavelength of UV radiation being responsible for such an unexpected measurement. Consequently, a simple device was designed using three commercially available anti-UV films, one of which could effectively prevent the UCD from direct sun exposure.
  • Koji Sugie, Takeshi Yoshimura, Masahide Wakita
    Limnology and Oceanography 2018年09月 [査読有り][通常論文]
  • 芳村毅, 小埜恒夫, 藤井賢彦
    月刊海洋 50 5 195 - 199 2018年 [査読無し][通常論文]
  • Takeshi Yoshimura, Jun Nishioka, Hiroshi Ogawa, Atsushi Tsuda
    JOURNAL OF MARINE SYSTEMS 177 1 - 7 2018年01月 [査読有り][通常論文]
     
    Phosphorus (P) is an essential element for all organisms and thus the P cycle plays a key role in determining the dynamics of lower trophic levels in marine ecosystems. P in seawater occurs conceptually in particulate and dissolved organic and inorganic (POP, PIP, DOP, and DIP, respectively) pools and clarification of the dynamics in these P pools is the basis to assess the biogeochemical cycle of P. Despite its importance, behaviors of each P pool with phytoplankton dynamics have not been fully examined. We measured the four operationally defined P pools (POPop, PIPop, DOPop, and SRP) during an iron-induced phytoplankton bloom (as part of the subarctic ecosystem response to iron enrichment study (SERIES)) in the eastern subarctic Pacific in summer 2002. During our observations of the iron-enriched patch from day 15 to day 26 after the iron infusion, chlorophyll-alpha concentration in the surface layer decreased from 6.3 to 1.2 mu g L-1, indicating the peak through decline phase of the phytoplankton bloom. At the bloom peak, P was partitioned into POPop, PIPop, and DOPop in proportions of 60, 27, and 13%, respectively. While chlorophyll-alpha and POPop showed similar temporal variations during the declining phase, PIPop showed a different peak timing with a 2 day delay compared to POPop, resulting in a rapid change in the relative proportion of PIPop to total particulate P (TPP = POPop + PIPop) at the peak (25%) and during the declining phase of the bloom (50%). A part of POPop was replaced by PIPop just after slowing down of phytoplankton growth. This process may have a significant role in the subsequent regeneration of P. We conclude that measurement of TPP alone is insufficient to show the interaction between P and phytoplankton dynamics and fractionation of TPP into POPop and PIPop provides useful insights to clarify the biogeochemical cycle of P.
  • Hisashi Endo, Koji Sugie, Takeshi Yoshimura, Koji Suzuki
    PLOS ONE 11 4 2016年04月 [査読有り][通常論文]
     
    Next-generation sequencing (NGS) technologies have enabled us to determine phytoplankton community compositions at high resolution. However, few studies have adopted this approach to assess the responses of natural phytoplankton communities to environmental change. Here, we report the impact of different CO2 levels on spring diatoms in the Oyashio region of the western North Pacific as estimated by NGS of the diatom-specific rbcL gene (DNA), which encodes the large subunit of RubisCO. We also examined the abundance and composition of rbcL transcripts (cDNA) in diatoms to assess their physiological responses to changing CO2 levels. A short-term (3-day) incubation experiment was carried out on-deck using surface Oyashio waters under different pCO(2) levels (180, 350, 750, and 1000 mu atm) in May 2011. During the incubation, the transcript abundance of the diatom-specific rbcL gene decreased with an increase in seawater pCO(2) levels. These results suggest that CO2 fixation capacity of diatoms decreased rapidly under elevated CO2 levels. In the high CO2 treatments (750 and 1000 mu atm), diversity of diatom-specific rbcL gene and its transcripts decreased relative to the control treatment (350 mu atm), as well as contributions of Chaetocerataceae, Thalassiosiraceae, and Fragilariaceae to the total population, but the contributions of Bacillariaceae increased. In the low CO2 treatment, contributions of Bacillariaceae also increased together with other eukaryotes. These suggest that changes in CO2 levels can alter the community composition of spring diatoms in the Oyashio region. Overall, the NGS technology provided us a deeper understanding of the response of diatoms to changes in CO2 levels in terms of their community composition, diversity, and photosynthetic physiology.
  • Koji Sugie, Takeshi Yoshimura
    ICES JOURNAL OF MARINE SCIENCE 73 3 680 - 692 2016年02月 [査読有り][通常論文]
     
    Iron availability in seawater, namely the concentration of dissolved inorganic iron ([Fe']), is affected by changes in pH. Such changes in the availability of iron should be taken into account when investigating the effects of ocean acidification on phytoplankton ecophysiology because iron plays a key role in phytoplankton metabolism. However, changes in iron availability in response to changes in ocean acidity are difficult to quantify specifically using natural seawater because these factors change simultaneously. In the present study, the availability of iron and carbonate chemistry were manipulated individually and simultaneously in the laboratory to examine the effect of each factor on phytoplankton ecophysiology. The effects of various pCO(2) conditions (similar to 390, similar to 600, and similar to 800 mu atm) on the growth, cell size, and elemental stoichiometry (carbon [C], nitrogen [N], phosphorus [P], and silicon [Si]) of the diatom Thalassiosira weissflogii under high iron ([Fe'] = similar to 240 pmol l(-1)) and low iron ([Fe'] = similar to 24 pmol l(-1)) conditions were investigated. Cell volume decreased with increasing pCO(2), whereas intracellular C, N, and P concentrations increased with increasing pCO(2) only under high iron conditions. Si:C, Si:N, and Si:P ratios decreased with increasing pCO(2). It reflects higher production of net C, N, and P with no corresponding change in net Si production under high pCO(2) and high iron conditions. In contrast, significant linear relationships between measured parameters and pCO(2) were rarely detected under low iron conditions. We conclude that the increasing CO2 levels could affect on the biogeochemical cycling of bioelements selectively under the iron-replete conditions in the coastal ecosystems.
  • 芳村 毅, 柳川 敏治, 藤原 淳, 村井 圭次, 野方 靖行, 古田 岳志, 小林 卓也
    Ｓｅｓｓｉｌｅ Ｏｒｇａｎｉｓｍｓ 33 2 38 - 43 日本付着生物学会 2016年 [査読有り][通常論文]
     
    Chlorination of seawater is a common practice to prevent biofouling on maritime structures. To enable control and reduce the environmental impacts of chlorine we measure the residual chlorine in wastewaters. In routine measurements of residual chlorine using N,N-diethyl-p-phenylenediamine (DPD), we found significant color development by DPD in non-chlorinated waters when large-sized phytoplankton appeared in high abundance. Through screening 27 microalgal isolates in laboratory culture, the diatom Coscinodiscus wailesii was shown to be the only species that causes the DPD color development in the procedure for total residual chlorine, although not for free residual chlorine, despite no chlorine additions to the culture medium. The result was also confirmed in field samples. Since two other species of genus Coscinodiscus did not show a similar result, this is considered to be highly species-specific. The color development was prevented by removing the C. wailesii cells using a 100 µm mesh cell strainer. Coscinodiscus wailesii did not react with 3,3′-dimethylbenzidine (o-tolidine), an alternative reagent to measure residual chlorine. We conclude that C. wailesii produces some materials which can oxidize DPD to produce the red color, and C. wailesii blooms in the field interfere with measurements of residual chlorine concentrations in natural and waste seawater.
  • Isao Kudo, Takatsugu Hisatoku, Takeshi Yoshimura, Yoshiaki Maita
    ESTUARINE COASTAL AND SHELF SCIENCE 158 12 - 19 2015年06月 [査読有り][通常論文]
     
    Primary production is supported by utilization of several forms of nitrogen (N), such as nitrate, ammonium, and urea. Nevertheless, only few studies have measured the concentration and uptake of urea despite its importance as a nitrogenous nutrient for phytoplankton. We measured primary productivity monthly at four depths within the euphotic zone using a clean technique and the C-13 method by a 24 h in situ mooring incubation over a year in Funka Bay, a subarctic coastal area in Japan, to make better updated estimates (re-evaluation) of annual primary production. Nitrogenous (N) nutrient assimilation rates (nitrate, ammonium and urea) were also measured to elucidate the relative contributions of these nutrients to autotrophic production and to distinguish between new and regenerated production. The estimated annual primary production was 164 g C m(-2), which was 40-60% higher than the previously reported values in the bay. Use of a clean technique and more frequent measurement during the spring bloom may have contributed to the higher rates. The production during the spring bloom was 56.5 g C m(-2), accounting for 35% of the annual production. The maximum daily productivity occurred in the bloom at 1.4 g C m(-2) d(-1), which is one of the highest values among the world embayments. The annual primary production in the bay was classified as mesotrophic state based on the classification by Cloern et al. (2014). The assimilation rate of nitrate was maximal at 54 nmol N L-1 h(-1) during the bloom. During the post-bloom periods with nitrate depleted conditions, assimilation rates of ammonium and urea increased and accounted for up to 85% of the total N assimilation. The assimilation rate of urea was almost comparable to that of ammonium throughout the year. Taking urea into account, the f-ratio ranged from 0.15 under the nitrate-depleted conditions to 0.8 under the spring bloom conditions. These ratios were overestimated by 50% and 10%, respectively, if urea uptake was eliminated. We provide a valuable data for the primary production dataset in the world's ecosystems, and show that urea plays an important role in supporting regenerated production during late spring and summer. (C) 2015 Elsevier Ltd. All rights reserved.
  • H. Endo, K. Sugie, T. Yoshimura, K. Suzuki
    BIOGEOSCIENCES 12 7 2247 - 2259 2015年 [査読有り][通常論文]
     
    Iron (Fe) can limit phytoplankton productivity in approximately 40% of the global ocean, including in high-nutrient, low-chlorophyll (HNLC) waters. However, there is little information available on the impact of CO2-induced seawater acidification on natural phytoplankton assemblages in HNLC regions. We therefore conducted an on-deck experiment manipulating CO2 and Fe using Fe-deficient Bering Sea water during the summer of 2009. The concentrations of CO2 in the incubation bottles were set at 380 and 600 ppm in the non-Fe-added (control) bottles and 180, 380, 600, and 1000 ppm in the Fe-added bottles. The phytoplankton assemblages were primarily composed of diatoms followed by haptophytes in all incubation bottles as estimated by pigment signatures throughout the 5-day (control) or 6-day (Fe-added treatment) incubation period. At the end of incubation, the relative contribution of diatoms to chlorophyll a biomass was significantly higher in the 380 ppm CO2 treatment than in the 600 ppm treatment in the controls, whereas minimal changes were found in the Fe-added treatments. These results indicate that, under Fe-deficient conditions, the growth of diatoms could be negatively affected by the increase in CO2 availability. To further support this finding, we estimated the expression and phylogeny of rbcL (which encodes the large subunit of RuBisCO) mRNA in diatoms by quantitative reverse transcription polymerase chain reaction (PCR) and clone library techniques, respectively. Interestingly, regardless of Fe availability, the transcript abundance of rbcL decreased in the high CO2 treatments (600 and 1000 ppm). The present study suggests that the projected future increase in seawater pCO(2) could reduce the RuBisCO transcription of diatoms, resulting in a decrease in primary productivity and a shift in the food web structure of the Bering Sea.
  • Takeshi Yoshimura, Koji Sugie, Hisashi Endo, Koji Suzuki, Jun Nishioka, Tsuneo Ono
    DEEP-SEA RESEARCH PART I-OCEANOGRAPHIC RESEARCH PAPERS 94 1 - 14 2014年12月 [査読有り][通常論文]
     
    Increase in seawater pCO(2) and the corresponding decrease in pH caused by the increasing atmospheric CO2 concentration (i.e., ocean acidification) may affect organic matter production by phytoplankton communities. Organic matter production forms the basis of marine food webs and plays a crucial role in oceanic CO2 uptake through the biological carbon pump, and hence will potentially affect future marine ecosystem dynamics. However, responses of organic matter production in open ocean plankton ecosystems to CO2 increase have not been fully examined. We conducted on-deck microcosm experiments using high nutrient, low chlorophyll (HNLC) waters in the western subarctic Pacific and oceanic Bering Sea basin in summer 2008 and 2009, respectively, to examine the impacts of elevated CO2 on particulate and dissolved organic matter (i.e., POM and DOM, respectively) production. Iron deficient natural plankton communities were incubated for 7-14 days under multiple CO2 levels with and without iron enrichments (hereafter +Fe and -Fe treatments, respectively). By combining with our previous experiments at two sites, we created a comprehensive dataset on responses of organic matter production to CO2 increase during macronutrient replete conditions in HNLC waters. Significant differences in net particulate organic carbon production among CO2 treatments were observed only in the -Fe treatments, whereas that in net dissolved organic carbon production were mainly observed in the +Fe treatments, suggesting that CO2 may affect different processes depending on the Fe nutritional status. However, impacts of CO2 were not consistent among experiments and were much smaller than the consistent positive effects of Fe enrichment. In contrast, no significant differences among the CO2 treatments were observed for organic carbon partitioning into POM and DOM, and carbon to nitrogen ratio of net produced POM. We conclude that CO2 does not play a primary role, but could have secondary effects on controlling the organic matter production under macronutrient replete conditions in HNLC waters. On the other hand, in a nutrient-depleted, declining phase of the phytoplankton bloom induced by Fe enrichment, carbon overconsumption was found in an experiment with elevated CO2 conditions suggesting that CO2 impacts might become more significant in such environments. (C) 2014 Elsevier Ltd. All rights reserved.
  • Takeshi Yoshimura, Jun Nishioka, Hiroshi Ogawa, Kenshi Kuma, Hiroaki Saito, Atsushi Tsuda
    MARINE CHEMISTRY 165 46 - 54 2014年10月 [査読有り][通常論文]
     
    A significant part of phosphorus (P) in seawater is found in the dissolved organic matter (DOM) fraction as DOP, which plays a key role in the marine biogeochemical cycle of P. The DOM pool size changes with biological activity, but DOP production and decomposition processes, unlike carbon (C) and nitrogen, have been only infrequently studied during phytoplankton blooms when rapid production and accumulation of organic matter occurs. We observed the DOP dynamics during two phytoplankton blooms dominated by centric diatoms, the first induced by an in situ mesoscale iron enrichment experiment in the western subarctic Pacific in summer 2001 (SEEDS) and the second that occurred naturally in spring 2003 in the Oyashio region. DOP concentration increased with the buildup of phytoplankton biomass with DOP/chlorophyll-a production ratios (mol/g) of 0.0027 +/- 0.0004 and 0.0044 +/- 0.0010 for the SEEDS and Oyashio blooms, respectively. During the SEEDS and Oyashio blooms the amount of net DOP production corresponded to (4.9 +/- 0.7) % and (4.5 +/- 0.6) % of the consumed soluble reactive P, and (5.5 +/- 0.8) % and (13 +/- 3) % of the newly accumulated organic P was partitioned into DOP, respectively. Seawater culture bottle experiments showed that newly produced DOP during the bloom development was decomposed by free living bacteria over a time scale of a month even under soluble reactive P available conditions. C:P for the decomposed DOM (molar ratio of 78-88) showed a similar value to in situ net produced DOM (66) and POM (83) but much lower than that for the bulk DOM (395-706), suggesting that the composition of the freshly produced DOM with high lability differs significantly from the bulk DOM. (C) 2014 Elsevier B.V. All rights reserved.
  • H. Endo, K. Sugie, T. Yoshimura, K. Suzuki
    Biogeosciences Discussions 11 12 18105  Copernicus {GmbH} 2014年 [査読無し][通常論文]
  • Takeshi Yoshimura, Koji Suzuki, Hiroshi Kiyosawa, Tsuneo Ono, Hiroshi Hattori, Kenshi Kuma, Jun Nishioka
    JOURNAL OF OCEANOGRAPHY 69 5 601 - 618 2013年10月 [査読有り][通常論文]
     
    Response of phytoplankton to increasing CO2 in seawater in terms of physiology and ecology is key to predicting changes in marine ecosystems. However, responses of natural plankton communities especially in the open ocean to higher CO2 levels have not been fully examined. We conducted CO2 manipulation experiments in the Bering Sea and the central subarctic Pacific, known as high nutrient and low chlorophyll regions, in summer 2007 to investigate the response of organic matter production in iron-deficient plankton communities to CO2 increases. During the 14-day incubations of surface waters with natural plankton assemblages in microcosms under multiple pCO(2) levels, the dynamics of particulate organic carbon (POC) and nitrogen (PN), and dissolved organic carbon (DOC) and phosphorus (DOP) were examined with the plankton community compositions. In the Bering site, net production of POC, PN, and DOP relative to net chlorophyll-a production decreased with increasing pCO(2). While net produced POC:PN did not show any CO2-related variations, net produced DOC:DOP increased with increasing pCO(2). On the other hand, no apparent trends for these parameters were observed in the Pacific site. The contrasting results observed were probably due to the different plankton community compositions between the two sites, with plankton biomass dominated by large-sized diatoms in the Bering Sea versus ultra-eukaryotes in the Pacific Ocean. We conclude that the quantity and quality of the production of particulate and dissolved organic matter may be altered under future elevated CO2 environments in some iron-deficient ecosystems, while the impacts may be negligible in some systems.
  • Koji Sugie, Takeshi Yoshimura
    JOURNAL OF PHYCOLOGY 49 3 475 - 488 2013年06月 [査読有り][通常論文]
     
    Partial pressure of CO2 (pCO2) and iron availability in seawater show corresponding changes due to biological and anthropogenic activities. The simultaneous change in these factors precludes an understanding of their independent effects on the ecophysiology of phytoplankton. In addition, there is a lack of data regarding the interactive effects of these factors on phytoplankton cellular stoichiometry, which is a key driving factor for the biogeochemical cycling of oceanic nutrients. Here, we investigated the effects of pCO2 and iron availability on the elemental composition (C, N, P, and Si) of the diatom Pseudo-nitzschia pseudodelicatissima (Hasle) Hasle by dilute batch cultures under 4 pCO2 (similar to 200, similar to 380, similar to 600, and similar to 800atm) and five dissolved inorganic iron (Fe; similar to 5, similar to 10, similar to 20, similar to 50, and similar to 100pmol center dot L-1) conditions. Our experimental procedure successfully overcame the problems associated with simultaneous changes in pCO2 and Fe by independently manipulating carbonate chemistry and iron speciation, which allowed us to evaluate the individual effects of pCO2 and iron availability. We found that the C:N ratio decreased significantly only with an increase in Fe, whereas the C:P ratio increased significantly only with an increase in pCO2. Both Si:C and Si:N ratios decreased with increasing pCO2 and Fe. Our results indicate that changes in pCO2 and iron availability could influence the biogeochemical cycling of nutrients in future oceans with high- CO2 levels, and, similarly, during the time course of phytoplankton blooms. Moreover, pCO2 and iron availability may also have affected oceanic nutrient biogeochemistry in the past, as these conditions have changed markedly over the Earth's history.
  • Takeshi Yoshimura, Shigeru Okada, Masaki Honda
    BIORESOURCE TECHNOLOGY 133 232 - 239 2013年04月 [査読有り][通常論文]
     
    Specific growth rates and hydrocarbon contents of Botryococcus braunii strain Showa were measured under a wide range of CO2, salinity, temperature, and irradiance conditions. The bubbling CO2 concentration of 0.2-5% and no addition of salinity were favorable conditions for growth. The strain cannot grow at 5 degrees C and above 35 degrees C under any irradiance levels. Maximum specific growth rate of 0.5 day(-1) (doubling time of 1.4 days), the highest value reported for B. braunii in the past studies, was observed at 30 degrees C and 850 mu mol photons m(-2) s(-1). Since hydrocarbon productivity, shown as the product of hydrocarbon content and specific growth rate, increased with the increasing specific growth rate, we conclude that more efficient hydrocarbon production by the mass culture of strain Showa can be achieved by maintaining higher specific growth rate based on the culture conditions presented in this study. (C) 2013 Elsevier Ltd. All rights reserved.
  • Takeshi Yoshimura
    LIMNOLOGY AND OCEANOGRAPHY-METHODS 11 APR 239 - 246 2013年04月 [査読有り][通常論文]
     
    Reference materials (RMs) are essential to ensure comparability of oceanographic chemical measurements. Since dissolved organic matter (DOM) plays a significant role in bioactive element cycles such as carbon (C), nitrogen (N), and phosphorus (P), measurements of dissolved organic C, N, and P (DOC, DON, and DOP) have been paid much attention. Although RMs that target DOC and DON analyses have been developed, no comparable studies have been conducted for DOP analysis. This study examined the selection of the most appropriate bottle to store DOP samples to develop an RM for DOP. Natural seawater samples were stored in bottles made from various plastic materials and glass, and the temporal change in DOP and DOC concentrations were measured during a storage period of months to a year in the dark under 25 degrees C. I found that severe contaminations of DOP and/or DOC occurred in some bottle types. Whereas DOP concentrations were unchanged in many bottle types, the concentrations of both DOP and DOC were stable only in glass and perfluoroalkoxy (PFA) bottles. I conclude that these two bottle types have potential as an appropriate sample bottle for RMs of DOP, this is because colloids formed via DOC contamination during storage can potentially bond with phosphate in the sample and thus inorganic phosphate may be measured as DOP contamination.
  • K. Sugie, H. Endo, K. Suzuki, J. Nishioka, H. Kiyosawa, T. Yoshimura
    Biogeosciences Discussions 10 3 4331  Copernicus {GmbH} 2013年03月 [査読無し][通常論文]
  • Hideki Kanda, Peng Li, Takeshi Yoshimura, Shigeru Okada
    FUEL 105 535 - 539 2013年03月 [査読有り][通常論文]
     
    We propose the use of liquid dimethyl ether (DME) as a solvent for extracting hydrocarbons and lipids from Botryococcus braunii Race B paste. The extraction yields and main elements of the extracts obtained from the B. braunii by our technique is nearly the same as by hexane Soxhlet extraction on dried mass. A qualitative analysis by silica gel thin layer chromatography suggested that DME also effectively extracted the specific hydrocarbons, botryococcenes from the wet B. braunii cells. Gas chromatography/mass spectrometry of DME extracts indicated the presence of a large amount of C32-C34 botryococcenes-the main components of B. braunii Race B. In contrast, hexane extracted not only C32-C34, but also small amounts of C30 and C31 botryococcenes-the minor components. The result shows that extraction differences between DME and hexane are not important. (C) 2012 Elsevier Ltd. All rights reserved.
  • K. Sugie, H. Endo, K. Suzuki, J. Nishioka, H. Kiyosawa, T. Yoshimura
    Biogeosciences 10 10 6309 - 6321 2013年 [査読有り][通常論文]
     
    Little is known concerning the effect of CO2 on phytoplankton ecophysiological processes under nutrient and trace element-limited conditions, because most CO2 manipulation experiments have been conducted under elements-replete conditions. To investigate the effects of CO2 and iron availability on phytoplankton ecophysiology, we conducted an experiment in September 2009 using a phytoplankton community in the iron limited, high-nutrient, low-chlorophyll (HNLC) region of the Bering Sea basin . Carbonate chemistry was controlled by the bubbling of the several levels of CO2 concentration (180, 380, 600, and 1000 ppm) controlled air, and two iron conditions were established, one with and one without the addition of inorganic iron. We demonstrated that in the iron-limited control conditions, the specific growth rate and the maximum photochemical quantum efficiency (< i> F< /i> v/< i> F< /i> m) of photosystem (PS) II decreased with increasing CO2 levels, suggesting a further decrease in iron bioavailability under the high-CO2 conditions. In addition, biogenic silica to particulate nitrogen and biogenic silica to particulate organic carbon ratios increased from 2.65 to 3.75 and 0.39 to 0.50, respectively, with an increase in the CO2 level in the iron-limited controls. By contrast, the specific growth rate, < i> F< /i> v/< i> F< /i> m values and elemental compositions in the iron-added treatments did not change in response to the CO2 variations, indicating that the addition of iron canceled out the effect of the modulation of iron bioavailability due to the change in carbonate chemistry. Our results suggest that high-CO2 conditions can alter the biogeochemical cycling of nutrients through decreasing iron bioavailability in the iron-limited HNLC regions in the future. © Author(s) 2013.
  • K. Sugie, H. Endo, K. Suzuki, J. Nishioka, H. Kiyosawa, T. Yoshimura
    BIOGEOSCIENCES 10 10 6309 - 6321 2013年 [査読有り][通常論文]
     
    Little is known concerning the effect of CO2 on phytoplankton ecophysiological processes under nutrient and trace element-limited conditions, because most CO2 manipulation experiments have been conducted under elements-replete conditions. To investigate the effects of CO2 and iron availability on phytoplankton ecophysiology, we conducted an experiment in September 2009 using a phytoplankton community in the iron limited, high-nutrient, low-chlorophyll (HNLC) region of the Bering Sea basin. Carbonate chemistry was controlled by the bubbling of the several levels of CO2 concentration (180, 380, 600, and 1000 ppm) controlled air, and two iron conditions were established, one with and one without the addition of inorganic iron. We demonstrated that in the iron-limited control conditions, the specific growth rate and the maximum photochemical quantum efficiency (F-v/F-m) of photosystem (PS) II decreased with increasing CO2 levels, suggesting a further decrease in iron bioavailability under the high-CO2 conditions. In addition, biogenic silica to particulate nitrogen and biogenic silica to particulate organic carbon ratios increased from 2.65 to 3.75 and 0.39 to 0.50, respectively, with an increase in the CO2 level in the iron-limited controls. By contrast, the specific growth rate, F-v/F-m values and elemental compositions in the iron-added treatments did not change in response to the CO2 variations, indicating that the addition of iron canceled out the effect of the modulation of iron bioavailability due to the change in carbonate chemistry. Our results suggest that high-CO2 conditions can alter the biogeochemical cycling of nutrients through decreasing iron bioavailability in the iron-limited HNLC regions in the future.
  • H. Endo, T. Yoshimura, T. Kataoka, K. Suzuki
    Journal of Experimental Marine Biology and Ecology 439 160 - 175 2013年01月 [査読有り][通常論文]
     
    On-deck CO2-Fe-manipulated incubation experiments were conducted using surface seawater collected from the Western Subarctic Gyre of the NW Pacific in the summer of 2008 to elucidate the impacts of ocean acidification and Fe enrichment on the abundance and community composition of phytoplankton and eubacteria in the study area. During the incubation, excluding the initial period, the mean partial pressures of CO2 in non-Fe-added bottles were 230, 419, 843, and 1124μatm, whereas those in Fe-added treatments were 152, 394, 791, and 1008μatm. Changes in the abundance and community composition of phytoplankton were estimated using HPLC pigment signatures with the program CHEMTAX and flow cytometry. A DGGE fingerprint technique targeting 16S rRNA gene fragments was also used to estimate changes in eubacterial phylotypes during incubation. The Fe addition induced diatom blooms, and subsequently stimulated the growth of heterotrophic bacteria such as Roseobacter, Phaeobacter, and Alteromonas in the post-bloom phase. In both the Fe-limited and Fe-replete treatments, concentrations of 19'-hexanoyloxyfucoxanthin, a haptophyte marker, and the cell abundance of coccolithophores decreased at higher CO2 levels (750 and 1000ppm), whereas diatoms exhibited little response to the changes in CO2 availability. The abundances of Synechococcus and small eukaryotic phytoplankton (< 10μm) increased at the higher CO2 levels. DGGE band positions revealed that Methylobacterium of Alphaproteobacteria occurred solely at lower CO2 levels (180 and 380ppm) during the post-bloom phase. These results suggest that increases in CO2 level could affect not only the community composition of phytoplankton but also that of eubacteria. As these microorganisms play critical roles in the biological carbon pump and microbial loop, our results indicate that the progression of ocean acidification can alter the biogeochemical processes in the study area. © 2012 Elsevier B.V.
  • 杉江 恒二, 芳村 毅, Koji Sugie, Takeshi Yoshimura, 財, 電力中央研究所環境科学研究所, 財)電力中央研究所環境科学研究所, Central Research Institute of Electric Power Industry, Central Research Institute of Electric Power Industry
    海の研究 = Umi no Kenkyu (Oceanography in Japan) 20 5 101 - 148 日本海洋学会 2011年09月15日 [査読無し][通常論文]
     
    海洋酸性化が海洋生物に及ぼす影響に関する研究が近年勢力的に行われている。本総説では,先ず,地球史における海水のpHの変遷と現代の海洋酸性化とを対比しながら植物プランクトンの動態について考察した。続いて,海洋酸性化の実験方法および植物プランクトンの生理生態と物質循環に及ぼす影響に関して近年の報告を中心にまとめ,以下の課題を抽出した。(1)過去の海洋酸性化の研究において亜寒帯や寒帯および外洋性の単離培養株が用いられていないこと,生息域や生活環に基づく実験が行われていないことは,自然環境における海洋酸性化の影響を把握する上での知識の欠如となっている。(2)pHの変化によって鉄と錯形成をする有機配位子の化学形態や2価鉄の濃度が変化するため,それらが生態系に及ぼす影響を評価する必要がある。(3)pHの低下により植物プランクトン細胞の有機炭素:リン比は増加する傾向,有機炭素:窒素比はほとんど変化しない傾向にある。一方では,pHの低下が溶存有機物およびケイ素の動態に与える影響には未解明な点が多いため,研究を促進させる必要がある。(4)pHの低下と鉄などの微量元素の利用性との複合作用がシアノバクテリアの窒素固定速度に及ぼす影響を明らかにし,窒素循環過程の理解を深化させる必要がある。Increasing atmospheric CO_2 over the Anthropocene due to human activities has reduced seawater pH and it continues as long as burning fossil fuels. In this review, we summarize and discuss the methodology of ocean acidification study and the effects of ocean acidification on phytoplankton and biogeochemical cycle of nutrients. In addition, we compare the pH variability and phytoplankton dynamics over the past 60 million years with the recent phenomenon of ocean acidification. We found that: (1) there is a significant lack of knowledge on the effect of ocean acidification because previous studies were not examined using subpolar to polar phytoplankton species and also not considered their habitat and life cycle. (2) We should study the effect of pH on dissolved inorganic Fe(II) bioavailability relative to dissolved inorganic Fe(III) species and the availability of Fe(III)-ligand complex. The role of Fe(II) and chemical structure of the ligand are also important issues. (3) Particulate organic carbon to phosphorus ratio increased in response to the decrease in pH whereas particulate organic carbon to nitrogen ratio was unaffected by pH. However, there is a lack of knowledge with regard to the effect of ocean acidification on dissolved organic matter and silicic acid dynamics. (4) We should study further the interactive effects of pH and trace metals especially for iron on N_2 fixation rate by cyanobacteria to improve our knowledge about nitrogen cycle.
  • Takeshi Yoshimura, Isao Kudo
    MARINE CHEMISTRY 126 1-4 182 - 192 2011年09月 [査読有り][通常論文]
     
    Temporal variations of phosphorus (P) pools (soluble reactive P (SRP), dissolved organic P (DOP), and particulate P (PP)), alkaline phosphatase activity (APA), and orthophosphate uptake were measured in Funka Bay to describe the annual P cycle in the bay and determine the phytoplankton and bacterial responses to the change in P availability. Observations over 4 years demonstrated that SRP concentrations in the surface waters decreased from the winter maximum of 1.0 mu mol L(-1) to 0.3 mu mol L(-1) at the end of the spring phytoplankton bloom, and then to under the detection limit by around August. The DOP concentration was relatively constant at 0.1 mu mol L(-1) throughout the year with higher concentrations up to 0.3 mu mol L(-1) just after the peak of the spring bloom. PP concentrations varied around 0.1 mu mol L(-1) with higher concentrations up to 0.6 mu mol L(-1) during the high chlorophyll a concentration period. SRP comprised 85% of the total P pool prior to the spring bloom, but the contribution decreased to less than 30% in the post-bloom period and DOP dominated the P pool in the surface. Most of the measured APA was less than 0.1 nmol L(-1) min(-1) in the whole water column, but exceptionally high values up to 1.4 nmol L(-1) min(-1) were observed in low SRP surface waters during summer. When SRP decreased from 0.3 mu mol L(-1) in spring to under the detection limit in summer, turnover time for orthophosphate at 10 m depth decreased from 61 days to 4.2 h. The high APA and short turnover time for orthophosphate suggested that microbes in the surface mixed layer experienced severe P stress in summer. Under severely P stressed conditions, APA was detected mostly in the >10 mu m fraction, suggesting that large phytoplankton would acquire P from DOP to overcome the P stress. This was supported by the result that large phytoplankton were outcompeted by the 0.2-1 mu m sized microbes for orthophosphate uptake. (C) 2011 Elsevier B.V. All rights reserved.
  • Jun Nishioka, Tsuneo Ono, Hiroaki Saito, Keiichiro Sakaoka, Takeshi Yoshimura
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 116 2 2011年02月 [査読有り][通常論文]
     
    Multiyear (2003-2008) time series observations along the A line provided information on the temporal variability of the dissolved iron (diss-Fe) concentration in the Oyashio region of the western subarctic Pacific, and the data indicated that there was an annual cycle in the concentration of surface diss-Fe occurring every year. Diss-Fe was supplied into the surface water in this region every winter and supports the spring phytoplankton bloom after development of the thermocline. The diss-Fe concentration was drawn down during the phytoplankton bloom period and was depleted in summer in some water masses. Then diss-Fe increased from autumn to winter with the increasing depth of the surface mixed layer. The high diss-Fe concentrations in the surface layer in winter were controlled by mesoscale oceanic intrinsic processes, such as vertical winter mixing and horizontal Fe-rich intermediate water transport. Difference in magnitude of the winter mixing processes among different water masses caused the heterogeneous distribution of diss-Fe concentration in the surface layer. Moreover, the vertical section profiles along a cross-Oyashio transect showed the occurrence of Fe-rich intermediate water, and upward transport of materials from the intermediate water to the surface layer via tidal and winter mixing processes are important mechanisms to explain the high winter surface diss-Fe concentrations. Additionally, the substantially higher diss-Fe/NO3 ratio in the winter surface layer in this studied area other than the high-nutrient low-chlorophyll region indicates that the winter surface water in the Oyashio and the Oyashio-Kuroshio transition zone has a high potential to stimulate phytoplankton growth.
  • Jun Nishioka, Tsuneo Ono, Hiroaki Saito, Keiichiro Sakaoka, Takeshi Yoshimura
    Journal of Geophysical Research: Oceans 116 2 2011年 [査読有り][通常論文]
     
    Multiyear (2003-2008) time series observations along the A line provided information on the temporal variability of the dissolved iron (diss-Fe) concentration in the Oyashio region of the western subarctic Pacific, and the data indicated that there was an annual cycle in the concentration of surface diss-Fe occurring every year. Diss-Fe was supplied into the surface water in this region every winter and supports the spring phytoplankton bloom after development of the thermocline. The diss-Fe concentration was drawn down during the phytoplankton bloom period and was depleted in summer in some water masses. Then diss-Fe increased from autumn to winter with the increasing depth of the surface mixed layer. The high diss-Fe concentrations in the surface layer in winter were controlled by mesoscale oceanic intrinsic processes, such as vertical winter mixing and horizontal Fe-rich intermediate water transport. Difference in magnitude of the winter mixing processes among different water masses caused the heterogeneous distribution of diss-Fe concentration in the surface layer. Moreover, the vertical section profiles along a cross-Oyashio transect showed the occurrence of Fe-rich intermediate water, and upward transport of materials from the intermediate water to the surface layer via tidal and winter mixing processes are important mechanisms to explain the high winter surface diss-Fe concentrations. Additionally, the substantially higher diss-Fe/NO3 ratio in the winter surface layer in this studied area other than the high-nutrient low-chlorophyll region indicates that the winter surface water in the Oyashio and the Oyashio-Kuroshio transition zone has a high potential to stimulate phytoplankton growth. Copyright © 2011 by the American Geophysical Union.
  • Nishioka J, Ono T, Saito H, Sakaoka K, Yoshimura T
    Journal of Geophysical Research 116 C2 C02021  2011年 [査読有り][通常論文]
  • 夏季ベーリング海における植物プランクトン群集へのCO2と鉄の効果がSiとNの利用比に及ぼす影響.
    杉江恒二, 遠藤寿, 鈴木光次, 芳村毅
    月間海洋 43 12 735 - 741 2011年 [査読無し][通常論文]
  • Takeshi Yoshimura, Jun Nishioka, Takeshi Nakatsuka
    DEEP-SEA RESEARCH PART I-OCEANOGRAPHIC RESEARCH PAPERS 57 11 1454 - 1464 2010年11月 [査読有り][通常論文]
     
    To clarify the iron (Fe) nutritional status of the phytoplankton assemblage in the Okhotsk Sea we conducted incubation experiments in summer 2006 Replicate surface seawater samples with the natural plankton community were incubated with three treatments Fe enrichment addition of the strong Fe chelator siderophore desferriferrioxime B (DFB) which strips Fe from the biologically accessible pool and as a control no addition To prevent macronutrient limitation we added surplus nutrients to all treatments At all 4 stations in Sakhalin Bay near the mouth of the Amur River and around the east of Sakhalin Island net specific growth rate showed no significant difference between the control and +Fe treatment and was repressed in +DFB treatment both in large- and small-sized phytoplankton These findings indicate that these waters contain sufficient bioavailable Fe and that the Amur River plume which is transported by the east Sakhalin current is a major source of the Fe In the Bussol Strait net specific growth rate in the control was significantly higher than +DFB treatment suggesting a supply of bioavailable Fe through intense vertical mixing at this site Iron enrichment treatment stimulated the net specific growth rate of large-sized phytoplankton indicating that Fe still limits the growth for the large-sized phytoplankton assemblage but not for small-sized phytoplankton in this area An Index of Fe availability was defined to quantify the degree of ambient Fe availability in each station and it revealed the spatial variability of ambient Fe availabilities among the sites (C) 2010 Elsevier Ltd All rights reserved
  • Takeshi Yoshimura, Jun Nishioka, Koji Suzuki, Hiroshi Hattori, Hiroshi Kiyosawa, Yutaka W. Watanabe
    JOURNAL OF EXPERIMENTAL MARINE BIOLOGY AND ECOLOGY 395 1-2 191 - 198 2010年11月 [査読有り][通常論文]
     
    Increasing CO2 in seawater (i.e. ocean acidification) may have various and potentially adverse effects on phytoplankton dynamics and hence the organic carbon dynamics. We conducted a CO2 manipulation experiment in the Sea of Okhotsk in summer 2006 to investigate the response of the organic carbon dynamics. During the 14-day incubation of nutrient depleted and 200 mu atm in situ pCO(2) surface water with a natural plankton assemblage under 150, 280, 480, and 590 mu atm pCO(2), the amount of net dissolved organic carbon accumulation was significantly lower at >480 mu atm pCO(2) than at 150 mu atm pCO(2), while differences in net particulate organic carbon accumulation between the treatments were small and did not show a clear relationship with the pCO(2). This is the first report to show a decreased net organic carbon production of natural plankton community under elevated pCO(2). Phytoplankton pigment analysis suggests that the relative contribution of fucoxanthin-containing phytoplankton such as diatoms to the phytoplankton biomass was lower at >280 mu atm pCO(2) than at 150 mu atm pCO(2). Different pCO(2) conditions may alter the organic carbon dynamics through changes in plankton processes. We conclude that the continuing increase in atmospheric CO2 in a time scale from the last half century to the end of this century has potential to affect the carbon cycle in nutrient depleted subpolar surface waters. (C) 2010 Elsevier B.V. All rights reserved.
  • Nishioka J, Ono T, Saito H, Sakaoka K, Yoshimura T, Matoba
    AGU Fall Meeting Abstracts 1 0331  2010年 [査読有り][通常論文]
  • Takeshi Yoshimura, Hiroshi Ogawa, Keiri Imai, Takafumi Aramaki, Yukihiro Nojiri, Jun Nishioka, Atsushi Tsuda
    DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 56 26 2863 - 2874 2009年12月 [査読有り][通常論文]
     
    The dynamics of organic carbon (C), nitrogen (N), and phosphorus (P) were examined during an in situ mesoscale iron-enrichment experiment in the western North Pacific in the summer of 2004. We separately determined the production of particulate organic matter (POM) and dissolved organic matter (DOM) and their subsequent removal during the bloom decline. As the iron-induced phytoplankton bloom progressed (days 0-14), POM increased in the surface mixed layer, while DOM did not increase significantly. The molar ratios for C:N, C:P, and N:P of the newly produced POM were estimated to be 4.9,190, and 37 in the surface mixed layer, whereas the dissolved inorganic nitrogen to soluble reactive phosphorus drawdown ratio was 17. Preferential remineralization of P over C and N from the POM was postulated during the developing phytoplankton bloom. During the bloom decline (days 16-25), surface POM decreased with a similar C:N of 5.2. The N:P ratio of surface DOM increased during the bloom decline. Below the surface mixed layer, DOC and DON increased moderately after the peak of the bloom. The time-series variation of DOC and DON was not identical. The C, N, and P dynamics through the accumulation and removal of POM and DOM were complex. Grazing by mesozooplankton during the experiment may have played a significant role in the uncoupling of the dynamics of C, N, and P. (C) 2009 Elsevier Ltd. All rights reserved.
  • Hiroaki Saito, Atsushi Tsuda, Yukihiro Nojiri, Takafumi Aramaki, Hiroshi Ogawa, Takeshi Yoshimura, Keiri Imai, Isao Kudo, Jun Nishioka, Tsuneo Ono, Koji Suzuki, Shigenobu Takeda
    DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 56 26 2852 - 2862 2009年12月 [査読有り][通常論文]
     
    Biogeochemical cycles of N and Si were examined in the surface mixed layer during the mesoscale iron-enrichment (IE) experiment in the high-nutrient low-chlorophyll (HNLC) western subarctic Pacific (SEEDS-II). Although the IEs increased nitrate uptake, silicic acid utilization was not stimulated. The nitrate drawdown in the iron-patch (IN-patch, 140.3 mmol m(-2) in the surface mixed layer, 0-30 in) was only 25% of the initial inventory, which was 1/3-2/5 of the previous IE experiments in the subarctic Pacific. This relatively weak response of nutrient drawdown to IEs was due to the high biomass of mesozooplankton (MZ) dominated by copepod Neocalanus; plumchrus. Feeding of MZ (247.2 mmol m(-2) during Day 0-21 from the first IE) in the IN-patch was higher than the nitrate drawdown and prevented further development of the phytoplankton bloom. In the later period of the experiment (Day 14-21) the increase in the feeding activity and resultant decrease in phytoplankton biomass induced the accumulation of dissolved organic nitrogen (DON) and ammonium. Among total growth of MZ (81.6 mmol N m(-2)), 89% (72.8 mmol N m(-2)) was transported to the depth by the ontogenetic downward migration of N. plumchrus. Although silicic acid drawdown was not increased by the IEs, Si export flux increased by 2.7 times. The increase in Si export was also due to the increase in MZ, which egested faecal pellets with higher Si:N ratio and faster sinking speed than diatoms. The export efficiency (78% of new production) and total amount of export flux (143.8 mmol N m(-2) 1392 mmol C m(-2)) were highest records within the IE experiments despite weak responses of nutrient drawdown to the IE. During SEEDS-II, the high biomass of MZ reduced the phytoplankton response and nutrient drawdown to the IEs but via grazing and ontogenetic vertical migration accelerated the export flux as well as accumulations of dissolved forms of N. Results of the present and previous IE experiments indicate that the ecosystem and biogeochemical responses to IEs in the HNLC region are quite sensitive to the ecosystem components, especially for grazers of diatoms such as copepods and heterotrophic dinoflagellates. More attention needs to be paid to the ecosystem components and their biogeochemical functions as well as physical and chemical properties of the ecosystems in order to hindcast or forecast the impacts of changes in atmospheric iron deposition. (C) 2009 Elsevier Ltd. All rights reserved.
  • Yoshimura T., Nishioka J., Suzuki K., Hattori H., Kiyosawa H., Watanabe Y. W.
    Biogeosciences Discussions 6 2 4143 - 4163 Copernicus Publications 2009年04月15日 
    Impacts of the increasing CO2 in seawater (i.e. ocean acidification) on phytoplankton physiology may have various and potentially adverse effects on phytoplankton dynamics and the carbon cycle. We conducted a CO2 manipulation experiment in the Sea of Okhotsk in summer 2006 to investigate the response of the phytoplankton assemblage and dynamics of organic carbon. During the 14-day incubation of nutrient-depleted surface water with a natural phytoplankton assemblage under 150, 280, 480, and 590μatm pCO2, the relative abundance of fucoxanthin-containing phytoplankton such as diatoms and prymnesiophytes decreased with increasing pCO2. The amount of DOC accumulation also decreased with increasing pCO2, while differences in POC accumulation between the treatments were small and did not show a clear trend with the pCO2. Change in the phytoplankton community composition under different pCO2 conditions will alter the organic carbon dynamics as found in the present experiment. Compared to results in the literature from nutrient-replete conditions indicating a potential enhancement of phytoplankton production with elevated pCO2, the present results indicated a different physiological response of phytoplankton under nutrient-depleted conditions. These results indicate that the continuing increase in atmospheric CO2 can significantly affect the structure of marine ecosystems and carbon cycle in nutrient-depleted subpolar surface waters.
  • Atsushi Tsuda, Shigenobu Takeda, Hiroaki Saito, Jun Nishioka, Isao Kudo, Yukihiro Nojiri, Koji Suzuki, Mitsuo Uematsu, Mark L. Wells, Daisuke Tsumune, Takeshi Yoshimura, Tatsuo Aono, Takafumi Aramaki, William P. Cochlan, Maki Hayakawa, Keiri Imai, Tomoshi Isada, Yoko Iwamoto, William K. Johnson, Sohiko Kameyama, Shungo Kato, Hiroshi Kiyosawa, Yoshiko Kondo, Maurice Levasseur, Ryuji J. Machida, Ippei Nagao, Fumiko Nakagawa, Takahiro Nakanish, Seiji Nakatsuka, Akira Narita, Yoshifumi Noiri, Hajime Obata, Hiroshi Ogawa, Kenji Oguma, Tsuneo Ono, Tomofumi Sakuragi, Motoki Sasakawa, Mitsuhide Sato, Akifumi Shimamoto, Hyoe Takata, Charles G. Trick, Yutaka W. Watanabe, Chi Shing Wong, Naoki Yoshie
    JOURNAL OF OCEANOGRAPHY 63 6 983 - 994 2007年12月 [査読有り][通常論文]
     
    mesoscale iron-enrichment study (SEEDS II) was carried out in the western subarctic Pacific in the summer of 2004. The iron patch was traced for 26 days, which included observations of the development and the decline of the bloom by mapping with sulfur hexafluoride. The experiment was conducted at almost the same location and the same season as SEEDS (previous iron-enrichment experiment). However, the results were very different between SEEDS and SEEDS II. A high accumulation of phytoplankton biomass (similar to 18 mg chl m(-3)) was characteristic of SEEDS. In contrast, in SEEDS II, the surface chlorophyll-a accumulation was lower, 0.8 to 2.48 mg m(-3), with no prominent diatom bloom. Photosynthetic competence in terms of F-v/F-m for the total phytoplankton community in the surface waters increased after the iron enrichments and returned to the ambient level by day 20. These results suggest that the photosynthetic physiology of the phytoplankton assemblage was improved by the iron enrichments and returned to an iron-stressed condition during the declining phase of the bloom. Pico-phytoplankton (< 2 mu m) became dominant in the chlorophyll-a size distribution after the bloom. We observed a nitrate drawdown of 3.8 mu M in the patch (day 21), but there was no difference in silicic acid concentration between inside and outside the patch. Mesozooplankton (copepod) biomass was three to five times higher during the bloom-development phase in SEEDS II than in SEEDS. The copepod biomass increased exponentially. The grazing rate estimation indicates that the copepod grazing prevented the formation of an extensive diatom bloom, which was observed in SEEDS, and led to the change to a pico-phytoplankton dominated community towards the end of the experiment.
  • Jun Nishioka, Tsuneo Ono, Hiroaki Saito, Takeshi Nakatsuka, Shigenobu Takeda, Takeshi Yoshimura, Koji Suzuki, Kenshi Kuma, Shigeto Nakabayashi, Daisuke Tsumune, Humio Mitsudera, W. Keith Johnson, Atsushi Tsuda
    Journal of Geophysical Research: Oceans 112 10 2007年10月08日 [査読有り][通常論文]
     
    Iron is an essential nutrient and plays an important role in the control of phytoplankton growth (Martin et al., 1989). Atmospheric dust has been thought to be the most important source of iron, supporting annual biological production in the western subarctic Pacific (WSP) (Duce and Tindale, 1991 Moore et al., 2002). We argue here for another source of iron to the WSP. We found extremely high concentrations of dissolved and particulate iron in the Okhotsk Sea Intermediate Water (OSIW) and the North Pacific Intermediate Water (NPIW), and water ventilation processes in this region probably control the transport of iron through the intermediate water layer from the continental shelf of the Sea of Okhotsk to wide areas of the WSP. Additionally, our time series data in the Oyashio region of the WSP indicate that the pattern of seasonal changes in dissolved iron concentrations in the surface-mixed layer was similar to that of macronutrients, and that deep vertical water mixing resulted in higher winter concentrations of iron in the surface water of this region. The estimated dissolved iron supply from the iron-rich intermediate waters to the surface waters in the Oyashio region was comparable to or higher than the reported atmospheric dust iron input and thus a major source of iron to these regions. Our data suggest that the consideration of this source of iron is essential in our understanding of spring biological production and biogeochemical cycles in the western subarctic Pacific and the role of the marginal sea. Copyright 2007 by the American Geophysical Union.
  • Isao Kudo, Takeshi Yoshimura, Choon-Weng Lee, Mitsuru Yanada, Yoshiaki Maita
    JOURNAL OF OCEANOGRAPHY 63 5 791 - 801 2007年10月 [査読有り][通常論文]
     
    Nutrient regeneration and oxygen consumption after a spring bloom in Funka Bay were studied on monthly survey cruises from February to November 1998 and from March to December 1999. A high concentration of ammonium (more than 4 mu mol 1-1) was observed near the bottom (80-90 m) after April. Phosphate and silicate gradually accumulated and dissolved oxygen decreased in the same layer. Salinity near the bottom did not change until summer, leading to the presumption that the system in this layer is semi-closed, so regenerated nutrients were preserved until September. Nitrification due to the oxidation of ammonium to nitrate was observed after June. Nitrite, an intermediate product, was detected at 4-7 mu mol L(-1) in June and July 1999. Assuming that decomposition is a first order reaction, the rate constant for decomposition of organic nitrogen was determined to be 0.014 and 0.008 P in 1998 and 1999, respectively. The ammonium oxidation rate increased rapidly when the ambient ammonium concentration exceeded 5,mu mol L-1. We also performed a budget calculation for the regeneration process. The total amount of N regenerated in the whole water column was 287.4 mmol N m(-2) in 4 months, which is equal to 22.8 gC m(-2), assuming the Redfield C to N ratio. This is 34% of the primary production during the spring bloom and is comparable to the export production of 25 gC m-2 measured by a sediment trap at 60 m.
  • Jun Nishioka, Tsuneo Ono, Hiroaki Saito, Takeshi Nakatsuka, Shigenobu Takeda, Takeshi Yoshimura, Koji Suzuki, Kenshi Kuma, Shigeto Nakabayashi, Daisuke Tsumune, Humio Mitsudera, W. Keith Johnson, Atsushi Tsuda
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 112 C10 2007年10月 [査読有り][通常論文]
     
    Iron is an essential nutrient and plays an important role in the control of phytoplankton growth ( Martin et al., 1989). Atmospheric dust has been thought to be the most important source of iron, supporting annual biological production in the western subarctic Pacific (WSP) (Duce and Tindale, 1991; Moore et al., 2002). We argue here for another source of iron to the WSP. We found extremely high concentrations of dissolved and particulate iron in the Okhotsk Sea Intermediate Water (OSIW) and the North Pacific Intermediate Water (NPIW), and water ventilation processes in this region probably control the transport of iron through the intermediate water layer from the continental shelf of the Sea of Okhotsk to wide areas of the WSP. Additionally, our time series data in the Oyashio region of the WSP indicate that the pattern of seasonal changes in dissolved iron concentrations in the surface-mixed layer was similar to that of macronutrients, and that deep vertical water mixing resulted in higher winter concentrations of iron in the surface water of this region. The estimated dissolved iron supply from the iron-rich intermediate waters to the surface waters in the Oyashio region was comparable to or higher than the reported atmospheric dust iron input and thus a major source of iron to these regions. Our data suggest that the consideration of this source of iron is essential in our understanding of spring biological production and biogeochemical cycles in the western subarctic Pacific and the role of the marginal sea.
  • Takeshi Yoshimura, Jun Nishioka, Hiroaki Saito, Shigenobu Takeda, Atsushi Tsuda, Mark L. Wells
    MARINE CHEMISTRY 103 1-2 112 - 121 2007年01月 [査読有り][通常論文]
     
    Several operationally defined fractions of phosphorus (P) were measured along a surface water transect in the North Pacific. The P content in all fractions was found to increase northward from the edge of the subtropical to the subarctic region. Particulate organic P (POP) concentrations increased from 9 to 110 nmol L-1, whereas the particulate inorganic P (PIP) concentrations increased from 1 to 13 nmol L-1. A significant correlation between POP, PIP and chlorophyll a suggested that these P pools are associated directly or indirectly with phytoplankton cells. PIP comprised 10-20% of the total particulate P pool across the transect, indicating it is an important component of the marine P cycle in this region. Dissolved non-reactive P (assumed to consist predominantly of non-reactive organic P compounds, thus referred to as DOP) concentration increased from 0.10 to 0.22 mu mol L-1, whereas soluble reactive P (SRP) concentration increased from 0.01 to 1.42 mu mol L-1 along the transect. The proportion of DOP and SRP varied widely, with a large proportion of DOP in areas with low total dissolved P concentrations in lower latitudes and a large proportion of SRP in areas with high total dissolved P concentrations in higher latitudes. High demand for DOP in the lower latitudinal region would diminish the concentration of this pool relative to higher latitudinal regions where SRP is more abundant and would be preferentially utilized. The availability of SRP could have a significant impact on the concentration and probably on the composition of DOP. We show that P fractionation provides an important insight for discussing the marine P cycle. (c) 2006 Elsevier B.V. All rights reserved.
  • Hiroaki Saito, Atsushi Tsuda, Yukihiro Nojiri, Jun Nishioka, Shigenobu Takeda, Hiroshi Kiyosawa, Isao Kudo, Yoshifumi Noiri, Tsuneo Ono, Yousuke Taira, Koji Suzuki, Takeshi Yoshimura, Philip W. Boyd
    DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 53 20-22 2168 - 2181 2006年 [査読有り][通常論文]
     
    This paper reports on the variations in nutrient concentrations and phytoplankton dynamics during the stationary and declining phases of a phytoplankton bloom induced by a mesoscale iron-enrichment conducted in the high-nutrient low-chlorophyll (HNLC) eastern subarctic Pacific. During the 26-d sampling period, the main pycnocline was located between 30 and 45 m with a shallow pycnocline developing at 10 m, 19 d after the first iron-enrichment. The iron-induced bloom dominated by diatoms peaked during days 15 and 18, a period of high chlorophyll a concentrations (ca. 5 mg m(-3)), and declined thereafter. Nitrogenous nutrients and phosphate were not depleted during the whole experiment. In contrast, silicic acid and iron concentrations became very low during the stationary phase of the diatom bloom (days 15-18) and F(v)/F(m) declined. These observations suggest that silicic acid and iron limitation probably prevented further development of the diatom bloom. The decline in chlorophyll a concentrations during days 19-24 was mostly due to the decrease in diatom abundance. On the other hand, cell abundances of pico- and nanophytoplankton exhibited little change until day 24. In the layer located between the main and the shallow pycnocline (10-30 m), ammonium and silicic acid concentration increased during days 19-26, suggesting recycling of these nutrients. The amount of silicic acid recycled during that period was estimated at 71.3-99.2 mmol m(-2), while the dissolution rate of biogenic silica (BSi) was estimated to be 5.9-9.2% d(-1) in the upper 50 m of the water column. These results show that the development of a shallow pycnocline during the experiment accelerated the iron and silicic acid depletion in the upper mixed layer and influenced the recycling of the organic matter assimilated during the iron-induced bloom in the ocean surface. (c) 2006 Elsevier Ltd. All rights reserved.
  • FA Whitney, DW Crawford, T Yoshimura
    DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 52 7-8 1055 - 1067 2005年 [査読有り][通常論文]
     
    The high-nitrate, low-chlorophyll (HNLC) waters of the Gulf of Alaska tend towards silicate rather than nitrate depletion as phytoplankton utilize nutrients during summer. This tendency is enhanced when iron supply is elevated through natural inputs such as from coastally generated rnesoscale eddies or through artificial enrichment as was carried out in an in situ experiment in July 2002. However, ship-board incubations with iron enrichment demonstrate nitrate rather than silicate depletion for these waters. The difference between in situ and in vitro experiments occurs at least in part because deck incubations do not allow export of particulate Si and N. Due to the more efficient recycling of nitrogen and carbon, export favours the removal of silicon from the upper ocean (the Si pump). Previous measurements at Ocean Station Papa (50 degrees N, 145 degrees W) show that similar to 25% of the Si, but only similar to 7% of the C and similar to 4% of the N utilized during spring growth, is exported to a depth of 200 m. These results in the Gulf of Alaska agree with the present understanding of phytoplankton controls in other HNLC regions and show that any estimates of carbon export from iron enrichment should be based on Si- rather than N-limitation. (c) 2005 Elsevier Ltd. All rights reserved.
  • N Ramaiah, S Takeda, K Furuya, T Yoshimura, J Nishioka, T Aono, Y Nojiri, K Imai, Kudo, I, H Saito, A Tsuda
    PROGRESS IN OCEANOGRAPHY 64 2-4 253 - 261 2005年 [査読有り][通常論文]
     
    Dynamics of transparent exopolymer particles (TEP) was studied during the first in situ iron-enrichment experiment conducted in the western subarctic Pacific in July-August 2001, with the goal of evaluating the contribution of TEP to vertical flux as a result of increased primary production following iron enrichment in open ocean ecosystems. Subsequent to the enhancement of phytoplankton production, we observed increase in TEP concentration in the surface layer and sedimentation of organic matter beneath it. Vertical profiles of TEP, chlorophyll a (Chl a) and particulate organic carbon (POC) were obtained from six depths between 5 and 70 m, from a station each located inside and outside the enriched patch. TEP and total mass flux were estimated from the floating sediment traps deployed at 200 m depth. Chl a and TEP concentrations outside the patch varied from 0.2 to 1.9 μ g L-1 and 40-60 μ g XG equiv. L-1, respectively. Inside the patch, Chl a increased drastically from day 7 reaching the peak of 19.2 μ g L-1 on day 13, which coincided with the TEP peak of 189 μ g XG equiv. L-1. TEP flux in the sediment trap increased from 41 to 88 mg XG equiv. m(-2) d(-1), with 8-14% contribution of TEP to total mass flux. This forms the basic data set on ambient concentrations of TEP in the western subarctic Pacific, and evaluation of the effect of iron enrichment on TEP. © 2005 Elsevier Ltd. All rights reserved.
  • PW Boyd, CS Law, CS Wong, Y Nojiri, A Tsuda, M Levasseur, S Takeda, R Rivkin, PJ Harrison, R Strzepek, J Gower, RM McKay, E Abraham, M Arychuk, J Barwell-Clarke, W Crawford, D Crawford, M Hale, K Harada, K Johnson, H Kiyosawa, Kudo, I, A Marchetti, W Miller, J Needoba, J Nishioka, H Ogawa, J Page, M Robert, H Saito, A Sastri, N Sherry, T Soutar, N Sutherland, Y Taira, F Whitney, SKE Wong, T Yoshimura
    NATURE 428 6982 549 - 553 2004年04月 [査読有り][通常論文]
     
    Iron supply has a key role in stimulating phytoplankton blooms in high-nitrate low-chlorophyll oceanic waters(1-5). However, the fate of the carbon fixed by these blooms, and how efficiently it is exported into the ocean's interior, remains largely unknown(1-5). Here we report on the decline and fate of an iron-stimulated diatom bloom in the Gulf of Alaska. The bloom terminated on day 18, following the depletion of iron and then silicic acid, after which mixed-layer particulate organic carbon (POC) concentrations declined over six days. Increased particulate silica export via sinking diatoms was recorded in sediment traps at depths between 50 and 125 m from day 21, yet increased POC export was not evident until day 24. Only a small proportion of the mixed-layer POC was intercepted by the traps, with more than half of the mixed-layer POC deficit attributable to bacterial remineralization and mesozooplankton grazing. The depletion of silicic acid and the inefficient transfer of iron-increased POC below the permanent thermocline have major implications both for the biogeochemical interpretation of times of greater iron supply in the geological past(6,7), and also for proposed geo-engineering schemes to increase oceanic carbon sequestration(3,8).
  • Jun Nishioka, Shigenobu Takeda, Isao Kudo, Daisuke Tsumune, Takeshi Yoshimura, Kenshi Kuma, Atsushi Tsuda
    GEOPHYSICAL RESEARCH LETTERS 30 14 2003年07月 [査読有り][通常論文]
     
    Comparison of vertical profiles of size-fractionated iron between the western and eastern subarctic North Pacific clearly showed higher labile particulate iron concentrations towards the west and this result strongly supports the higher iron supply in the western region. Additionally, the results of the SEEDS experiment, the first meso-scale iron enrichment experiment in the subarctic North Pacific, clearly showed that artificially enriched iron in the dissolved fraction (mainly in colloidal fraction) was rapidly transformed to suspended labile particulate iron during phytoplankton growth and was retained in the surface mixed layer. Probably, this same rapid transformation process occurs naturally after sporadic atmospheric iron supply and the labile particulate iron is retained in the western region. Furthermore, this transformation process reduces dissolved concentration of iron and its bioavailability. Therefore, the transformation process is important for understanding how phytoplankton became iron limited and the biogeochemical iron cycle in the western subarctic North Pacific.
  • A Tsuda, S Takeda, H Saito, J Nishioka, Y Nojiri, Kudo, I, H Kiyosawa, A Shiomoto, K Imai, T Ono, A Shimamoto, D Tsumune, T Yoshimura, T Aono, A Hinuma, M Kinugasa, K Suzuki, Y Sohrin, Y Noiri, H Tani, Y Deguchi, N Tsurushima, H Ogawa, K Fukami, K Kuma, T Saino
    SCIENCE 300 5621 958 - 961 2003年05月 [査読有り][通常論文]
     
    We have performed an in situ test of the iron limitation hypothesis in the subarctic North Pacific Ocean. A single enrichment of dissolved iron caused a large increase in phytoplankton standing stock and decreases in macronutrients and dissolved carbon dioxide. The dominant phytoplankton species shifted after the iron addition from pennate diatoms to a centric diatom, Chaetoceros debilis, that showed a very high growth rate, 2.6 doublings per day. We conclude that the bioavailability of iron regulates the magnitude of the phytoplankton biomass and the key phytoplankton species that determine the biogeochemical sensitivity to iron supply of high-nitrate, low-chlorophyll waters.
  • 芳村毅, 工藤勲
    海の研究 12 2 185 - 193 2003年03月05日 [査読無し][通常論文]
  • Takeshi Yoshimura, Isao Kudo
    Japanese Journal of Limnology 62 3 205 - 217 2001年 [査読有り][通常論文]
     
    Nutrient concentrations and alkaline phosphatase activity were measured in Lake Ohnuma from June 1995 to March 1997. Sufficient amounts of dissolved inorganic nitrogen remained throughout the year, whereas soluble reactive phosphorus concentrations were extremely low (close to the detection limit). The N:P ratio in the standing stock of nutrients ranged 74-3000 (by atoms), far richer in nitrogen than the Redfield ratio. Chlorophyll a concentrations were high (5-22 μ g L-1) during icefree seasons, but the elemental composition of phytoplankton exhibited a moderate phosphorus deficiency. A high seasonal variability in alkaline phosphatase activity suggested that not only was phytoplankton growth highly limited by phosphorus, but that the degree of this limitation varied. Alkaline phosphatase activity served as a simple and an effective indicator for judging the phosphorus status of phytoplankton.
  • 工藤勲, 芳村毅
    沿岸海洋研究 38 1 47 - 54 2000年08月25日 [査読無し][通常論文]
  • Kudo, I, T Yoshimura, M Yanada, K Matsunaga
    MARINE ECOLOGY PROGRESS SERIES 193 45 - 51 2000年 [査読有り][通常論文]
     
    Time series observations over 36 h were carried out in Funka Bay, Japan, to complement a 6 yr nutrient-dynamics study during a diatom spring bloom, especially focusing on nitrate and silicate as limiting nutrients. At the beginning of the study, nutrient concentrations were high (NO3 = 10 mu M, SiO4 = 20 mu M and PO4 = 0.8 mu M) Nitrate was depleted first during the night while silicate and phosphate remained at 10 and 0.3 mu M, respectively. High nitrate reductase activity was observed the following morning, indicating the assimilation of nitrate in situ by the dominant phytoplankter, Chaetoceros socialis. The consumption ratio of SiO4:NO3 increased by 3 times from 0.83 before the peak to 2.5 after the peak of the bloom. This change was also obvious in the elemental composition of phytoplankton, showing an increase in Si:C and Si:N ratios by a factor of 2 to 4 after the peak of the bloom, while C:N ratio remained constant at around 7. As a consequence of this change in the consumption rate of SiO4 and NO3, both silicate and nitrate were finally depleted in the photic zone.
  • Takeshi Yoshimura, Isao Kudo, Mitsuru Yanada, Katsuhiko Matsunaga
    Limnology 1 1 63 - 68 2000年 [査読有り][通常論文]
     
    Annual variations in nutrients, algal biomass, and primary production were investigated in Lake Ohnuma, Japan, in 1996 in order to compare them with 1977. Chlorophyll a concentrations gradually increased after the ice melted and reached a maximal value of 20.7 μg1-1 in August. Phosphate concentrations in the lake were close to the detection limit throughout the study period, whereas sufficient nitrate remained even in the productive summer season. In contrast, in the summer of 1977, both nutrients were exhausted, and primary production was less than 0.2g Cm-2 day-1. Primary production in 1996 ranged from 0.4 to 5.8g Cm-2 day-1, which was 2 to 30 times higher than 20 years ago. These results indicate that the lake has become eutrophic in the last two decades. A comparison of the nutrients in the inflowing river between 1977 and 1996 indicated that nitrate and ammonium concentrations were markedly elevated in the rivers, and therefore the nitrogen loading to the lake tripled.

MISC

所属学協会

  • 日本水環境学会   日本海洋学会   ASLO, Association for the Sciences of Limnology and Oceanography   日本地球惑星科学連合   日本海洋学会沿岸海洋研究会   

Works（作品等）

共同研究・競争的資金等の研究課題

  • 日本学術振興会：科学研究費助成事業
    研究期間 : 2021年04月 -2024年03月 
    代表者 : 芳村 毅, 工藤 勲, 今井 圭理
     
    海洋によるCO2の吸収・固定量は植物プランクトンによる炭素と栄養素の利用比率によって変化します。その比率としてレッドフィールド比が良く知られており，気候変化を予測する生態系モデルにおいて固定値として使われていますが，実測データの蓄積は十分ではありません。また，プランクトン群集はサイズの異なる多くの種で形成されますが，それらをひとまとめにした比率しか議論されてきませんでした。海洋環境の変化によりプランクトン群集組成が変化すると予測されていますが，その変化が生態系に与える影響を評価できません。そこで，本研究は有機物粒子のサイズ分画手法を確立したうえで，各サイズ画分の主要4元素組成比（炭素：窒素：リン：ケイ素）に関するデータを取得します。 研究一年目の2021年度は海水中の有機物粒子をサイズ毎に分画するための手法を検討しました。撹拌式セルを用いた限外ろ過システムを応用した「限外ろ過方式」と，孔径の異なるフィルターに連続的に通過させる「逐次ろ過方式」を並行して検討しました。その結果，限外ろ過方式はフィルター上に粒子が捕捉されてしまい，有機物粒子量を過小評価することがわかりました。有機物粒子のサイズ分画手法としては逐次ろ過方式が適していることがわかりました。逐次ろ過方式はリンおよびケイ素の分析試料を作成するのに適していますが，炭素および窒素を分析するためにはフィルター上に集めた有機物粒子を液体に懸濁させて回収する必要があります。フィルターに付着する有機物粒子を完全に回収する手法を検討した結果，超音波洗浄を利用することで達成できることがわかりました。これらの検討により，孔径の異なるメンブレンフィルターを用いた逐次ろ過方式によって有機物粒子を三つにサイズ分画し，超音波洗浄を活用して回収した有機物粒子の主要4元素を分析することで，各サイズ画分の元素組成比を分析するための道筋が完成しました。
  • 日本学術振興会：科学研究費助成事業
    研究期間 : 2021年04月 -2024年03月 
    代表者 : 木田 新一郎, 田中 潔, 芳村 毅, 伊佐田 智規
  • 日本学術振興会：科学研究費助成事業
    研究期間 : 2018年04月 -2021年03月 
    代表者 : 工藤 勲, 芳村 毅
     
    ホタテガイ垂下養殖漁業が行われている青森県陸奥湾で、近年栄養塩濃度が減少する「貧栄養化」が起こっていることが明らかとなった。本研究は、持続的な生物生産活動の維持を脅かしかねない海域の「貧栄養化」のメカニズムを解明し、貧栄養化の効果的な方策を提言することを目的として行われた。夏季に栄養塩は枯渇状態にあり、この時期の基礎生産（ホタテガイの餌生産）は栄養塩が不足していた。 海底耕耘により海底に埋没した栄養塩の回帰効果があることが確認され、その量は海水中に存在する栄養塩の10-20％に相当すると見積もられた。このことから「貧栄養化」の対策として海底耕耘は有効な手段であることが明らかになった。
  • 日本学術振興会：科学研究費助成事業
    研究期間 : 2012年06月 -2017年03月 
    代表者 : 神田 穣太, 喜多村 稔, 西川 淳, 青野 辰雄, 山口 篤, 高木 省吾, 土屋 光太郎, 立田 穣, 林 敏史, 野田 明, 渡辺 豊, 茂木 正人, 田中 祐志, 守屋 繁春, 小林 卓也, 芳村 毅, 石丸 隆, 五十嵐 敏, 須賀 次郎, 山川 紘, 大津 秀暁, 伊藤 友加里, 高澤 伸江, 内山 香織, 久保 篤史, 今井 圭理
     
    海洋生態系における福島第一原子力発電所事故由来の放射性物質について、以下の研究を行った。１）福島沿岸域において生物と環境の放射性セシウムについて、年に2回の船舶観測による経時的なデータセットを得た。また外洋域の動物プランクトンについても複数の海域で時系列データを得た。２）岩礁性の底生魚に重点を置いて、生態系への移行経路を検討した結果、懸濁・沈降粒子や堆積物の有機物画分等からの移行は小さいことを確認した。３）現場観測データを用い、生態系内の放射性セシウム推移をモデルにより再現し、海洋生物の放射能レベルの今後の推移について検討した。
  • 日本学術振興会：科学研究費助成事業
    研究期間 : 2011年05月 -2014年03月 
    代表者 : 青山 道夫, 村田 昌彦, 芳村 毅, 日置 昭治, 内田 裕, ハイド デイビット, ディクソン アンドリュー, 小杉 如央
     
    海洋での栄養塩の正確な空間分布を明らかにするための基礎となるデータセット作成を行った。栄養塩標準を用い分析の不確かさが0.2%より良い測点間の比較可能性が確保されている航海を６航海行うとともに複数のデータセットから栄養塩データが存在している航海を抽出した。これらのデータから、栄養塩標準を使用し比較可能性が明示的に確保されている航海を基準として、全球243点で交点での解析をおこない栄養塩濃度を補正した後、さらに当初の予定にはなかった溶存酸素量のデータについても栄養塩データを同様な処理を行い、深度面で整合性のある0.5度メッシュ、平均50m厚み136層の格子点データセットを作成した。
  • 日本学術振興会：科学研究費助成事業
    研究期間 : 2011年 -2013年 
    代表者 : 服部 寛, 佐々木 洋, 芳村 毅, 小埜 恒夫
     
    地球温暖化が原因となる海洋酸性化が植物プランクトン、特に炭酸カルシウム殻をもつ円石藻類に対する影響を調べるための酸性化予測実験を行った。現場における分布密度の把握と酸性化予測実験の試料分析は全て終わっていないが、これまでの結果では現存量が多い円石藻類の種ほど酸性化の影響を受けると推定出来る結果を得ていて、現在この原因の理論的背景を構築している段階である。 北太平洋と比較を目的に南極海で酸性化実験を実施しする機会に恵まれ、現在はその試料の解析も行なっている。南極海では植物プランクトン現存量で優先する珪藻類が多いことから、円石藻類の応答に加え、珪藻類にも着目して、試料の分析を勧めている。
  • 日本学術振興会：科学研究費助成事業
    研究期間 : 2010年 -2012年 
    代表者 : 芳村 毅
     
    単離株を用いた室内培養実験と現場群集を用いた船上培養実験により,CO2分圧の上昇が植物プランクトンの比増殖速度と元素組成比や有機物生産に与える影響を調査した。CO2分圧の上昇に対する応答は種特異的であり,水温・光量の条件によっても変化することが明らかとなった。現場群集への影響は顕著ではないが,CO2増加に対する明確なトレンドを示すケースがあった。CO2分圧の上昇は実海域での植物プランクトンの動態を変化させる要因となり得ることがわかった。
  • 日本学術振興会：科学研究費助成事業
    研究期間 : 2004年 -2006年 
    代表者 : 小埜 恒夫, 葛西 広海, 西岡 純, 渡辺 豊, 芳村 毅
     
    通常のCTDを利用したクリーン採水技術を確立し、親潮域および混合域において年数回の鉄の反復断面観測を実施した。世界で初めて栄養塩と同一の時空間分解能で得られた、鉄濃度周年変動データの解析から、以下の事があきらかになった。 1]親潮域表層における溶存鉄の各月の海域平均濃度は、主要栄養塩と定性的に同じ季節変動パターンを示すだけでなく、硝酸と定量的に一定比(ΔDFe/ΔN=27x10^<-6>[mol/mol])を保って増減している。表層鉄濃度のピークは冬期の最大混合深度期(1-3月)であり、黄砂飛来時期(4月)とは明瞭に異なる。 2]親潮表層の冬期溶存鉄濃度はアラスカ湾より遙かに高いが、両者の亜表層域の鉄濃度も同様に異なる為に、両海域の冬期表層鉄濃度は、亜表層からのエントレインメントと鉛直渦拡散を仮定した同じモデルで再現可能である。親潮表層の溶存鉄:硝酸費も同様にアラスカ湾より高く、北太平洋中層水(NPIW)のそれと同一値をしめす。 3]これらの事から、西部北太平洋表層に存在する鉄は数年-数十年程度の平均滞留時間を持っており、したがって周年-経年変動のスケールでは、鉄の動態は主要栄養塩と同様であることが推定される。つまり大気ダストからの鉄の供給は従来の定説に反してそれほど重要ではなく、生物による取り込みと中層での再分解、そして鉛直混合による表層への回帰という海洋内部での再循環だけで、ほぼ加不足なく溶存鉄の周年変動は説明出来る。 4]春季ブルーム終了時の表層溶存鉄濃度がほぼ0.25nMで一定となる事から、ブルーム形成ケイ藻種の成育限界鉄濃度が0.25nM程度と高く、このためブルーム期には硝酸でなく鉄が主な制限栄養素になっている事が推定される。一方周年を通じた硝酸・溶存鉄プロットは鉄側に切片を持つ事から、鉄も硝酸も低濃度となる夏期には硝酸が親潮表層の制限栄養素となっている。

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