基本情報

所属

• 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

職名

• 教授

学位

• 水産学博士（北海道大学）

J-Global ID

• 200901096611872459

研究キーワード

• 植物プランクトン生理学   海洋環境化学   生物地球化学   Phytoplankton physiology   Marine Chemistry   Biogeochemistry   

研究分野

• ライフサイエンス / 植物分子、生理科学
• ライフサイエンス / 水圏生産科学
• 環境・農学 / 環境政策、環境配慮型社会
• 環境・農学 / 環境影響評価

担当教育組織

•  学士課程　水産学部
•  修士課程　環境科学院
•  博士課程　環境科学院

職歴

• 2016年04月 - 現在 北海道大学大学院水産科学研究院 教授
• 2007年04月 - 2016年03月 北海道大学大学院水産科学研究院 海洋生物資源科学 准教授
• 2000年04月 - 2007年03月 北海道大学大学院水産科学研究院 海洋生物資源科学部門 助教授
• 1987年04月 - 2000年03月 北海道大学水産学部 水産化学科 助手
• 1987年 - 1999年 北海道大学

学歴

•         - 1987年   北海道大学   水産学研究科   水産化学
•         - 1987年   北海道大学
•         - 1982年   北海道大学   水産学部   水産化学科
•         - 1982年   北海道大学

所属学協会

• 日本プランクトン学会   American Geophysical union   The American Society of Limnology and Oceanography   日本海洋学会   

研究活動情報

論文

• Effect of water mass mixing on phytoplankton dynamics in the scallop culture areas off Okhotsk Sea: A microcosm experiment

  Frolan A. Aya, Isao Kudo

  Regional Studies in Marine Science 55 102568 - 102568 2022年09月 [査読有り]
  
• Application of chemostat culture to nutrient uptake rate measurements by the macroalgae Saccharina japonica var. religiosa ( Phaeophyceae ) and Ulva australis ( Ulvophyceae )

  Ryosuke Okazaki, Narumi Teramoto, Andrew K. Carlson, Kiyoko Nakanishi, Isao Kudo

  Phycological Research 70 3 142 - 150 2022年03月30日 [査読有り]
  
• Tight coupling between primary productivity, export production, and the growth of benthic scallops in the coastal region of the Okhotsk Sea along Hokkaido

  Takayuki Terumoto, Isao Kudo, Koji Miyoshi, Akiyoshi Shinada, Akira Miyazono

  Fisheries Oceanography 2022年01月26日 [査読有り]
  
• Behaviors of the Yukon river sediment plume in the Bering sea: Relations to glacier-melt discharge and sediment load

  Kazuhisa A. Chikita, Tomoyuki Wada, Isao Kudo, Sei Ichi Saitoh, Toru Hirawake, Mitsuhiro Toratani

  Water (Switzerland) 13 19 2021年10月01日 

  Sediment plumes, released to the Bering Sea from the delta front of the Yukon River, Alaska, are initiated mainly by glacier-melt sediment runoffs in the glacierized regions of the Yukon River drainage basin. The surface sediment plumes are extended around the fan-shaped Yukon River delta, which is followed by the northwestward dispersion. During continuous measurements of the Yukon River discharge and sediment load, behaviors of the sediment plumes were explored by shipboard and coastal observations in the Bering Sea. At the high river sediment load of ca. 2500 kg/s, the plume partially plunged into the sea bottom layer. The plunging probably originated in the nepheloid-layer formation from the flocculation of river-suspended sediment, of which more than 90% wt. is silt and clay (grain size d < 63 µm). In order to numerically obtain the area of the surface sediment plumes, a satellite image analysis was performed by using three near-infrared bands in MODIS/Aqua or MODIS/Terra. The plume area was significantly correlated (R2 = 0.735, p < 0.01) to the sediment load averaged for the two days with time lags of 20 days and 21 days to the date of a certain satellite image. Hence, the dispersion of plume-suspended sediment appears to be controlled by the sediment runoff events in the Yukon River rather than the northward “Alaskan Coastal Water”.
• The impact of eutrophication towards selected bacterial process rates in tropical coastal waters

  Joon Hai Lim, Choon Weng Lee, Chui Wei Bong, Isao Kudo

  Marine Pollution Bulletin 169 112524 - 112524 2021年08月
• Effects of River Discharge and Sediment Load on Sediment Plume Behaviors in a Coastal Region: The Yukon River, Alaska and the Bering Sea

  Kazuhisa A. Chikita, Tomoyuki Wada, Isao Kudo, Sei-Ichi Saitoh, Mitsuhiro Toratani

  Hydrology 8 1 45 - 45 2021年03月12日 

  In the Bering Sea around and off the Yukon River delta, surface sediment plumes are markedly formed by glacier-melt and rainfall sediment runoffs of the Yukon River, Alaska, in June– September. The discharge and sediment load time series of the Yukon River were obtained at the lowest gauging station of US Geological Survey in June 2006–September 2010. Meanwhile, by coastal observations on boat, it was found out that the river plume plunges at a boundary between turbid plume water and clean marine water at the Yukon River sediment load of more than ca. 2500 kg/s. Grain size analysis with changing salinity (‰) for the river sediment indicated that the suspended sediment becomes coarse at 2 to 5‰ by flocculation. Hence, the plume’s plunging probably occurred by the flocculation of the Yukon suspended sediment in the brackish zone upstream of the plunging boundary, where the differential settling from the flocculation is considered to have induced the turbid water intrusion into the bottom layer.
• Behavior of Water Mass Beneath the Tsushima Warm Current in the Japan Sea

  Xiaorong Fang, Yutaka Isoda, Isao Kudo, Takafumi Aramaki, Keiri Imai, Naoto Ebuchi

  Water 12 8 2184 - 2184 2020年08月03日 

  To better understand the behavior of water mass beneath the Tsushima Warm Current (TWC), we use the vertical cross-sections of potential temperature, salinity and dissolved oxygen in the Japan Sea obtained by the T/V Oshoro Maru of the Hokkaido University during 8–29 June in 2011 to analyze its origins and variations. The results show that the potential temperature and salinity beneath the TWC varies little, but the dissolved oxygen varies largely with the geographical location. There are two deep water masses with different dissolved oxygen content below the TWC. One is on the coastal side with the low dissolved oxygen, and the other is on the offshore side with the high dissolved oxygen. It is inferred that the former one is relatively old water and the latter is the new water. By using the phosphate (PO4) and the apparent oxygen utilization (AOU) relationship, we calculate the PO40 (preformed PO4) as a water mass tracer. These results suggest that the water masses beneath the TWC with high and low dissolved oxygen originate from the same surface water mass in the central Japan Sea.
• 植物プランクトンの生理と窒素代謝に与える鉄の影響

  工藤 勲, 國分治代, 宮本真希子, 野入善史

  沿岸海洋研究 56 105 - 114 2019年08月 [査読有り]
  
• Long-term comparison of dissolved nitrogen species in tropical estuarine and coastal water systems

  Joon Hai Lim, Yi You Wong, Choon Weng Lee, Chui Wei Bong, Isao Kudo

  Estuarine, Coastal and Shelf Science 222 103 - 111 2019年06月
• Metagenomics of Coral Reefs Under Phase Shift and High Hydrodynamics

  Pedro Milet Meirelles, Ana Carolina Soares, Louisi Oliveira, Luciana Leomil, Luciana Reis Appolinario, Ronaldo Bastos Francini-Filho, Rodrigo Leão de Moura, Renato Tenan de Barros Almeida, Paulo S. Salomon, Gilberto Menezes Amado-Filho, Ricardo Kruger, Eduardo Siegle, Diogo A. Tschoeke, Isao Kudo, Sayaka Mino, Tomoo Sawabe, Cristiane C. Thompson, Fabiano L. Thompson

  Frontiers in Microbiology 9 2018年10月04日
• Distributions of particulate and dissolved phosphorus in aquatic habitats of Peninsular Malaysia

  Joon Hai Lim, Choon Weng Lee, Chui Wei Bong, Yang Amri Affendi, Yii Siang Hii, Isao Kudo

  Marine Pollution Bulletin 128 415 - 427 2018年03月01日 [査読有り][通常論文]
   

  Particulate phosphorus was the dominant phosphorus species and accounted for 72 ± 5% of total phosphorus in coastal habitats, 63 ± 4% in estuaries, 58 ± 6% in lakes and 80 ± 7% in aquaculture farms whereas dissolved inorganic phosphorus (DIP) and dissolved organic phosphorus (DOP) were minor components. Correlation analyses (DIP vs Chl a R2 = 0.407, df = 31, p < 0.001) suggested phosphorus limiting conditions in lakes, which was corroborated with the highest alkaline phosphatase activity (APA) that fluctuated from 0.38 to 41.14 nmol L−1 min−1. In contrast, APA was elevated in coastal habitats and estuaries only when DIP concentration decreased below 0.9 μM. Moreover size-fractionation experiment showed that the highest APA was detected in the 0.2–2 μm pico-size fraction. Our results suggested that the main APA in coastal habitats and estuaries was from phototrophic pico-eukaryotes and heterotrophic bacteria, and regulated largely by DIP availability.
• Nitrogen stable isotopes reveal age-dependent dietary shift in the Japanese scallop Mizuhopecten yessoensis

  Frolan A. Aya, Isao Kudo

  ISOTOPES IN ENVIRONMENTAL AND HEALTH STUDIES 53 1 80 - 90 2017年03月 [査読有り][通常論文]
   

  Ontogenetic niche shifts in diet are a consequence of changes in body size or resource partitioning between age classes. To better resolve the feeding patterns of the Japanese scallop Mizuhopecten yessoensis, we examined the relative importance of age and size in the diet of this species using stable isotope ratios of carbon (C-13) and nitrogen (N-15) from 2006 to 2009. Contribution of food sources was quantified using an isotope mixing model by comparing the muscle tissue isotope ratios to those of suspended particulate organic matter (SPOM) and their zooplankton prey (e.g. micro- and meso-zooplankton). Unlike the C-13 values, which remained constant with age and size, muscle N-15 values were more positively correlated with age accounting for 69% of variations than size with only 46%. Increasing N-15 values with age suggested that shifts in diet from SPOM to micro- and meso-zooplankton occurred during ontogeny in M. yessoensis. Results of the isotope mixing model indicated that SPOM contribution to scallop's diet decreased from 68 to 8% while those of zooplankton increased from 15 to 50% with increasing age. This study concludes that age-related dietary shift explains the enrichment of N-15, as a result of predation on zooplankton by M. yessoensis.
• Significant contribution of lytic mortality to bacterial production and DOC cycles in Funka Bay, Japan

  Eri Kakuta, Kudo Isao

  Journal of oceanography 72 2 177 - 187 Springer 2016年04月 [査読有り]
   

  A modified dilution experiment was conducted to evaluate the relative contribution of viral lysis and protozoan grazing to the mortalities of heterotrophic bacteria in Funka Bay, a subarctic coastal bay. The experiment included the stepwise dilution of the original seawater with virus-free seawater (10 kDa ultrafiltered) to change the encounter rate of both virus and protozoa to heterotrophic bacteria, incubation for 48 h and monitoring the change in the abundance of heterotrophic bacteria. In a parallel experiment, the original seawater was replaced by 1.0 A mu m fractionated seawater to eliminate protozoa, and the same dilution was conducted with the virus-free seawater to estimate only lytic mortality. The viral lysis and protozoan grazing rates in the surface water ranged from 0.40 to 1.19 and 0.08 to 0.27 days(-1), respectively. Viral lysis was the main cause for the bacterial mortality (79.8 +/- A 3.2 %). The net (in situ) growth rate of heterotrophic bacteria was about 0.15 days(-1). In the bottom water (90 m), both mortalities were lower than those at the surface and the net growth rate was mostly a negative value. The contribution of released dissolved organic matter (DOM) through lysis to the bacterial carbon demand (BCD) was evaluated. The lysed bacterial cells might release DOM to the ambient environment, in which bacterial organic matter is recycled in the subsequent bacterial production. The potential contribution was estimated to range from 25 to 27 % in the surface water and to be 31 % in the bottom water, suggesting that the lytic mortality significantly fueled DOM to the subsequent bacterial production.
• 森林流域における主要無機イオンの流出機構： 観測とモデリング

  宮本拓人, 知北和久, 阪田義隆, 落合泰大, M.Hossain, 大八木英夫, 工藤勲

  日本水文科学会誌 46 1 39 - 57 日本水文科学会 2016年04月 [査読有り][通常論文]
   

  本研究では，北海道・十勝地方沿岸域にある生花苗川（おいかまないがわ）流域（面積 62.47 km2）を対象とし，淡水の電導度モニタリングに基づく解析を行っている。生花苗川流域の土地利用被覆率は，森林88.3%，農用地10.6%でほとんどが森林である。森林土壌は，30–40 cm深に1667年樽前山噴火によるテフラTa-b層，さらに深部には約40,000年前の支笏火山大噴火によるテフラSpfa-1層と角レキ層が基盤の上に分布している。これらの層は極めて透水性が高く，側方浸透流の流出経路の役割をもつ。また，レゴリス下の地質基盤は大部分が新第三紀中新世の海成層で透水性の高い砂岩・レキ岩を含み，これには日高造山運動によってできた断層も分布している。今回は，試水の化学分析から河川水の主要無機イオンMg2＋, Ca2＋, Na＋, SO42－, HCO3－の濃度（mg/L）と25℃電気伝導度（EC25と表記）との間に相関の高い線形関係を見出した。他方，2013年の降雨期に生花苗川で水位・電気伝導度をモニタリングし，溶存イオン濃度とEC25との関係を用いて，上記五種のイオン濃度およびその負荷量の時系列を得た。本研究では，得られた流量と主要無機イオン負荷量の時系列にタンクモデルとベキ関数を適用し，河川への各種イオン物質の流出経路について解析を行った。流出解析の結果，2013年は表面流出と中間流出で流出全体の74.2%を占め，流域外への地下水漏出は16.8%を占めた。また，イオン物質負荷量に対するモデル解析から，全負荷量に対する表面流出，中間流出，地下水流出，河川流出による負荷量の寄与が求められた。結果として，五つのイオン種全てで中間流出による寄与が40.0～70.0%と最大であった。これは，中間流出に対応する基盤上の角レキ層と支笏テフラ層での側方浸透流による溶出が卓越していることを示唆している。
• アラスカ・ユーコン河流域における物質輸送機構 : 観測とモデリング (陸海結合システム)

  知北 和久, 和田 知之, 工藤 勲

  低温科学 74 43 - 54 低温科学第74巻編集委員会 2016年 

  2006 年6 月～2009 年9 月の3 年余の期間，アラスカ・ユーコン河最下流部のUSGS 水位観測点（PLS地点）で浮遊物質濃度，懸濁態炭素（POC）濃度，懸濁態窒素（PON）濃度の1 時間連続データを得た．ユーコン河流域（面積8.57×105 km2）の約75％は不連続永久凍土をもつ森林域，1.1％は高山氷河域である．6 月～9 月の夏期には，ユーコン河の流量と浮遊物質濃度は氷河と凍土の融解と数日間にわたる数回の降雨によって大きく変動する．4 月下旬～5 月初旬には，融雪増水に伴う河氷の破砕（breakup）が生じ，この後は6 月初旬まで融雪出水が続く．2007～2009 年の3 回の融雪出水について，年間の流量・土砂流出量・POC フラックス・PON フラックスに対する寄与を評価した．結果として，年間の流量に対し14.1-24.8％，土砂流出量に対し8.7-22.5％，POC フラックスに対し11.6-23.7％，PON フラックスに対し10.3-24.5％の寄与があった．2006～2008 年夏期の日平均流量と日平均土砂流出量の時系列について，それぞれタンク・モデルおよび計算流量のベキ関数を適用して，その再現を試みた．結果として，モデル計算は観測結果との間にr＝0.88－0.94（流量）とr＝0.73－0.93（浮遊物質濃度）の高い相関を示し，モデルによる再現が共に妥当であることがわかった．土砂流出量から得られる浮遊物質濃度に対する数値解析から，流送土砂の給源が氷河域と河道にあると推定された．感度解析では，温暖化による気温上昇が氷河融解流出に与える効果について議論した．Hourly time series of discharge, suspended sediment concentration, POC (particulate organic carbon) and PON(particulate organic nitrogen) were obtained at the lowest gauging station, PLS (Pilot Station) of U. S. Geological Surveyin the Yukon River basin, Alaska, in June 2006 - September 2009. The river basin (area, 8.57×105 km2) is occupied by1.1 ％ glacierized mountainous regions and ca. 75 ％ lower forest regions with discontinuous permafrost. Thereby,the Yukon River discharge in summer consists of glacier-melt and permafrost-melt discharges over the summer and afew rainfall runoffs for several days. Focusing on the snowmelt runoff in late April or early May to early June, itscontribution to annual discharge, sediment load, POC flux and PON flux was estimated. As a result, the threesnowmelt runoffs in 2007, 2008 and 2009 occupied 14.1 - 24.8％ of annual discharge, 8.7 - 22.5％ of annual sedimentload, 11.6 - 23.7％ of annual POC flux, and 10.3 - 24.5％ of annual PON flux. The discharge and suspendedsediment concentration time series in the summers of 2006-2008 were simulated by the tank model and a powerfunction of modeled discharge, respectively. The simulated results were agreeable to the observed ones with thecorrelation coefficient, r＝0.88－0.94 for discharge and r＝0.73－0.93 for suspended sediment concentration. In thesimulation of suspended sediment concentration, it was suggested that the sediment source is located in the glacierized regions and river channels. As a sensitivity analysis, the effect of an increase in air temperature, due to globalwarming, on the glacier-melt discharge is discussed.
• 東風時に弱化され易い宗谷暖流

  小野 祐嗣, 磯田 豊, 工藤 勲, 宮園 章, 有田 駿

  北海道大学水産科学研究彙報 65 2 47 - 59 北海道大学大学院水産科学研究院 2015年08月 

  The subinertial variation of the Soya Warm Current(SWC) is investigated through the analysis of current records obtained from the moored meters settled at three sites along the east coast of Hokkaido, during a period from April to October 2011. We found that the rapid decreases of southeastward SWC were frequently observed due to the strong easterly wind blowing, but the inverse(westerly) wind with the similar amplitude never grew the strong SWC. To understand such asymmetrical response of the SWC to the wind forcing, the numerical model experiments including the nonlinear drag law of bottom friction were carried out. In the circumstances combined with the SWC generated by sea level difference across the Soya Strait and wind-induced current, the model result suggests that the asymmetrial action of bottom friction term causes the different dynamic response. When the momentum balance between easterly wind stress and sea level gradient across the atrait is just realized, the SWC effectively is weakened, and hence such dynamic responce becomes the frictional free. In the case of westerly wind forcing, the SWC is superimposed by the eastward wind-induced current so that the southeastward flow speed tends to increase. However, its increase rate is gradually suppressed, because the bottom friction term on the momentum balance dominants in proportional as the square of flow speed.
• Primary productivity and nitrogen assimilation with identifying the contribution of urea in Funka Bay, Japan

  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.
• High-rate growth of bottom-cultured Japanese scallop Mizuhopecten yessoensis in the coastal area of northeastern Hokkaido, Japan

  Koji Miyoshi, Akiyoshi Shinada, Akira Miyazono, Yasuhiro Kuwahara, Masahide Tada, Takayuki Terumoto, Isao Kudo

  NIPPON SUISAN GAKKAISHI 81 3 468 - 470 2015年05月 [査読有り][通常論文]
  
• Temporal variation of phytoplankton growth and grazing loss in the west coast of Peninsular Malaysia

  Joon Hai Lim, Choon Weng Lee, Isao Kudo

  Environmental Monitoring and Assessment 187 5 246  2015年05月 [査読有り][通常論文]
   

  Phytoplankton growth (mu) and grazing loss (g) rates were measured monthly by the Landry-Hassett dilution method over a 2-year period at both estuarine (Klang) and coastal water (Port Dickson) systems along the Straits of Malacca. Chlorophyll a (Chl a) concentration ranged from 0.20 to 4.47 mu g L-1 at Klang except on two occasions when Chl a spiked above 10 mu g L-1. In contrast, Chl a concentrations were relatively stable at Port Dickson (0.14 to 2.76 mu g L-1). From the rate measurements, mu was higher (t=2.01, df=43, p<0.05) at Klang (0.30 to 2.26 day(-1)) than at Port Dickson (0.18 to 1.66 day(-1)), but g was not significantly different (p>0.80). g ranged from 0.30 to 1.50 and 0.21 to 1.51 day(-1) at Klang and Port Dickson, respectively. In this study, grazing loss was coupled to phytoplankton growth, and the ratio of g/mu or grazing pressure which estimates the proportion of primary production grazed was 50 % at Klang and lower than at Port Dickson (68 %; t=2.213, df=36, p<0.05). We found that the higher growth rates in a eutrophic system, i.e., Klang, were not matched by higher grazing loss, and this may have implications for the biogeochemical cycling in coastal waters.
• 2013年北海道オホーツク海沿岸域における地まきホタテガイの高成長と餌料環境. 日本水産学会誌、81: 468-470.

  三好晃治, 品田晃良, 宮園 章, 桒原康裕, 多田匡秀, 照本昂之, 工藤 勲

  日本水産学会誌 81 3 468 - 470 2015年 [査読有り][通常論文]
  
• Different contributions of riverine and oceanic nutrient fluxes supporting primary production in Ishikari Bay

  Julius I. Agboola, Isao Kudo

  CONTINENTAL SHELF RESEARCH 88 140 - 150 2014年10月 [査読有り][通常論文]
   

  We computed a ratio of riverine nutrient flux (RNF) to bottom nutrient flux (BNF) to determine the relative importance of oceanic and riverine nutrient fluxes on primary production dynamics in Ishikari Bay, which is composed of oligotrophic subarctic coastal water. Across spring, summer and autumn, the RNF:BNF ratio (R:B ratio) was significantly greater than 1.0, especially in spring and autumn for DIN and Si(OH)(4), suggesting that riverine nutrients mostly supported primary production. A strong inverse relationship (r=-0.927) between Chl a and salinity in autumn and a corresponding increase in the apparent utilization of DIN and primary production indicated that the contribution of DIN from the Ishikari River on primary production was maximal in autumn. However the R:B ratio for PO4 was significantly less than 1.0, especially in summer (0.1) and autumn (03), suggesting a larger contribution of bottom upwelling nutrient sources. In spring, when the ratio was close to 1 (0.8), PO4 supply from both bottom (upwelling) and surface (river) was equivalent, since PO4 concentration of river end-member was the lowest. Although riverine nutrient fluxes were a major source of DIN and Si(OH)(4) nutrient supply in the bay, oceanic nutrient contribution from bottom upwelling and horizontal advection was a major source of PO4. While riverine nutrients significantly fuel primary production, the estuarine circulation process may contribute significantly to compensating for the inadequate supply of riverine PO4 in an oligotrophic system like Ishikari Bay. Also, unlike the usual estuarine system in which nutrient concentration at a deeper layer is high due to the regeneration of nutrients at depth, concentration in Ishikari Bay was very low due to an influence of oligotrophic waters. We conclude that riverine nutrient flux contributes a large portion of the total flux in Ishikari Bay. (C) 2014 Elsevier Ltd. All rights reserved.
• 陸奥湾の逆エスチャリー循環流（1）夏季の湾口海洋観測

  河野航平, 磯田 豊, 工藤 勲, 藤原将平, 有田 駿, 小林直人, 吉田 達, 扇田いずみ, 高津哲也

  海と空 90 1 11 - 16 2014年08月31日 [査読有り][通常論文]
  
• Light utilization efficiency of phytoplankton in the Western Subarctic Gyre of the North Pacific during summer

  Yuichi Nosaka, Tomonori Isada, Isao Kudo, Hiroaki Saito, Hiroshi Hattori, Atsushi Tsuda, Koji Suzuki

  JOURNAL OF OCEANOGRAPHY 70 1 91 - 103 2014年02月 [査読有り][通常論文]
   

  We investigated the water-column light utilization efficiency (I) of phytoplankton photosynthesis in the Western Subarctic Gyre (WSG) of the North Pacific during summer 2008. The I values (0.64-1.86 g C [g Chl a](-1) [mol photon](-1) m(2)) obtained were observed to increase significantly with decreasing daily photosynthetic available radiation (PAR) and were generally higher than those of previous studies, not only from the subarctic Pacific but also from the world's oceans. To examine the effect of iron availability on I in the WSG, I values were estimated from the data of two in situ iron fertilization experiments: the Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study I (SEEDS-I) and II (SEEDS-II). We found that iron availability did not affect I values. Overall, this study revealed that I values changed remarkably in the WSG during the summer, and that higher values were found at the stations where moderate PAR levels (ca. 10-30 mol photons m(-2) day(-1)) were observed and where autotrophic flagellates predominated in the phytoplankton assemblages.
• 陸奥湾における貧栄養化と二枚貝養殖の関連性.

  工藤 勲, 吉村 真理, 橋岡 香織, 足立 敏成, 磯田 豊

  沿岸海洋研究 52 1 83 - 92 日本海洋学会沿岸海洋研究部会 2014年 [査読有り][通常論文]
  
• Clearance rates and ingestion efficiency of the Japanese scallop Patinopecten yessoensis

  Frolan A. Aya, Yasuharu Hidaka, Isao Kudo

  Plankton and Benthos Research 8 3 134 - 140 2014年 [査読有り][通常論文]
   

  In coastal ecosystems, variations in food quantity may have significant effects on the clearance and ingestion rates of suspension-feeding bivalves. In this study, clearance rates and ingestion efficiencies were determined for Japanese scallop (Patinopecten yessoensis) juveniles (60.6 ±4.5 mm in shell height) under laboratory conditions. Scallops were kept individually in glass beakers at 15°C and fed with different cell numbers of Pavlova sp. (0.8 to 57.60 x10< sup> 6< /sup> cells) to provide a wide range of food quantity as particulate organic carbon (POC). Clearance rates (CR) and ingestion efficiencies (IE) were estimated by monitoring POC concentration over a two-day period, and from 2 to 14 days of feeding, respectively. Both CR and IE were significantly influenced by POC concentration. CR ranged from 15.8 to 38.5 mL ind< sup> -1< /sup> h< sup> -1< /sup> (or 8.9 to 49.6 mg C h< sup> -1< /sup> g dry weight< sup> -1< /sup> ) with maximum values at high POC concentrations. IE varied from 40 to 71% and differed significantly between the lowest (2,900 //g C L< sup> -1< /sup> ) and highest (8,000 //g C L< sup> -1< /sup> ) food rations. The feeding response of juvenile scallops to different POC concentrations was fitted to a power curve equation: IE (%)=0.9272xPOC< sup> 0.5105< /sup> , r=0.98. Extrapolated field-based estimates of IE ranged from 7.8 to 12.7% in response to seasonal changes in POC concentration (64.5 to 168.6 μg C L< sup> -1< /sup> ). It is concluded that particle filtration rates by juvenile scallops are related to food quantity, as suggested by both field and laboratory-derived feeding rates.
• Seasonality and environmental drivers of biological productivity on the western Hokkaido coast, Ishikari Bay, Japan

  Julius I. Agboola, Mario Uchimiya, Isao Kudo, Masato Osawa, Kazuo Kido

  ESTUARINE COASTAL AND SHELF SCIENCE 127 12 - 23 2013年07月 [査読有り][通常論文]
   

  We report on a 16-month time series of the primary productivity, chlorophyll biomass and environmental variables in Ishikari Bay, the western Hokkaido coast, from September 2006 to December 2007. During productive summer and autumn seasons, the average phytoplankton biomass (as chlorophyll a) and productivity were 4.7 +/- 0.4 mu g Chl a L-1 and 229.2 +/- 113.4 mu g C L-1 d(-1), and 5.9 +/- 6.0 mu g Chl a L-1 and 138.9 +/- 202.6 mu g C L-1 d(-1), respectively. The water column salinity profiles suggested that nutrient upwelling from bottom waters, especially in summer and autumn, when autotrophic production was at a maximum, was the major source of nutrients fueling the productivity in near-surface waters. Size-fractionation measurements showed that the micro-size fraction (>10 mu m) accounted for > 80% of the total phytoplankton biomass and productivity. The strong correlation of POC with Chl a (r = 0.875; P < 0.001; n = 16) suggests that living phytoplankton in POC may outweigh detrital and bacterial biomass. Stoichiometrical analysis of inorganic nutrients revealed a seasonal low (13) and high (25) DIN-to-Pal ratio and a seasonal low (30) and high (37) Si-to-PO4 ratio, suggesting that phytoplankton growth was possibly limited by nitrate and phosphate and not by silicic acid. We conclude that bottom-advection of nutrients resulting from wind-driven mixing may have been a significant source of nutrient input in this oligotrophic coastal system, and zooplankton grazing or other losses might be a crucial factor in controlling the phytoplankton biomass and production, particularly in spring. Also, it is probable that phytoplankton growth was not prevented by insufficient light in Ishikari Bay considering the values of both mean light intensity in the mixed layer (I-m) and the mixing and euphotic depth ratio (Z(mix):Z(eu)). (C) 2013 Elsevier Ltd. All rights reserved.
• 流出特性と土地利用形態に基づく十勝川水系の栄養塩供給機構

  岩波 秀晃, 和田 知之, 坂本 和佳, 工藤 勲, 知北 和久

  日本水文科学会誌 43 1 3 - 24 THE JAPANESE ASSOCIATION OF HYDOROLOGICAL SCIENCES 2013年 [査読有り][通常論文]
   

  北海道十勝川水系の支川（利別川・札内川・音更川・然別川・猿別川）において，2004 ～ 2006 年に河川水の栄養塩濃度を求めた。また，各河川における栄養塩濃度の季節変動を解析するため，タンクモデルを用いて雨水・融雪水の流出過程（表面，中間，基底）を推定した。解析には，河川流量，降水量，気温，湿度，気圧，風速と日射量のデータを用いた。また，流域からの栄養塩の供給源を理解するため，流域の土地利用形態・地質・土壌の分布を考慮した。結果として，冬期の低流量時には，流出成分として基底流出と中間流出が全流量の 80％以上を占めた。 3月から 6月の雪解けによる急激な流量増加期は，表面流出の寄与が全流量の 50％以上まで増加した。有効雨量と表面流出率の間に強い正の相関が存在し，これは降雨イベント後に表面流出としてすみやかに流量に反映されることを示唆している。利別川については，低流量時のケイ酸濃度は全河川における最高値を示し，春の融雪期にはケイ酸濃度は減少した。また，ケイ酸濃度と表面流出率との間に有意な負の相関が存在した。このことは，利別川のケイ酸濃度は高濃度の地下水からの基底流出と低濃度の表面流出との混合度合いによって規定されることを示唆している。しかし，流域に火山性土壌が多い利別，音更，然別川では，1 年を通してケイ酸濃度が高い傾向（200- 600 μmol l^-1）が存在し，他方，札内，猿川ではケイ酸は低濃度であった（10- 300 μmol l^-1）。硝酸塩は，流出経路の違いによる濃度変化はほとんど認められなかったが，河川ごとの濃度が大きく異なり，猿別川で高く（ 300- 400 μmol l^-1），利別川で低濃度（20- 50 μmol l^-1）であった。 河川の硝酸塩濃度は，各河川の流域における農地の割合との間に正の相関があり，これは農地および酪農地からの未利用の余剰肥料および家畜排泄物が河川への主要な起源であることを示唆する。以上から，十勝川水系の河川水中の栄養塩濃度の変動は，流出経路の違いよりも各河川の流域における土地利用形態や地質の影響をより強く受けていることが明らかとなった。
• Viability of Alexandrium tamarense cysts in the sediment of Funka Bay, Hokkaido, Japan: Over a hundred year survival times for cysts

  Akira Miyazono, Satoshi Nagai, Isao Kudo, Kosho Tanizawa

  HARMFUL ALGAE 16 81 - 88 2012年04月 [査読有り][通常論文]
   

  The abundance of Alexandrium tamarense cyst was investigated vertically in a sediment core in Funka Bay, Hokkaido, Japan. Germination experiments were conducted to estimate the germination ability of the cysts at different depth layers of the sediment. Molecular identification with loop-mediated isothermal amplification (LAMP) of A. tamarense cysts and core dating with Pb-210 methods (CRS model) were performed. The results indicated A. tamarense has presented since at least 100 years in Funka Bay and intensive bloom started occurring in the early 1960s and has continued until the late 1980s. Cysts from shallow layers of the sediment core displayed greater viabilities than those from deeper layers. However, successful germination of A. tamarense cysts was observed even at the deepest layer, indicating the long-term survival ability (approximately 100 years), and this is the longest record for survival of resting cysts on this species at present. The abundance of viable cysts deposited during the past intensive bloom period was approximately 1.4 times higher than that of recently deposited cysts, suggesting that the cysts in the sub-surface dense layers could potentially function as seed populations in Funka Bay. Thus, dredging the sediment in Funka Bay may involve the risk for initiation of toxic blooms. (C) 2012 Elsevier B.V. All rights reserved.
• 山岳氷河の質量・水収支の経年変動 (池田隆司教授記念号)

  知北 和久, 和田 知之, 工藤 勲, キム ヨンオン

  北海道大学地球物理学研究報告 75 1 - 13 北海道大学大学院理学研究院自然史科学部門（地球物理学） 2012年03月 

  Meteorological and hydrological data for a drainage basin of a mountain glacier, the GulkanaGlacier, Alaska, monitored by U. S. Geological Survey (USGS) for more than 40 yrs, are correctedand retrieved. Interannual variations of daily mean air temperature and daily precipitation nearthe glacier and daily runoff rate at a proglacial stream averaged over the ablation period (June toSeptember) indicate that the increasing rates of 1968－2010 and 1985－2010 are 0.012 ℃/yr and0.014 ℃/yr, 0.024 mm/yr and 0.041 mm/yr, and 0.092 mm/yr and 0.25 mm/yr, respectively. Thus,the runoff rate and precipitation tend to increase greatly in the recent years. The daily glacier-meltrunoff rate averaged over the ablation period was obtained on the assumption of zero englacialwater storage change over the period. As a result, the glacier-melt runoff rate exhibits the linearrelationship with the daily mean air temperature near the glacier. The summer ablation of the glacierwas larger than the winter accumulation in many years, consequently indicating that the netmass balance is negative in almost all the years. Especially since 1988, the net mass balance is negative in any years. The tendency of greatly negative cumulative net mass balance since 1988suggests that the glacier thickness become zero in ca. 2160. Meanwhile, the glacier area tends todecrease greatly and linearly since 1993 at the rate of -0.18 km2/yr. This indicates that the glacierarea would be zero in ca. 2096. The large decrease of cumulative net mass balance since 1980'sis common to the three glaciers, Gulkana, Wolverine and South Cascade, monitored by USGSsince 1960's. The decreasing rate of glacier thickness in 1980's to 2004 is -0.74 m/yr for GulkanaGlacier, -0.78 m/yr for Wolverine Glacier, and -0.71 m/yr for South Cascade Glacier, thusshowing the similar tendency in thinning.
• The intra-annual variability of discharge, sediment load and chemical flux from the monitoring: The Yukon River, Alaska.

  Chikita, K. A, Wada, T, Kudo, I, Kim Y

  Journal of Water Resource and Protection 4 173 - 179 2012年 [査読有り][通常論文]
  
• Glacial Effects on Discharge and Sediment Load in the Subarctic Tanana River Basin, Alaska

  Tomoyuki Wada, K. A. Chikita, Y. Kim, I. Kudo

  ARCTIC ANTARCTIC AND ALPINE RESEARCH 43 4 632 - 648 2011年11月 [査読有り][通常論文]
   

  About 5.6% of the drainage area of the Tanana River, Alaska, is covered by mountainous glacierized regions, and most of the other area by forests (51%) and wetlands (9%) with discontinuous permafrost. The water discharge and sediment load from glacierized and non-glacierized regions within the drainage area were represented by observed data of the proglacial Phelan Creek and the non-glacial Chena River, respectively, which are both the tributaries of the Tanana River and ultimately drain to the Yukon river basin. In the glacier-melt periods of 2007 and 2008, the runoff rate and suspended sediment concentration in Phelan Creek was 15 times and 36 times as high as those in the non-glacial Chena River, respectively. As a result, the mean sediment yield in the glacier-melt periods of 2007 and 2008 for Phelan Creek (24.8 t km(-2) day(-1)) was estimated to be 640 times as high as that in the Chena River (0.039 t km(-2) day(-1)). Hence, the glacierized regions were considered to be a major source of the fluvial sediment. In order to quantify the contribution of water discharge and sediment load from the glacierized regions to those of the Tanana River, the time series of water discharge, Q, and sediment load, L, in the glacier-melt periods were simulated by a tank model coupled with the L-Q equations (Nash-Sutcliffe efficiency coefficients, 0.41 to 0.82). The model indicates that the glacier-melt discharge accounted for 26-57% of the Tanana discharge, while the sediment load from the glacierized regions solely accounted for 76-94% of the Tanana sediment load. The remaining contribution (6-24%) of the sediment load was probably due to the fluvial resuspension of glacial sediment deposited previously in the river channels.
• Seasonal phosphorus depletion and microbial responses to the change in phosphorus availability in a subarctic coastal environment

  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.
• オホーツク海沿岸域の海洋構造と生物生産

  工藤 勲, フローラン アヤ, 高田 兵衛, 小林 直人

  沿岸海洋研究 49 13 - 21 2011年 [査読有り][通常論文]
  
• 簡略モデルを用いた噴火湾底泥ＡＶＳ変化の検討

  奥村 裕弥, 磯田 豊, 工藤 勲, 宮園 章

  沿岸海洋研究 49 1 91 - 21 日本海洋学会沿岸海洋研究部会 2011年 [査読有り][通常論文]
   

  2007年から2008年の噴火湾において,海底近傍の溶存酸素(DO)と表層泥中の酸揮発性硫化物(AVS)の季節的な関係を調べるための調査を実施した.DOには一年周期の変化がみられたものの(秋季に極小,春季に極大),AVSの一年周期の変化は不明瞭であり,むしろ数カ月の時間スケールをもった短期変動が卓越していた.DO供給に対するAVS応答を理解するために,一定の還元作用と可変の酸化作用を表現した二つのパラメータを有する簡略モデルを提案した.モデル結果は次の3つの特徴を示し,これらは一年周期のAVS変化を定性的に説明できる応答特性である.(1)正弦波のDO強制に対して,AVSは指数関数的(または双曲線関数的)な急増急減を示す.(2)AVSが極大となる位相は,DOが極小となる位相から0°～90°の範囲で遅れる.(3)酸化還元速度が小さくなるほど,AVSの応答振幅は小さくなる.AVSの短期的な急減を説明するためには,通常時(非攪乱時)よりも1オーダ大きな酸化速度を伴う何らかの底泥攪乱が必要であることが推測されたが,具体的な攪乱要因については不明である.
• PARAMETERS DETERMINING WATER TEMPERATURE OF A PROGLACIAL STREAM: THE PHELAN CREEK AND THE GULKANA GLACIER, ALASKA

  Kazuhisa A. Chikita, Ryo Kaminaga, Isao Kudo, Tomoyuki Wada, Yongwon Kim

  RIVER RESEARCH AND APPLICATIONS 26 8 995 - 1004 2010年10月 [査読有り][通常論文]
   

  The heat budget of a proglacial stream, Phelan Creek, from the Gulkana Glacier, Alaska, is estimated by using the hourly time series of stream water temperature, discharge and meteorology obtained in the summer of 2006. As an analytical result, the net shortwave radiation and bed friction occupied 32.1 and 38.2% of the total thermal input to the stream, respectively. The time series of the water temperature were simulated by a deterministic model with the coupling of the heat budget and the thermal advective diffusion equation. The simulated result is agreeable to the observed one with the Nash-Sutcliffe efficiency (NASH) of 0.747 and the root-mean-square error (RMSE) of 0.236 degrees C over the observation period. However, under condition that the rainfall of more than 1mm h(-1) occurs continually for more than a day, the simulation was less reasonable with NASH=0.225 and RMSE=0.226 degrees C. This is probably because the relatively warm subsurface flow input to the stream channel from the non-glacial area. Copyright (C) 2009 John Wiley & Sons, Ltd.
• Isotopic shifts with size, culture habitat, and enrichment between the diet and tissues of the Japanese scallop Mizuhopecten yessoensis (Jay, 1857)

  Frolan A. Aya, Isao Kudo

  MARINE BIOLOGY 157 10 2157 - 2167 2010年10月 [査読有り][通常論文]
   

  Use of stable isotope signatures to trace diet patterns in cultured marine bivalves, particularly when changing culture habitat, requires knowledge of the isotopic shift and enrichment between diet and consumer's tissues. The aim of this study was to determine the patterns of isotope change and the variability of enrichment values (a dagger delta(13)C and a dagger delta(15)N) in different tissues (muscle, gonad, digestive gland) of the Japanese scallop (Mizuhopecten yessoensis). It was hypothesized that the isotopic signatures of a consumer's tissues changed during settlement and that the changes were related to variations in the isotopic signatures of food sources and gut contents. Particular attention was paid to the isotope enrichment between the diet and a consumer's tissues using isotope analysis of gut content. Muscle delta(15)N values decreased significantly 3-5 months post-settlement in a nearshore seabed, concomitant with the ingestion of lower delta(15)N food. For juvenile scallops, sinking particles (SP) were considered a more important dietary source than suspended particulate organic matter (SPOM), based on the correspondence between SP and gut contents delta(13)C. Enrichment values (a dagger delta(13)C and a dagger delta(15)N) varied with tissue and season. a dagger delta(15)N was 2.4aEuro degrees in muscle, 1.2aEuro degrees in gonad, and 0.7aEuro degrees in the digestive gland. a dagger delta(13)C was 3.2aEuro degrees in muscle, 2.3aEuro degrees in gonad, and -0.5aEuro degrees in the digestive gland. a dagger delta(15)N was the lowest in summer (0.3aEuro degrees), and a dagger delta(13)C was the highest in autumn (2.8aEuro degrees). a dagger delta(15)N values were significantly influenced by age, but not a dagger delta(13)C. Patterns of isotope ratios and enrichment values may be related to physiological attributes and differences in diet. This is the first study to demonstrate isotopic shift and enrichment encountered in different tissues of a cultured scallop when changing culture habitat.
• Dynamics of pelagic variables in two contrasting coastal systems in the western Hokkaido coast off Otaru port, Japan

  Julius I. Agboola, Mario Uchimiya, Isao Kudo, Kazuo Kido, Masato Osawa

  ESTUARINE COASTAL AND SHELF SCIENCE 86 3 477 - 484 2010年02月 [査読有り][通常論文]
   

  Human impact on adjacent coastal waters, leading to alteration in nutritional environment and hence affecting phytoplankton biomass (Chlorophyll a), will probably be enhanced by the nearby presence of ports. The main goal of this study is to assess the influence of nearby presence of port on phytoplankton biomass build-up and the physical-chemical environmental characteristics in two contrasting coastal systems (Otaru port, S-IN and an exposed coastal area, S-OUT) in the western Hokkaido coast off Otaru port, Japan. Sampling was conducted on "bi-weekly and monthly" basis during the period of September 2006-December 2007 and data comprising 11 pelagic variables were obtained. In most instance, phytoplankton biomass, nutrients' (NH4, NO3, PO4, and Si(OH)(4)) concentrations and nutrients' molar ratios were higher at the Otaru port location. Physical parameters (temperature, salinity, hydrogen ion concentration (pH), photosynthetically active radiation (PAR) and dissolved oxygen, (DO)) were not significantly different (P > 0.05) between the two locations. With the exception of salinity, pH and DIC, all variables measured showed significant variation (P < 0.05) with season. While the coefficient of variation (CV) of physical parameters and phytoplankton biomass were relatively higher in Otaru port location (SIN), the exposed coastal location (S-OUT) showed a higher variation in chemical parameters. Other variables showed different patterns between the two locations. We conclude that ports, due to its activities and restricted circulation favour high nutrient loading and phytoplankton biomass build-up in adjacent coastal systems, thus, suggesting the need for continuous field observation data in order to advance our knowledge on possible future human impact on coastal environment and the need to monitor and control port activities. (C) 2009 Elsevier Ltd. All rights reserved.
• Iron-induced alterations of bacterial DMSP metabolism in the western subarctic Pacific during SEEDS-II

  Martine Lizotte, Maurice Levasseur, Isao Kudo, Koji Suzuki, Atsushi Tsuda, Ronal, P. Kiene, Michael G. Scarratt

  Deep-Sea Research Part II: Topical Studies in Oceanography 56 2889 - 2898 2009年12月15日 [査読無し][通常論文]
   

  The effect of added iron on bacterial cycling of the climate-active gas dimethylsulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) was tested during the second Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study (SEEDS II) from 19 July to 21 August 2004 aboard the R/V Hakuho-Maru. The study area in the northwest Pacific Ocean (48°N 165°E) was enriched with Fe and the conservative tracer, SF6, allowing the fertilized patch to be tracked. Microbial DMSP cycling rates were determined in the surface mixed layer (5 m) during incubations using the 35S-DMSP technique. The addition of iron resulted in a 4-fold increase in concentrations of chlorophyll a (chl a) within the surface mixed layer (5 m depth), and the length of the sampling period allowed the observation of both bloom and post-bloom conditions. Inside the fertilized patch, the alleviation of resource limitation gave rise to the concurrent increase in bacterial abundance and production. Changes in the phytoplankton community within the Fe-enriched patch translated into a sustained decrease in chl a-normalized particulate DMSP (DMSPp) concentrations, suggesting a preferential stimulation of the growth of DMSPp-poor phytoplankton species. Despite short-lived peaks of DMSPp within the Fe-enriched area, concentrations of DMSPp generally remained stable during the entire sampling period inside and outside the fertilized patch. During the Fe-induced bloom, microbial DMSP-sulfur (DMSP-S) assimilation efficiency increased 2.6-fold inside the Fe-enriched area, which indicated that as bacterial production increased, a greater proportion of DMSP-S was assimilated and possibly diverted away from the bacterial cleavage pathway (i.e. production of DMS). Our results suggest that iron-induced stimulation of weak DMSPp-producers and DMSP-assimilating bacteria may diminish the potential production of DMS and thus limit its flux towards the atmosphere over the subarctic Pacific Ocean. © 2009 Elsevier Ltd.
• Primary productivity, bacterial productivity and nitrogen uptake in response to iron enrichment during the SEEDS II

  Isao Kudo, Yoshifumi Noiri, William P. Cochlan, Koji Suzuki, Takafumi Aramaki, Tsuneo Ono, Yukihiro Nojiri

  DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 56 26 2755 - 2766 2009年12月 [査読有り][通常論文]
   

  Primary productivity (PP), bacterial productivity (BP) and the uptake rates of nitrate and ammonium were measured using isotopic methods ((13)C, (3)H, (15)N) during a mesoscale iron (Fe)-enrichment experiment conducted in the western subarctic Pacific Ocean in 2004 (SEEDS 11). PP increased following Fe enrichment, reached maximal rates 12 days after the enrichment, and then declined to the initial level on day 17. During the 23-day observation period, we observed the development and decline of the Fe-induced bloom. The surface mixed layer (SML) integrated PP increased by 3-fold, but was smaller than the 5-fold increase observed in the previous Fe-enrichment experiment conducted at almost the same location and season during 2001 (SEEDS). Nitrate uptake rates were enhanced by Fe enrichment but decreased after day 5, and became lower than ammonium uptake rates after day 17. The total nitrogenous nutrient uptake rate declined after the peak of the bloom, and accumulation of ammonium was obvious in the euphotic layer. Nitrate utilization accounted for all the requirements of N for the massive bloom development during SEEDS, whereas during SEEDS II, nitrate accounted for >90% of total N utilization on day 5, declining to 40% by the end of the observation period. The SML-integrated BP increased after day 2 and peaked twice on days 8 and 21. Ammonium accumulation and the delayed heterotrophic activity suggested active regeneration occurred after the peak of the bloom. The SML-integrated PP between days 0 and 23 was 19.0g Cm(-2). The SML-integrated BP during the same period was 2.6 g C m(-2), which was 14% of the SML-integrated PR Carbon budget calculation for the whole experimental period indicated that 33% of the whole (particulate plus dissolved) PP (21.5 gCm(-2)) was exported below the SML and 18% was transferred to the meso-zooplankton (growth). The bacterial carbon consumption (43% of the whole PP) was supported by DOC or POC release from phytoplankton, zooplankton, protozoa and viruses. More than a half (56%) of the whole PP in the Fe patch was consumed within the SML by respiration of heterotrophic organisms and returned to CO(2). (C) 2009 Elsevier Ltd. All rights reserved.
• Production and air-sea flux of halomethanes in the western subarctic Pacific in relation to phytoplankton pigment concentrations during the iron fertilization experiment (SEEDS II)

  Shinya Hashimoto, Shuji Toda, Koji Suzuki, Shungo Kato, Yasusi Narita, Michiko K. Kurihara, Yoko Akatsuka, Hiroshi Oda, Takahiro Nagai, Ippei Nagao, Isao Kudo, Mitsuo Uematsu

  DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 56 26 2928 - 2935 2009年12月 [査読有り][通常論文]
   

  Iron could play a key role in controlling phytoplankton biomass and productivity in high-nutrient, low-chlorophyll regions. As a part of the iron fertilization experiment carried out in the western subarctic Pacific from July to August 2004 (Subarctic Pacific iron Experiment for Ecosystem Dynamics Study II-SEEDS II), we analysed the concentrations of trace gases in the seawater for 12 d following iron fertilization. The mean concentrations of chlorophyll a in the mixed layer (5-30m depth) increased from 0.94 to 2.81 mu g L(-1) for 8 d in the iron patch. The mean concentrations of methyl bromide (CH(3)Br; 5-30 in depth) increased from 6.4 to 13.4 pmol L(-1) for 11 d; the in-patch concentration increased relative to the out-patch concentration. A linear correlation was observed between the concentrations of 19'-hexanoyloxyfucoxanthin, which is a biomarker of several prymnesiophytes, and CH(3)Br in the seawater. After fertilization, the air-sea flux of CH(3)Br inside the patch changed from influx to efflux from the ocean. There was no clear evidence for the increase in saturation anomaly of methyl chloride (CH(3)Cl) due to iron fertilization. Furthermore, CH(3)Cl fluxes did not show a tendency to increase after fertilization of the patch. In contrast to CH(3)Br, no change was observed in the concentrations of bromoform (in-patch day 11 and out-patch day 11: 1.7 and 1.7 pmol L(-1)), dibromomethane (2.1 and 2.2 pmol L(-1)), and dibromochloromethane (1.0 and 1.2 pmol L(-1), respectively). The concentration of isoprene, which is known to have a relationship with chlorophyll a, did not change in this study. The responses of trace gases during SEEDS II differed from the previous findings (in situ iron enrichment experiment-EisenEx, Southern Ocean iron experiment-SOFeX, and Subarctic Ecosystem Response to Iron Enrichment Study-SERIES). Thus, in order to estimate the concomitant effect of iron fertilization on the climate, it is important to assess the induction of biological activity and the distributions/air-sea fluxes of trace gases by iron addition. (C) 2009 Published by Elsevier Ltd.
• The response of the virus community to the SEEDS II mesoscale iron fertilization

  Julie L. Higgins, Isao Kudo, Jun Nishioka, Atsushi Tsuda, Steven W. Wilhelm

  DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 56 26 2788 - 2795 2009年12月 [査読有り][通常論文]
   

  Although the important role of viruses in marine biogeochemical cycles has been established in recent years, virus activity (including changes in this activity) has been largely ignored during mesoscale iron (Fe)-fertilization experiments relative to other processes. This is of particular interest as viruses have been shown to be critical to the transformation of Fe from the particulate (i.e., biological) to the dissolved pools. The goal of the present study was to evaluate changes in the virus-mediated lysis of heterotrophic bacterial cells following a shift in ecosystem trophic status brought about by a mesoscale Fe addition in the subarctic Pacific Ocean. Virus production rates, estimated by a reduction and reoccurrence assay, were coupled with transmission electron microscopy estimates of burst size and direct counts of virus and bacterial abundance. Fe fertilization of the upper mixed layer resulted in significant yet weak increases in virus production rates during the 12 days of observation immediately after fertilization, although the burst size (viruses produced per lytic event) and the percentage of visibly infected cells remained constant. The results imply that increases in virus production rates were most likely tied to a decreased lytic cycle length or the stimulation of lysogenized cells following the stimulation of primary and secondary productivity by the addition of Fe. The results also indicate that virus-induced cell-lysis regenerated an estimated nearly 200 pmol L(-1) Fe daily, providing a significant return of Fe back to the water column, which may be critical in the maintenance of this added Fe as resident. (C) 2009 Elsevier Ltd. All rights reserved.
• Responses of DMS in the seawater and atmosphere to iron enrichment in the subarctic western North Pacific (SEEDS-II)

  Ippei Nagao, Shinya Hashimoto, Koji Suzuki, Shuji Toda, Yasushi Narita, Atsushi Tsuda, Hiroaki Saito, Isao Kudo, Shungo Kato, Yoshizumi Kajii, Mitsuo Uematsu

  DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 56 26 2899 - 2917 2009年12月 [査読有り][通常論文]
   

  Simultaneous measurements of dimethylsulfide (DMS) in the seawater and atmosphere were conducted during SEEDS-II to investigate the responses of DMS to iron (Fe) fertilization in the subarctic North Pacific. No significant increases in the seawater DMS (DMSw) concentration were observed inside the fertilized patch compared to those outside the patch, while particulate dimethylsulfioniopropionate (DMSPp) concentration inside the patch increased 2-fold compared to those outside the patch in the phytoplankton bloom of major DMSP producers such as prasinophytes, cryptophytes, diatoms and prymnesiophytes. In the decline phase of the bloom, maximum DMSw was observed both inside the patch (ca. 6.2 nM) and outside the patch (ca. 9.3 nM). In this period, increases in mesozooplankton and decreases in the DMSP producers (prymnesiophytes and diatoms) were observed both sides of the patch, but larger inside the patch than outside the patch. Large decreases in the DMSPp inside the patch, which was probably related to the large increases in mesozooplankton inside the patch, did not result in increases in the DMSw concentration. Considering biological and nonbiological parameters, we discussed these results, although they could not be completely explained. Unfortunately, the impact of Fe fertilization on the atmospheric DMS (DMSa) concentration was not detected due to no significant changes in DMSw. However, it is noted that DMSa concentrations were dependent on the sea-air DMS flux in the air from higher latitudes and/or the Eurasian continent, though the DMS flux was a minor role to the budget of DMSw. Therefore if DMSw were significantly changed by Fe fertilization, DMSa might be affected through changes in the sea-air flux in this condition. (C) 2009 Elsevier Ltd. All rights reserved.
• Temporal changes in community composition of heterotrophic bacteria during in situ iron enrichment in the western subarctic Pacific (SEEDS-II)

  Takafumi Kataoka, Koji Suzuki, Maki Hayakawa, Isao Kudo, Seigo Higashi, Atsushi Tsuda

  DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 56 26 2779 - 2787 2009年12月 [査読有り][通常論文]
   

  Little is known about the effects of iron enrichment in high-nitrate low-chlorophyll (HNLC) waters on the community composition of heterotrophic bacteria, which are crucial to nutrient recycling and microbial food webs. Using denaturing gradient gel electrophoresis (DGGE) of 16S rDNA fragments, we investigated the heterotrophic eubacterial community composition in surface waters during an in situ iron-enrichment experiment (SEEDS-II) in the western subarctic Pacific in the summer of 2004. DGGE fingerprints representing the community composition of eubacteria differed inside and outside the iron-enriched patch. Sequencing of DGGE bands revealed that at least five phylotypes of alpha-proteobacteria including Roseobacter, Cytophaga-Flavobacteria-Bacteroides (CFB), gamma-proteobacteria, and Actinobacteria occurred in almost all samples from the iron-enriched patch. Diatoms did not bloom during SEEDS-II, but the eubacterial composition in the iron-enriched patch was similar to that in diatom blooms observed previously. Although dissolved organic carbon (DOC) accumulation was not detected in surface waters during SEEDS-II, growth of the Roseobacter clade might have been particularly stimulated after iron additions. Two identified phylotypes of CFB were closely related to the genus Saprospira, whose algicidal activity might degrade the phytoplankton assemblages increased by iron enrichment. These results suggest that the responses of heterotrophic bacteria to iron enrichment could differ among phylotypes during SEEDS-II. (C) 2009 Elsevier Ltd. All rights reserved.
• Biogeochemical cycling of N and Si during the mesoscale iron-enrichment experiment in the western subarctic Pacific (SEEDS-II)

  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.
• The interactions between the Yukon River and Bering Sea

  K. A. Chikita, Y. Kim, I. Kudo, S. Saito, I. Wada, H. Miyazaki

  FROM HEADWATERS TO THE OCEAN: HYDROLOGICAL CHANGES AND WATERSHED MANAGEMENT 647 - 651 2009年 [査読有り][通常論文]
   

  As a first step to explore the effects of water, sediment, chemical and heat fluxes of the Yukon River on the ecosystem and sea ice formation in the Bering Sea, discharge, water turbidity and water temperature were monitored at a downstream site in June 2006-September 2007. The monitoring revealed that suspended sediment concentration fluctuates in the ice-covered season by rainfall sediment runoff in the upstream in late fall and probably by the movement of covered ice mass in early spring. During the high river sediment load in summer, the sediment plume was formed around the Yukon river delta. In order to clarify the dynamic behaviors of river water, the sediment plume was simulated by using a CFD program with a three-dimensional topographic model. The simulations were agreeable to observations off the Yukon delta and to the dispersal pattern of sediment plume by a MODIS image.
• Simultaneous estimation of viral lysis and protozoan grazing on bacterial mortality using a modified virus-dilution method

  Yosuke Taira, Mario Uchimiya, Isao Kudo

  MARINE ECOLOGY PROGRESS SERIES 379 23 - 32 2009年 [査読有り][通常論文]
   

  A modified dilution method designed to simultaneously estimate bacterial mortality due to viral infection and protozoan grazing was developed and compared with the standard dilution protocol. Various fractions of original seawater (non-filtered) and 1.0 mu m-filtered seawater (grazer-free) collected from coastal waters in Hokkaido, Japan, were diluted with 10 kDa filtered seawater (Virus-free) to set up 4 gradients of predator-prey interaction, and monitored every 12 h for bacterial abundance during a 48 h incubation. In more diluted fractions, bacterial abundance increased rapidly, and a good linear fit with a negative slope (as mortality) was found between the apparent. growth rate and fractions of original water. The resulting slopes of the regression in samples prepared from the original seawater were significantly higher than grazer-free samples, which denotes both viral lysis and protozoan grazing in the former, and only viral lysis in the latter. Ranges of specific lytic and grazing rate were 0.53 to 0.98 and 0.05 to 0.13 d(-1), respectively, and lytic pressure accounted for 87 to 91% of the total mortality. Comparison Of Our method (using virus-free diluent) with the standard dilution protocol (using 0.2 mu m diluent) showed significant. differences in slope and y-intercept (potential growth rate without mortality) found at 0,26 d(-1) and 0.88 d(-1), respectively, The above results suggest. that using the standard dilution protocol might. underestimate instantaneous growth rate, particularly in environments where lytic pressure is relatively high.
• Seasonal change of riverine nutrients and distribution of chlorophyll a in Ishikari Bay, subarctic oligotrophic coastal environment of Japan.

  Agboola, I. J, Yoshi, S, Kudo, I

  La mer 47 1 1 - 17 2009年 [査読有り][通常論文]
  
• Evidence for the grazing hypothesis: Grazing reduces phytoplankton responses of the HNLC ecosystem to iron enrichment in the western subarctic pacific (SEEDS II)

  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.
• Nutrient regeneration at bottom after a massive spring bloom in a subarctic coastal environment, Funka Bay, Japan

  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.
• Isotopic determination of Japanese scallop Patinopecten (Mizuhopecten) yessoensis (Jay) tissues shows habitat-related differences in food sources

  Frolan A. Aya, Isao Kudo

  JOURNAL OF SHELLFISH RESEARCH 26 2 295 - 302 2007年08月 [査読有り][通常論文]
   

  Using stable isotope analysis. this study examined the potential food sources of the Japanese scallop, Patinopecten (Mizuhopecten) yessoensis (Jay) and determined whether the isotopic ratios inferred from tissues of cultured scallops varied according, to Culture systems (hanging and sowing). Different tissues (i.e., muscle, ctenidia, and digestive gland) of scallops nesting in Saloma Lake (18.3-113.0 mm shell height) and Tokoro (66.8-121.4 mm shell height) and their potential food sources were analyzed for stable carbon and nitrogen isotope ratios. Average isotope ratios obtained in scallop muscle and ctenidia (-20.6 and -21.0 degrees(infinity) for delta C-13: 3.3 and 3.0 parts per thousand for delta N-15) from Tokoro area (sowing Culture; similar to 65 in depth) had close affinities to Suspended particulate organic matter (-21.6 parts per thousand and 2.7 parts per thousand for delta C-13 Lind delta N-15, respectively) suggesting that the scallops from Tokoro area depends oil detritus as in important food Source. Saloma Lake (hanging Culture; similar to 20 m depth) scallop muscle and ctenidia tissues however had relatively enriched delta C-13 values (-17.7 and -18.1 parts per thousand) and were relatively closer to either net plankton (-21.1 parts per thousand) and particulate organic matter (-22.1 parts per thousand) delta N-15 values were also higher in particulate organic matter taken in Saloma Lake (5.0 parts per thousand) and showed a 5.1 parts per thousand difference when muscle tissues in two sites are compared. Digestive gland tissues however showed depleted delta C-13 and delta N-15 signatures relative to other tissues in both systems. The observed variations in isotopic ratios in scallops rea red in two systems thus reflect differences in carbon and nitrogen sources related to culture habitats.
• 亜寒帯河川流域の流出に与える山岳氷河の影響について

  和田知之, 知北和久, 工藤 勲, 城戸大作, 成田悠一

  日本水文科学会誌 37 3 103 - 113 THE JAPANESE ASSOCIATION OF HYDOROLOGICAL SCIENCES 2007年 [査読有り][通常論文]
   

  The subarctic Tanana River basin, Alaska, is occupied by ca. 5.6% glacierized region and the other discontinuous permafrost region. Time series of discharge, suspended sediment concentration (SSC) and water chemistry in the Tanana River were obtained in the glacier-melt seasons of 2000 to 2005. In order to estimate each contribution of glacier-melt to discharge, SSC and water chemistry, all the time series were reproduced by the tank model. As a result, the reasonable simulations (correlation coefficient R = 0.68-0.97) for the discharge time series revealed that glacial-melt discharge occupies 33-58% of the Tanana river discharge. The SSC and SiO₂ time series were also reasonably simulated (R = 0.70-0.87). These calculations led to the conclusion that the major sediment and SiO₂ sources are located in the glacierized and permafrost regions, respectively.
• Effect of high iron concentrations on iron uptake and growth of a coastal diatom Chaetoceros sociale

  Shouei Iwade, Kenshi Kuma, Yutaka Isoda, Masahiko Yoshida, Isao Kudo, Jun Nishioka, Koji Suzuki

  AQUATIC MICROBIAL ECOLOGY 43 2 177 - 191 2006年06月 [査読有り][通常論文]
   

  The growth and iron uptake of the coastal marine diatom Chaetoceros sociale were experimentally measured in batch experiments at 10 degrees C to which an acidic Fe(III) stock solution was added. The direct input of Fe(III) into the culture media induced the highest iron uptake rate (similar to 3.4 to 4.2 x 10(-16) mol Fe cell(-1) d(-1)) by C. sociale during the first day of the incubation, resulting from the supply of bioavailable inorganic Fe(III) species at levels above its expected equilibrium value (similar to 0.1 nmol l(-1)) with solid amorphous Fe(III) hydroxide in seawater. The iron uptake rate during the first day of incubation in solid amorphous Fe(Ill) hydroxide medium aged for 1 d at 10 degrees C was approximately 50% lower than that in the direct Fe(III) input media. We used a modified approach in which further iron uptake by C. sociale from external iron in the direct Fe(III) input media was prevented by adding hydroxamate siderophore desferrioxamine B (DFB) during cultivation. After the addition of DFB, the highest growth rate (similar to 0.5 to 0.6 d(-1)) of C. sociale by intracellularly stored Fe in the direct Fe(III) input media was maintained for a few days since no iron uptake was observed after the DFB addition. The growth rate was independent of the amount of intracellularly stored Fe, However, the maximal cell yields appeared to be relatively dependent on the amount of intracellularly stored Fe, suggesting the presence of a critical concentration of intracellular Fe (minimum cellular Fe for growth) for phytoplankton growth (similar to 1 x 10(-16) mol Fe cell(-1) for C. sociale). In the present study, maximal and minimal Fe quotas were 3.4 to 4.2 x 10(-16) and similar to 1 x 10(-16) mol Fe cell(-1) (the maximal/minimal Fe ratio of 3.4 to 4.2), respectively. The high iron uptake and storage capacity in C. sociale allows this species to accumulate excess iron at high concentrations of bioavailable inorganic Fe species and to support up to 1.8 to 2.1 cell divisions without any additional iron uptake. In addition, we attempted to model the effect of luxury uptake on growth, as biodilution of cellular Fe eventually decreases the Fe quota to a critical threshold.
• Distribution and sources of organic matter in surficial sediments on the shelf and slope off Tokachi, western North Pacific, inferred from C and N stable isotopes and C/N ratios

  T Usui, S Nagao, M Yamamoto, K Suzuki, Kudo, I, S Montani, A Noda, M Minagawa

  MARINE CHEMISTRY 98 2-4 241 - 259 2006年02月 [査読有り][通常論文]
   

  Organic carbon (C) and total nitrogen (N) contents and corresponding isotope ratios were determined in surficial sediment (0-3 cm) at 94 stations ranging from 21 to 1995 m water depth off Tokachi, Hokkaido, Japan, to elucidate the distribution and source of sedimentary organic matter. Suspended particulate organic matter (POM) in the seawater and suspended POM and sediment in the Tokachi River were also examined. delta C-13, delta N-15 and C/N ratios of the samples in the Tokachi River suggest that the spring snowmelt is an important process for the transport of terrestrial organic matter to the coastal waters. delta C-13 values of suspended POM in the Surface seawater were higher in May and November than in August, while delta N-15 values of the POM were higher in May and August than in November. These changes are attributed to seasonal changes in phytoplankton growth rate and nitrate availability. delta C-13 and delta N-15 values in the sediments off Tokachi were lowest near the Tokachi River mouth, and increased offshore to constant values that persisted from 134 to 1995 m water depth. The spatial variation in C/N ratios in the sediment mirrored those of delta C-13 and delta N-15. Comparison of delta C-13, delta N-15 and C/N ratios in the sediments off Tokachi with those in the Tokachi River and seawater indicates that about half of the organic matter in the sediment was of terrestrial origin near the Tokachi River mouth, and the sedimentary organic matter from 134 to 1995 in water depth was of marine origin. The organic C content in the sediment was high near the Tokachi River mouth, and also around 1000 m water depth. The C content was significantly correlated with silt plus clay content, with different regression lines for those stations shallower and deeper than 134 m, owing to several stations of higher C content with the elevated C/N ratio on the inner shelf. These results suggest that transport and deposition of organic-rich fine sediment particles by hydrodynamic processes were major factors controlling C content off Tokachi. In addition, the supply of a fraction of terrestrial organic matter with high C/N probably also affected C content on the inner shelf. (c) 2005 Elsevier B.V. All rights reserved.
• Effect of high iron level on iron uptake and growth of marine phytoplankton

  Iwade, S, K. Kuma, Y. Isoda, M. Yoshida, I. Kudo, J. Nishioka, K. Suzuki

  Aquatic Microbial Ecology 43 177 - 191 2006年 [査読有り][通常論文]
  
• Phytoplankton community response to Fe and temperature gradients in the NE (SERIES) and NW (SEEDS) subarctic Pacific Ocean

  Isao Kudo, Yoshifumi Noiri, Jun Nishioka, Yousuke Taira, Hiroshi Kiyosawa, Atsushi Tsuda

  DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 53 20-22 2201 - 2213 2006年 [査読有り][通常論文]
   

  Ship-board iron enrichment bottle experiments were carried out with samples collected at the mesoscale iron fertilization experimental site (SERIES) in the subarctic NE Pacific in the summer of 2002. Samples were collected on Day 14 of the experiment outside the patch that was in a typical high nitrate and low chlorophyll (HNLC) condition. The iron concentration in the incubation bottles ranged from 0.1 to 2.0 nM by adding FeCl(3) solution. The increase in chlorophyll-a (chl-a) in the micro (> 10 mu m) and nano-sized (2-10 mu m) fraction was observed as a function of the added iron. Chl-a in the pico-sized fraction (0.7-2 pm) showed no increase with time. Nitrate and silicate were exhausted in the Fe-amended bottles, while those in the control bottle remained at the end of incubation. The relative consumption ratio of silicate to nitrate for the control bottles was significantly higher than that for the Fe-amended bottles. As a hyperbolic relation was found between iron concentration and the rate of increase in Chl-a (specific growth rate) for the micro and nano-sized fraction, the Monod equation was fit to obtain a maximum growth rate and a half-saturation constant for iron (K(Fe)). The mu(max) values were 0.72 and 0.48 d(-1) for the micro and nano-sized fraction, respectively. The K(Fe) values were 0.10 and 0.08 nM for the micro and nano-sized fraction, respectively. The mu(max) agreed with the rate of increase in Chl-a observed in situ for the mesoscale iron fertilization experiment. The P-a value for micro-sized fraction at 12 degrees C was half of that in the western subarctic Pacific Ocean (SEEDS experiment in 2001), indicating the Chl-a increase rate (potential growth rate) after iron enrichment was much higher in SEEDS than that in SERIES. The K(Fe) values were much lower than that in SEEDS, suggesting that the phytoplankton community in the NE subarctic Pacific Ocean acclimates to a lower ambient Fe concentration. This difference in K(Fe) between SERIES (NE) and SEEDS (NW) may reflect the previously suggested gradient in Fe flux to the subarctic Pacific Ocean. A temperature gradient was also applied to investigate the effect of temperature on the growth response of the phytoplankton community. No obvious effect of temperature increase to 16 degrees C was found in SERIES, while mu(max) and K(Fe) changed significantly with temperature in SEEDS. (c) 2006 Elsevier Ltd. All rights reserved.
• Nutrient and phytoplankton dynamics during the stationary and declining phases of a phytoplankton bloom induced by iron-enrichment in the eastern subarctic Pacific

  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.
• Mesozooplankton response to iron enrichment during the diatom bloom and bloom decline in SERIES (NE Pacific)

  Atsushi Tsuda, Hiroaki Saito, Jun Nishioka, Tsuneo Ono, Yoshifumi Noiri, Isao Kudo

  DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 53 20-22 2281 - 2296 2006年 [査読有り][通常論文]
   

  A mesoscale iron-fertilization experiment was carried out in the eastern subarctic Pacific during summer 2002. The iron patch was traced for 26 days after the enrichment, and the abundance and behavior of mesozooplankton was compared with those outside of the patch during the first half of the experiment (days 2-18) by Sastri and Dower [2006. Mesozooplankton community response during the SERIES iron enrichment experiment in the subarctic NE Pacific. Deep-Sea Research Part II.) and during the post-enrichment diatom bloom and its period of decline (days 15-26; this paper). The surface chlorophyll-a concentration in the patch was high between days 15 and 17 (6 mg m(-3)) and decreased to 1.4 mg m(-3) at the end of the observation. Dominant zooplankton species in the upper 200 m were copepods: Eucalanus bungii, Pseudocalanus spp., Neocalanus plumchrus, N. cristatus, and Metridia pacifica. Species composition did not change significantly in the patch over the observation period. However, shallower distribution depths of E bungii, N. cristatus and M. pacifica were observed in the patch during and after the diatom bloom. Especially, E. bungii was mainly distributed in the subsurface layer outside of the patch, but it was mainly in the surface mixed layer inside the patch, where it also had an enhanced development rate and increased biomass. We also propose the accumulation mechanism of zooplankton in the patch due to the upward immigration. Moreover, the abundance of the first copepodite stage of E. bungii and calyptopis larvae of euphausiids increased several fold in the patch compared to the densities outside the patch. The increases in both species are considered to be due to lowered mortality during the egg and naupliar stages, which was caused by lowered relative importance of eggs and nauplii in the diets of the suspension-feeding omnivores in the patch due to increased diatom abundance during the diatom bloom. Gut-pigment contents of dominant copepods in the patch increased 6-8 times, and the maximum values were observed during the bloom peak. The grazing impact on phytoplankton was low during the bloom period, but increased in the declining period of the diatom bloom. (c) 2006 Elsevier Ltd. All rights reserved.
• 噴火湾における低次生産過程と貝毒プランクトンの中長期変動

  工藤 勲, 宮園 章, 嶋田 宏, 磯田 豊

  沿岸海洋研究 43 1 33 - 38 日本海洋学会沿岸海洋研究部会 2005年 [査読有り][通常論文]
   

  ホタテ貝の安全流通の観点から,麻痺性貝毒と下痢性貝毒について主要産地である噴火湾において1970年代から北海道(水産林務部)によってモニタリングが行われている.1980年代に高い麻痺性貝毒値が春から夏にかけて検出されたが,1990年代以降は,ほとんど検出されなくなった.これは,この貝毒の原因種である独立栄養型渦鞭毛藻Alexandrium tamarenseの消長と一致している.また堆積物中のA.tamarenseの休眠胞子は,海底から5-7cmと12-14cmに高密度を示す鉛直分布を示した.このことは,噴火湾におけるA.tamarenseは,数十年周期で消長を繰り返していることを示唆している.つまり,この種は,珪藻による春季ブルーム後に出現することから,この種の消長は春季ブルームの発生時期,規模の変動による栄養塩消費と循環過程などの栄養塩動態あるいは水温などの気候変動を反映している可能性がある.
• Influence of iron and temperature on growth, nutrient utilization ratios and phytoplankton species composition in the western subarctic Pacific Ocean during the SEEDS experiment

  Y Noiri, Kudo, I, H Kiyosawa, J Nishioka, A Tsuda

  PROGRESS IN OCEANOGRAPHY 64 2-4 149 - 166 2005年 [査読有り][通常論文]
   

  On-board bottle incubation experiments were carried out by adding different amounts of iron as part of the SEEDS (Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study) experiment to elucidate the phytoplankton community response to various iron concentrations. A temperature gradient from 5 to 18 &DEG; C was also applied to the incubation experiment because temperature also affects growth rate and metabolic functions such as enzyme reactions. Chlorophyll-a increased with increasing iron concentration, while the increase in chlorophyll-a in the control bottles (no iron addition) was nominal. Size-fractionated chlorophyll-a indicated that micro-phytoplankton (> 10 μ m) became dominant in the iron-enriched bottles below 13 &DEG; C, which was consistent with the results from in situ observations in the simultaneous mesoscale iron-enrichment experiment and similar experiments for the other high nitrate and low chlorophyll (HNLC) waters. Macronutrients (silicate, nitrate and phosphate) were consumed in the bottles below 13 &DEG; C. The consumption ratio of silicate to nitrate was around 1:1 for iron-enriched bottles, but this ratio was slightly higher for control bottles, indicating a physiological stress due to low iron concentration on the diatoms. In contrast, nanophytoplankton (< 10 μ m) were dominant in the iron-enriched bottles at 18 &DEG; C. There was no silicate utilization, but nitrate and phosphate were consumed in these bottles. Bacillariophyceae were dominant in the bottles below 13 &DEG; C, but Prymnesiophyceae became dominant at 18 &DEG; C. This is the first report demonstrating that temperature influences phytoplankton composition after iron addition in HNLC waters. The Chlorophyll-a specific increase rate for micro-plankton (> 10 μ m) doubled from 5 to 8 &DEG; C and was the highest between 8 and 13 &DEG; C at the same iron concentration. The surface mixed layer temperature was 8 &DEG; C at the beginning of iron fertilization, but it had been 5 &DEG; C just 2 weeks before the experiment. This dramatic increase in growth rate with temperature is hypothesized to be the reason for SEEDS 2001 biomass changes being the highest reported response to a mesoscale Fe-enrichment experiment in HNLC waters. © 2005 Elsevier Ltd. All rights reserved.
• Primary productivity and nitrogenous nutrient assimilation dynamics during the Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study

  Kudo, I, Y Noiri, K Imai, Y Nojiri, J Nishioka, A Tsuda

  PROGRESS IN OCEANOGRAPHY 64 2-4 207 - 221 2005年 [査読有り][通常論文]
   

  Primary productivity and uptake rates of nitrate and ammonium were measured during an iron enrichment-experiment in the western subarctic Pacific Ocean using stable C-13 and N-15 isotope methods with simulated, on-board, for 24-h incubations. Nitrate, phosphate and silicate before the iron enrichment in the surface mixed layer (0-10 m) were abundant at 17, 1.5 and 34 μ M, respectively. Primary productivity in the upper 20 m ranged from 10 to 30 mg C m(-3) d(-1) on Day 0 (the day of iron fertilization) and did not change Outside the iron patch during 13 days of the observation. Primary productivity in the iron patch increased more than 10-fold after Day 7 and the maximum of 310 mg C m(-3) d(-1) was observed on Day 9. The elevated primary productivity continued until Day 13 (end of the observation). The integrated primary productivity in the photic zone was about 2 g C m(-2) d(-1) after Day 7. During 13 days of the observation, 16.5 g C m(-2) was fixed in the patch, while C fixation was 4.5 g C m(-2) outside. Absolute nitrate uptake at 5 m depth was 0.2 μ M d(-1) on Day 0, almost same as the ammonium uptake rate. Nitrate uptake sharply increased 20 times after Day 7. The maximum rate of 4.2 μ M d(-1) was observed on Day 9. Total nitrate uptake at 5 m between Days 4 and 9 was 10.8 μ M, which was comparable to the net decrease of nitrate at 8.5 μ M at this depth. Uptake rate of ammonium was 0.2 μ M d(-1) on Day 0 and had not changed by Day 13. The f-ratio (a ratio of nitrate uptake to total nitrogen (nitrate and ammonium) uptake) was 0.4-0.6 before Day 4, but increased to 0.95 on Day 7 and remained high until Day 13. These results suggest that the nitrate assimilation pathway was induced by the iron enrichment to meet the high nitrogen demand from rapid growth of the dominant diatom Chaetoceros dedilis. Relative uptake rate among nutrients was estimated from their apparent concentration change at 5 m in the patch. Nitrate and phosphate decreased at a molar ratio of 15.5, close to Redfield's stoichiometry. Silicate to nitrate uptake ratio, however, changed sharply on Day 9 from 1.6 to 3.0, indicating physiological stress on growing diatoms. Photosynthetically active radiation (PAR) at 5 in decreased from 400 to < 100 μ mol m(-2) s(-1) after Day 8 due to the high biomass of phytoplankton within the patch. Dissolved iron concentration also decreased to less than 0.3 nM after Day 9. This concentration was below the half saturation level for iron (K-s) obtained from on-board iron-addition incubations. These results suggest that low light and low iron concentration caused the physiological stress on diatoms after Day 8. © 2005 Elsevier Ltd. All rights reserved.
• Export fluxes of particulate organic carbon estimated from Th-234/U-238 disequilibrium during the Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study (SEEDS 2001)

  T Aono, M Yamada, Kudo, I, K Imai, Y Nojiri, A Tsuda

  PROGRESS IN OCEANOGRAPHY 64 2-4 263 - 282 2005年 [査読有り][通常論文]
   

  The Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study (SEEDS) was carried out in the western subarctic gyre of the North Pacific to verify the iron-limitation hypothesis. Activities of dissolved Th-234 in the upper 200 m of the water column and of particulate Th-234 collected in a drifting sediment trap were measured during SEEDS 2001. Dissolved Th-234 activities decreased and suspended particulate Th-234 increased in the upper 20 m of the water column with the formation of an iron-induced phytoplankton bloom. The calculated residence times of the dissolved and particulate Th-234 in the upper 20 m of the water column ranged from 2 to 13 d and 13 to 29 d, respectively. The ratios of particulate organic carbon (POC)/Th-234(p) (mu mol dpm(-1)) in the settling particles at 200 m depth varied between 1.94 and 4.02, and these levels were 10% those for the suspended particles. No large increase of the POC export flux was observed for the drifting sediment trap deployments during the observation period. In addition, the POC export fluxes derived from Th-234 with a non-steady-state model varied from 10.1 to 38.3 mmol m(-2) d(-1). The ratio of the estimated POC export flux to primary production, Th-E, was nearly constant (0.23-0.24) from July 20 to 27 (D2 and D9 after iron-enrichment) and decreased to 0.18 on July 31 (D13 after iron-enrichment). Regardless of whether the POC export fluxes increased, there was no change in Th-E. The increase of the POC export fluxes estimated from Th-234 was much smaller than the increase of primary production. Thus, no large POC export fluxes are concluded to have occurred from the iron-enrichment patch, though the removal flux of Th-234 in settling particles increased with particulate biogenic silica fluxes as the phytoplankton bloom progressed. (c) 2005 Elsevier Ltd. All rights reserved.
• Effect of iron enrichment on the dynamics of transparent exopolymer particles in the western subarctic Pacific

  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.
• The decline and fate of an iron-induced subarctic phytoplankton bloom

  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).
• Change in the uptake and cellular Si : N ratio in diatoms responding to the ambient Si : N ratio and growth phase

  Kudo, I

  MARINE BIOLOGY 143 1 39 - 46 2003年07月 [査読有り][通常論文]
   

  A change in the Si:N ratio of diatom cells during growth was examined for Chaetoceros socialis and Thalassiosira sp., with different initial silicate to nitrate (Si:N) ratios in the media. During exponential growth, C. socialis assimilated silicate and nitrate with a molar ratio of 0.5, independent of the ratio in the media, but after the depletion of nitrate, silicate continued to be taken up, and the Si:N ratio in the stationary phase increased to 2 as a function of the Si:N ratios in the media. In contrast, the ratio of silicate to nitrate taken up by Thalassiosira sp. increased with an increase in the Si:N ratio in the media. The Si:N ratio in the cells during the stationary phase increased in response to an increase in this ratio in the media. The Si:chl a ratio also increased with the increase in the initial Si:N ratio in the media, while the N:chl a ratio did not change to a great extent, indicating the changes in the cellular Si:N ratio was derived from changes in the Si content of the cells. These results indicated that the cellular Si:N ratio changed with the Si:N ratio in the medium, and the Si:N uptake ratio during the growth phase was different depending on diatom species. Thus, the dominance of different diatom species may affect nutrient composition and dynamics in the ocean.
• A mesoscale iron enrichment in the western Subarctic Pacific induces a large centric diatom bloom

  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.
• Size-fractionated iron distributions and iron-limitation processes in the subarctic NW Pacific

  Nishioka J, Takeda S, Kudo I, Tsumune D, Yoshimura T, Kuma K, Tsuda A

  Geophysical Research Letters 30 14 2003年 [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.
• 噴火湾における河川からの栄養塩負荷量およびそれらが湾内の基礎生産に与える影響

  芳村 毅, 工藤 勲

  海の研究 12 2 185 - 193 The Oceanographic Society of Japan 2003年 [査読有り][通常論文]
   

  河川から噴火湾への栄養塩負荷量を明らかにするために,噴火湾に流人する8河川および発電所排水路1か所において,流量および栄養塩濃度を一年にわたり観測した。その結果,河川流量は雪解け時の4月に特異的に高い明瞭な季節変化を示した。また,河川水中の栄養塩濃度は河川によって大きく異なったが,全河川の加重平均濃度は硝酸塩が24μM,アンモニウム塩が2.9μM,リン酸塩が0.30μM,ケイ酸塩が270μMであり. DIN:DIP比が非常に高いことが明らかとなった。河川から噴火湾への一年間の淡水負荷量は2.1×10⁹m³であり,溶存無機窒素,リン酸塩,ケイ酸塩負荷量はそれぞれ57×l0⁶mol,0.6.3×10⁶mol. 570x10⁶molだった。これらの栄養塩負荷量が噴火湾の基礎生産に与える影響は総生産を考慮した場合は非常に小さなものだった。しかしながら,新生産を夏期のリン酸塩の減少量から見積もった結果,河川由来の栄養塩が噴火湾の夏期の新生産の最大10%を支えていることが明らかとなった。
• Dynamics of bacterial respiration and related growth efficiency, dissolved nutrients and dissolved oxygen concentration in a subarctic coastal embayment

  CW Lee, Kudo, I, T Yokokawa, M Yanada, Y Maita

  MARINE AND FRESHWATER RESEARCH 53 1 1 - 7 2002年 [査読有り][通常論文]
   

  Temporal variations in dissolved oxygen, nutrient concentrations and oxygen utilization rates (< 0.7 μm fraction) were recorded for almost two years in the subarctic Funka Bay, Japan. Dissolved inorganic nitrogen ranged from 0 to 26 μM, phosphate from 0 to 2 μM and silicate from 1 to 64 μM. There was evidence of nitrogen limitation. Dissolved oxygen concentration ranged from 130 to 440 μM, and decreased to <150 muM in the bottom layer in summer. The rate of oxygen utilization, attributed to bacterial respiration, ranged from 0.6 to 9.3 muM day(-1) at 10 m depth, and from 0.8 to 5.0 muM day(-1) at 90 m depth, and was the principal mechanism causing the decrease in dissolved oxygen in the bottom layer in the summer. Bacterial growth efficiencies calculated for the 10 m and 90 m depths were similar: 1.6-17.2% and 1.4-23.6%, respectively. With the bacterial growth efficiencies <25%, the bacteria in Funka Bay acted as a net sink of carbon, where >75% of the organic matter flux through bacteria could be mineralized to CO2.
• Vertical distributions of iron(III) hydroxide solubility and dissolved iron in the northwestern North Pacific Ocean.

  S Nakabayashi, M Kusakabe, K Kuma, Kudo, I

  GEOPHYSICAL RESEARCH LETTERS 28 24 4611 - 4614 2001年12月 [査読有り][通常論文]
   

  Detailed vertical distributions of Fe(III) hydroxide solubilities and dissolved Fe concentrations, which are strongly related to the concentration and affinity of natural organic Fe(III) chelators in seawater, were measured at three typical stations in the northwestern North Pacific Ocean. Iron(III) hydroxide solubility in the surface mixed layer was generally high and variable (0.3-2.4 nM), corresponding with the depth of high chlorophyll a concentrations, and the solubility minima (0.2-0.4 nM) occurred at 75-125 m depth, The vertical profiles of Fe(III) hydroxide solubility in mid-depth and deep waters are characterized by mid-depth maxima (0.6-0.7 nM) and, subsequently, a slight decrease to 0.4-0.5 nM with depth, which is markedly similar to nutrient and dissolved Fe depth profiles. The solubility profiles reveal that dissolved Fe concentrations in deep ocean waters are controlled primarily by the Fe(III) complexation with natural organic ligands, which were released through the oxidative decomposition and transformation of biogenic organic matter in mid-depth and deep waters.
• Bacterial abundance and production and heterotrophic nanoflagellate abundance in subarctic coastal waters (Western North Pacific Ocean)

  CW Lee, Kudo, I, M Yanada, Y Maita

  AQUATIC MICROBIAL ECOLOGY 23 3 263 - 271 2001年02月 [査読有り][通常論文]
   

  We studied the temporal variations of bacterial abundance and production in subarctic Funka Bay (Hokkaido, Japan) for a 1 yr period, and related this data to chlorophyll a (chl a) concentration (highest cone. = 5.9 mug l(-1)), particulate organic carbon (POC) concentration (ranging from 60 to 360 mug C l(-1)), heterotrophic nanoflagellate (HNF) abundance (<0.5 to 5.6 x 10(3) cells ml(-1)), and temperature. Bacterial abundance ranged from 2.6 to 9.1 x 10(5) cells ml(-1), whereas bacterial production ranged from 0.1 to 22.9 <mu>g C l(-1) d(-1). The calculated bacterial growth rate ranged from 0.02 to 2.57 d(-1), with an average turnover time of 1.6 d, and 'attached' bacteria (>1 mum fraction) contributed more than half the total bacterial production (mean +/- 95 % CL = 54 +/- 12%). Bacterial abundance was relatively stable throughout the year (coefficient of variation, CV = 20 %) even though the CV for bacterial production was high (CV = 100%). Grazing (top-down) is an important control mechanism in Funka Bay, and HNF seemed to be directly cropping bacterial production. Annual bacterial production was estimated at 140 g C m(-2) yr(-1)), and was relatively high compared to the annual primary production (100 to 170 g C m(-2) yr(-1)), indicating the importance of the microbial loop in Funka Bay. After bacterial respiration loss was accounted for, bacterial carbon demand was higher than primary production. The inflow of Tsugaru warm water, a branch of the Kuroshio Current brought organic matter, but the concentration of POC and the timing of the inflow suggested that this organic matter addition was insignificant, and we concluded that other sources, including riverine input, are more important sources of organic matter.
• 渡島大沼における栄養塩濃度の季節変化と植物プランクトンの生長制限要因

  芳村 毅, 工藤 勲

  陸水学会誌 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.
• Examining the relationship between bacteria and heterotrophic nanoflagellates in Funka Bay (Japan) using the size-fractionation method

  Choon Weng Lee, Isao Kudo, Hiroyasu Morishita, Mitsuru Yanada, Yoshiaki Maita

  Aquatic Ecology 35 1 1 - 8 2001年 [査読有り][通常論文]
   

  The chemical and biological conditions, and the bacteria-heterotrophic nanoflagellate (HNF) relationship were investigated in the vicinity of Funka Bay, southwest of Hokkaido, Japan during early spring 1999. At the time of sampling, chlorophyll a concentration, bacteria, phycoerythrin rich-cyanobacteria, and HNF abundance were in the following ranges: 0.3-3.6 μg 1-1 2.5-5.6 × l05 cells ml-1, 0.6-1.2 × 103 cells ml-1 and 2.2-4.2 × 103 cells ml-1, respectively. Dissolved inorganic nitrogen, phosphate and silicate concentrations were in the ranges: 8.7-12.2 μM, 0.9-2.0 μM, and 21.6-25.5 μM, respectively. Primary production ranged from 6.4 to 76.3 mg C m-3 d-1. Using water samples from regions of different productivity levels (in and outside bay), the bacteria - HNF relationship was uncoupled experimentally by the size-fractionation technique. Higher primary production (19.9 mg C m-3 d-1) in the bay supported higher bacterial growth rate (0.029 h-1). However, outside the bay both primary production (6.4 mg C m-3 d-1) and bacterial growth rate (0.007 h-1) were lower. The HNF growth rates and grazing rates were similar for both but by comparing both HNF grazing capacity and bacterial production, there was net decrease in bacterial abundance outside the bay and net increase inside the bay. The microbial parameters (rates and abundance) and the amount of carbon flow estimated through the phytoplankton - dissolved organic matter (DOM) - bacteria loop were different between the coastal station and the open ocean station. However HNF grazing and growth rates was similar for both stations.
• Combined effects of temperature and iron on the growth and physiology of the marine diatom, Phaeodactylum tricornutum (Bacillariophyceae)

  Kudo, I, M Miyamoto, Y Noiri, Y Maita

  JOURNAL OF PHYCOLOGY 36 6 1096 - 1102 2000年12月 [査読有り][通常論文]
   

  Phaedactylum tricornutum Bohlin (Bacillariophyceae) was maintained in exponential growth under Fe-replete and stressed conditions over a range of temperatures from 5 to 30 degrees C, The maximum growth rate (CR) was observed at 20 degrees C (optimal temperature) for Fe-replete and stressed cells, There was a gradual decrease in the CR decreasing temperatures below the optimum temperature; however, the growth rate dropped sharply as temperature increased above the optimum temperature, Fe-stressed cells grew at half the growth rate of Fe-replete cells at 20 degrees C, whereas this difference became larger at lower temperatures, The change in metabolic activities showed a similar pattern to the change in growth rate temperature aside from their optimum temperature, Nitrate reductase activity (NRA) and respiratory electron transport system activity (ETS) per cell were maximal between 15 and 20 degrees C, whereas cellspecific photosynthetic rate (P-cell) was maximal at 20 degrees C for Fe-replete cells, These metabolic activities were influenced by Fe deficiency, which is consistent with the theoretical prediction that these activities should have an Fe dependency, The degree of influence of Fe deficiency, however, was different for the four metabolic activities studied: NRA > P-cell > ETS = GR, NRA in Fe-stressed cells was only 10% of that in Fe-replete cells at the same temperature, These results suggest that cells would have different Fe requirements for each metabolic pathway or that the priority of Fe supply to each metabolic reaction is related to Fe nutrition. Tn contrast, the order of influence of decreasing the temperature from the optimum temperature was ETS > P-cell > NRA > GR, For NRA, the observed temperature dependency could not be accounted for by the temperature dependency of the enzyme reaction rate itself that was almost constant with temperature, suggesting that production of the enzyme would be temperature dependent, For ETS, both the enzyme reactivity and the amount of enzyme accounted for the dependency, This is the first report to demonstrate the combined effects of Fe and temperature on three important metabolic activities (NRA, P-cell, and ETS) and to determine which activity is affected the most by a shortage of Fe, Cellular composition was also influenced by Fe deficiency, showing lower chi a content in the Fe-stressed cells. Chi a per cell volume decreased by 30% as temperature decreased from 20 to 10 degrees C under Fe-replete conditions, but chi a decreased by 50% from Fe-replete to Fe-stressed conditions,
• 噴火湾における春季ブルームの一次生産とその行方

  工藤 勲, 芳村 毅

  沿岸海洋研究 38 1 47 - 54 2000年 [査読無し][通常論文]
  
• Exhaustion of nitrate terminates a phytoplankton bloom in Funka Bay, Japan: change in SiO4,: NO3 consumption rate during the bloom

  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.
• Change in the water quality in Lake Ohnuma, Hokkaido, Japan: A comparison of 1977 and 1996

  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.
• Seasonal variation of two different heterotrophic bacterial assemblages in subarctic coastal seawater

  M Yanada, T Yokokawa, CW Lee, H Tanaka, Kudo, I, Y Maita

  MARINE ECOLOGY PROGRESS SERIES 204 289 - 292 2000年 [査読有り][通常論文]
   

  The numbers of heterotrophic bacteria in the subarctic coastal seawater of Funka Bay, Japan, were measured by now cytometry, and 2 different bacterial assemblages were detected: cells with low DNA content (Group I) and cells with high DNA content (Group II). Although the abundances of both Group I and Group II bacteria showed similar regression slopes against seawater temperature (p < 0.7), Group II bacteria consisted of high growth capacity cells, growing actively in an enriched incubation experiment, whereas Group I bacteria consisted of inactive cells, not responding to an enriched environment. The seasonal variation of the total bacterial abundance observed in Funka Bay was caused almost exclusively by Group II bacteria.
• Environmental factors affecting the occurrence and production of the spring phytoplankton bloom in Funka Bay, Japan

  Isao Kudo, Katsuhiko Matsunaga

  Journal of Oceanography 55 4 505 - 513 1999年08月 [査読有り][通常論文]
   

  The concentration of nutrients was measured during the spring phytoplankton bloom in Funka Bay over a 5-year period (1988-92). During the winter mixing period, nutrient concentrations were similar in every year except in 1990 when a high concentration of silicate was observed. There was interannual variation in the onset of the bloom, presumably depending on the stability of the water column. The bloom developed in early March when the Oyashio water (OW), which has a lower density than the existing winter water, flowed into the bay and the pycnocline formed near the bottom of the euphotic zone. In this case, high chl a was found only in the euphotic zone and nutrient utilization was limited to this zone. In the year when the inflow of OW was not observed by April, the bloom took place at the end of March without strong stratification and high chla was found in the whole water column, accompanied by a decrease in nutrients. Interannual differences were found not only at the beginning of the decrease, but also in the thickness of the layer which showed a decrease in nutrients. Primary production from the beginning to the end of the spring bloom was estimated from the nutrient budget before and after the spring bloom. The integrated production over the spring bloom period ranged from 25 to 73 g C m-2, which accounts for 19-56% of the annual production in this bay. We found that the timing of the bloom was strongly dependent on the inflow of OW, but the amount of production was not clearly related to this timing.
• Behavior of Cu, Ni and Cd during nutrient depletion in a spring bloom in Funka Bay

  Isao Kudo, Katsuhiko Matsunaga

  Journal of Oceanography 54 6 619 - 627 1998年 [査読有り][通常論文]
   

  The concentrations of Cu, Ni and Cd were determined in Funka Bay during a spring phytoplankton bloom, consisting of diatoms. Just after the bloom, both dissolved Cd and nutrients were removed in the euphotic zone. However, the removal ratio of Cd to phosphate was very different from that in seawater. The removal of Cd took place at a Cd/phosphate ratio of 0.07 x 10-3, which was lower than in seawater before the bloom (0.25 x 10-3), leading to an increase in this ratio in seawater exceeding 0.7 x 10-3 at the need of the bloom. Elevated concentrations of Cd and phosphate were observed in the deeper layer after the bloom due to the decomposition of detrital materials produced in the bloom. The ratio of Cd/phosphate in the regeneration step was 0.24 x 10-3 which was different from the removal ratio of 0.07 x 10-3. These observations suggest that the high Cd/phosphate ratio in the regeneration would reflect a relatively high regeneration rate of Cd than that of phosphate. No significant decrease in Cu and Ni concentrations was observed during the development of the bloom, suggesting that biological removal of these metals was not so significant during the spring bloom. The concentrations of Cd, Cu and silicate in surface waters increased after the bloom with decreasing salinity due to the influence of a spring thaw.
• Effect of iron nutrition on the marine cyanobacterium Synechococcus grown on different N sources and irradiances

  Kudo, I, PJ Harrison

  JOURNAL OF PHYCOLOGY 33 2 232 - 240 1997年04月 [査読有り][通常論文]
   

  The effects of Fe deficiency on the marine cyanobacterium Synechococcus sp. were examined in batch cultures grown on nitrate or ammonium as a sole nitrogen source under two different irradiances. Fe-stressed cells showed lower chlorophyll a content and cellular C and N quotas. Light limitation increased the critical iron concentration below which both suppression of growth rate and changes in cellular composition were observed. At a limiting irradiance (26 mu mol . m(-2). s(-1)), this critical value was similar to 10 nM, a 10 times increase compared to high-light cultures. Moreover, at low light the cellular chlorophyll a concentration was higher than at saturating light (110 mu mol . m(-2). s(-1)), this difference being most pronounced under Fe-stressed conditions. Cells grown on ammonium showed a lower half-saturation constant for Fe (K-s) compared to cells grown on nitrate, indicating Synechococcus sp. has the ability to grow faster on ammonium than on nitrate in a low Fe environment at high light. Consequently, in high-nutrient and low-chlorophyll regions where Fe limits new production, cyanobacteria most likely grow on regenerated ammonium, which requires less energy for assimilation. The K-s for growth on Fe at low light was significantly higher than at high light compared with the cells grown on the same N source, suggesting the cells require more Fe at low light. Therefore, if cells that are already fe-limited also become light-limited, their iron stress level will increase even more. For cyanobacteria this is the first report of a study combining the interactions of Fe limitation, light limitation, and nitrogen source (NO3- vs. NH4+).
• Cadmium in the southwest Pacific Ocean two factors significantly affecting the Cd-PO4 relationship in the ocean

  Kudo, I, H Kokubun, K Matsunaga

  MARINE CHEMISTRY 54 1-2 55 - 67 1996年09月 [査読有り][通常論文]
   

  Vertical profiles of dissolved Cd were determined in the southwest Pacific Ocean, which includes the Tasman Sea, the Solomon Sea, and the Coral Sea. Cd shows a nutrient type distribution, but weakly correlates with phosphate compared to the North Pacific Ocean when all data are plotted. The Cd/PO4 ratio in surface layer is < 0.1 nM/mu M and the ratio in the intermediate and deep water shows an increase approaching the Pacific value of similar to 0.3 nM/mu M. The Cd/PO4 ratio in the regeneration process has been estimated using an AOU (apparent oxygen utilization) respiration model. This ratio is affected by that in surface layer, so deep water receives a regeneration flux with a low Cd/PO4 ratio. The preformed Cd and phosphate as estimated by the AOU relation are low in the surface layer and high in the deep layers. The intermediate water is a mixture of surface water and Antarctic Intermediate Water (AAIW). The characteristic Cd-PO4 relationship in this area would be a consequence of mixing with two water masses which have different original Cd/PO4 ratios. The factors affecting the Cd-PO4 relationship in the world ocean are physical mixing and biological cycling, based on the results of the southern Pacific Ocean.
• Chemical fractionation of phosphorus and cadmium in the marine diatom Phaeodactylum tricornutum

  Kudo, I, H Kokubun, K Matsunaga

  MARINE CHEMISTRY 52 3-4 221 - 231 1996年05月 [査読有り][通常論文]
   

  Fractionation of P and Cd within the cell of Phaeodactylum tricornutum grown by semi-continuous cultures in natural low concentrations of dissolved phosphate and cadmium was examined by the chemical separation method. Cellular Cd content increased with the concentration of Cd in the medium and was independent of growth rate, but it was present only in the acid-soluble fraction and no growth inhibition was observed even at 90 nM. Cellular P content in the acid-soluble fraction varied with both growth rate and concentration in the medium. The residue fraction (acid-insoluble) depended on concentration in the medium. Proportion of these fractions, however, changed only as a function of growth rate. As a consequence of these changes, cellular Cd/P ratio decreased with growth rate. The distributions of Cd and P in the phytoplankton cells were not always the same portion. The Cd/P ratio in phytoplankton cells is affected by the physiological state of the cells and ambient Cd/PO4 ratio in the medium. Thus, biological activity would influence the biogeochemical cycling of Cd and PO4 and the Cd/PO4 ratio in the ocean.
• PHOTOREDUCTION OF MANGANESE-DIOXIDE IN SEAWATER BY ORGANIC-SUBSTANCES UNDER ULTRAVIOLET OR SUNLIGHT

  K MATSUNAGA, T OHYAMA, K KUMA, KUDO, I, Y SUZUKI

  WATER RESEARCH 29 2 757 - 759 1995年02月 [査読有り][通常論文]
   

  The reduction of particulate Mn(IV) in the presence of phytoplankton culture solution or hydroxycarboxylic acids, which might be derived from photodecomposition of phytoplankton cells, under ultraviolet (u.v.) or sunlight irradiation was investigated. The measurement of the oxidation number of the dissolved Mn during these experiments showed 2.1, indicating the reduction of particulate Mn(IV) to dissolved Mn(II). The reduction proportion of Mn(IV) to Mn(II) was 65, 30 and 30% for glucaric, gluconic and tartaric acids, respectively. The present study suggests that the reduction of particulate Mn(IV) to dissolved Mn(II) in seawater occurs under natural condition.
• Iron requirement of the brown macroalgae Laminaria japonica, Undaria pinnatifida (Phaeophyta) and the crustose coralline alga Lithophyllum yessoense (Rhodophyta), and their competition in the northern Japan Sea

  Suzuki, Y, K. Kuma, I. Kudo, K. Matsunaga

  Phycologia 34 201 - 205 1995年 [査読有り][通常論文]
  
• Diffusion of Fe(II) from an iron propagation cage and its effect on tissue iron and pigments of macroalgae on the cage

  Matsunaga, K, Y. Suzuki, K. Kuma, I. Kudo

  Journal of Applied Phycology 6 397 - 403 1994年 [査読有り][通常論文]
  
• EXISTENCE OF STABLE FE(II) COMPLEX IN OXIC RIVER WATER AND ITS DETERMINATION

  Y SUZUKI, K KUMA, KUDO, I, K HASEBE, K MATSUNAGA

  WATER RESEARCH 26 11 1421 - 1424 1992年11月 [査読有り][通常論文]
   

  Significant amounts of Fe(II) in river water exist as a fulvic-Fe complex even under oxic conditions. However, only small amounts of Fe(II) are detected even at pH 5.5 when a color complexing reagent commonly used for measuring Fe(II) is directly added to filtered samples without prior treatment. This situation might be attributed to the presence of fulvic acid, a main fraction of dissolved organic matter in river water, which forms a very stable fulvic acid-Fe(II) complex. In order to separate Fe(II) from the fulvic acid-Fe(II) complex, a filtered water sample is initially passed through a silica column coated with 8-hydroxyquinoline. The Fe(II) is eluted by an acid solution and determined spectrophotometrically by nitroso-5-(N-propyl-N-sulfopropylamine) phenol (PSAP) reagent specific for Fe(II). The Fe(II) determined by this method was greater than 70% of the total dissolved iron in river water with high total organic carbon (TOC).
• Stabilization and determination of Fe(II) in seawater and its concentration in Harimanada

  Suzuki, Y, S. Nakabayashi, I. Kudo, K. Kuma, K. Matsunaga

  Bulletin of the Japanese Society of Fisheries Oceanography 56 2 271 - 275 1992年 [査読有り][通常論文]
  
• Uptake rate of iron by macroalgae from the Sea of Japan (Laminaria religiosa Miyabe and Undaria pinnatifida)

  Matsunaga, K, Y. Suzuki, K. Kuma, I. Kudo, S. Nakabayashi

  Bulletin of the Japanese Society of Fisheries Oceanography 55 4 349 - 353 1991年 [査読有り][通常論文]
  
• 光合成により生物体内に摂取される鉄の化学形態

  松永勝彦, 久万健志, 工藤勲, 中林成人, 鈴木祥広, 戸屋健治

  水処理技術 31 3 9 - 12 1990年 [査読無し][通常論文]
  
• Existence of dissolved Fe2+ in a spring bloom at Funka Bay

  Nakabayashi, S, I. Kudo, K. Kuma, K. Toya, K. Matsunaga

  Bulletin of the Japanese Society of Fisheries Oceanography 53 2 128 - 130 水産海洋学会 1989年 [査読有り][通常論文]
  
• On the nutrient concentrations in the Northern Japan Sea

  Toya, K, K. Kuma, I. Kudo, H. Yamaguchi, T. Meguro, G. Anma, K. Matsunaga

  Bulletin of the Faculty of Fisheries, Hokkaido University 39 4 297 - 303 北海道大学 1988年 [査読無し][通常論文]
  

作品等

• 海洋の湧昇域における生物生産規模と鉄の関係に関する研究

  1993年

その他活動・業績

• 北日本のホタテガイ養殖水域における海洋酸性化の現状把握

  森亜弓美, 芳村毅, 工藤勲 日本地球惑星科学連合大会予稿集(Web) 2020 2020年
• 北日本におけるホタテガイ養殖施設近傍の海洋酸性化の現状把握

  森亜弓美, 芳村毅, 工藤勲, 阪口耕一 日本海洋学会大会講演要旨集 2020 2020年
• 陸奥湾における貧栄養化対策としての海底耕耘の有効性

  市村瞭, 工藤勲, 工藤勲, 芳村毅, 芳村毅 日本ベントス学会・日本プランクトン学会合同大会講演要旨集 2018 2018年
• 宗谷暖流域における日周潮流と順圧不安定波の相互作用

  有田 駿, 磯田 豊, 工藤 勲 沿岸海洋研究 = Bulletin on coastal oceanography 52 (2) 183 -195 2015年02月
• 陸奥湾の逆エスチャリー循環流(2)数値モデル実験

  河野 航平, 磯田 豊, 工藤 勲 海と空 = Sea and sky 90 (1) 17 -25 2014年08月
• 2013年のホタテガイの成長と餌料環境

  三好晃治, 桑原康裕, 宮崎亜希子, 多田匡秀, 宮園章, 品田晃良, 照本昴之, 工藤勲 日本水産学会北海道支部大会講演要旨集 2013 2013年
• 北海道西岸沖のホタテ貝種苗生産を支える産卵及び浮遊幼生の輸送過程

  磯貝 安洋, 磯田 豊, 下野 学, 小林 直人, 工藤 勲, 干場 康博 北海道大学水産科学研究彙報 = Bulletin of Fisheries Sciences, Hokkaido University 60 (2) 23 -37 2010年12月
• Primary production, bacterial production and nitrogen assimilation dynamics during the SEEDS II experiment

  Kudo, I, Y. Noiri, W.P. Cochlan, K. Suzuki, T. Aramaki, T. Ono, Y. Nojiri Deep-Sea Research Part II 56 2755 -2766 2009年 [査読無し][通常論文]
  
• 西部北太平洋亜寒帯域における鉄散布実験(SEEDS‐II)での従属栄養性細菌群集の変化

  片岡剛文, 鈴木光次, 早川真紀, 工藤勲, 東正剛, 津田敦 日本海洋学会大会講演要旨集 2008 238 2008年03月14日 [査読無し][通常論文]
  
• 海洋生物生産の加速と海洋大気 鉄添加による植物プランクトンの応答と細胞内での鉄の機能

  工藤勲, 芳村毅 月刊海洋 34 (3) 196 -201 2002年03月01日 [査読無し][通常論文]
  
• 現代海洋化学 II 3章 海洋表層の生物地球化学 海洋における鉄の生物地球化学

  工藤勲, 芳村毅 月刊海洋 78 -82 2001年05月01日 [査読無し][通常論文]
  
• Bacterial abundance and production and their relation to primary production in Funka Bay

  Lee Choon Weng, Kudo Isao, Yanada Mitsuru, MAITA Yoshiaki Plankton biology and ecology 48 (1) 1 -9 2001年02月
• 海洋における鉄の生物地球化学 (総特集 現代海洋化学・2) -- (3章 海洋表層の生物地球化学)

  工藤 勲, 芳村 毅 号外海洋 (25) 78 -82 2001年
• The fate of primary production during spring bloom in Funka Bay

  38 (1) 47 -54 2000年 [査読無し][通常論文]
  
• 噴火湾における全微小生物量の指標としてのATP濃度の時間変動

  簗田 満, 米田 義昭, 工藤 勲 北海道大學水産學部研究彙報 50 (2) 139 -147 1999年08月
• TRACE DETERMINATION OF SUGAR ACIDS (GLUCONIC ACID) IN SEA-WATER BY LIQUID-CHROMATOGRAPHY

  S NAKABAYASHI, KUDO, I, K KUMA, K MATSUNAGA, K HASEBE ANALYTICA CHIMICA ACTA 271 (1) 25 -29 1993年01月 [査読無し][通常論文]
   

  A liquid chromatographic (LC) method with fluorescence detection was developed with sufficient sensitivity to determine organic acids in sea water and sediments. Sugar acids, except for uronic and alpha-keto acids, have not previously been measured in sea water because of their low concentrations. After desalting with a column, gluconic acid was esterified with 9-anthryldiazomethane. The reaction mixture containing the gluconic acid derivative was directly chromatographed by LC using an octadecylsilane reversed-phase column and fluorescence detection. The peak areas were linearly related to gluconic acid concentration. The detection limit was less than 10 nM. The reproducibility and recovery were 8% at 200 nM gluconic acid and 99.8%, respectively.
• BEHAVIOR AND DYNAMIC BALANCE OF MANGANESE DURING SPRING BLOOM IN FUNKA BAY, JAPAN

  KUDO, I, T OHYAMA, S NAKABAYASHI, K KUMA, K MATSUNAGA MARINE CHEMISTRY 40 (3-4) 273 -289 1992年12月 [査読無し][通常論文]
   

  The distribution of manganese was investigated during a spring bloom in Funka Bay by chemically segregating dissolved, soluble and refractory Mn. The concentration of dissolved Mn changed slightly owing to assimilation by phytoplankton and dissolution of aerosol particles. A simple mass balance was adopted in this bloom period to clarify the dynamics of Mn in the euphotic zone. From this result, the assimilated Mn was estimated to be 10.6 mumol m-2 day-1. However, it was observed that the total Mn increased significantly despite the removal of Mn by settling. This Mn increase was attributable to atmospheric transport of fine particles from land close to this bay.
• PHOTO-REDUCTION OF FE (III) BY DISSOLVED ORGANIC-SUBSTANCES AND EXISTENCE OF FE (II) IN SEAWATER DURING SPRING BLOOMS

  K KUMA, S NAKABAYASHI, Y SUZUKI, KUDO, I, K MATSUNAGA MARINE CHEMISTRY 37 (1-2) 15 -27 1992年03月 [査読無し][通常論文]
   

  Fe(III) in seawater was readily reduced to Fe(II) in sunlight in the presence of hydroxycarboxylic acids, such as sugar acids. Tartaric, gluconic, glucaric acids and glucaric acid-1,4-lactone gave higher reduced Fe(II) concentrations and a more rapid increase for a short illumination time than other acids. The reduced Fe(II) concentration was found to be greater at 5-degrees-C than at 20-degrees-C. In sunlight the reduction of Fe(III) in seawater occurred even at low concentration of the hydroxycarboxylic acids (1-mu-M). From field observations, 0.02-0.04-mu-M of dissolved Fe(II) appeared in oxic surface seawater during spring blooms in Funka Bay (Japan) during 1987, 1988 and 1989 as a result of vertical water mixing by which Fe(III) from sediments was transported to the surface. These observations may result from photo-reduction of Fe(III) in the presence of hydroxycarboxylic acids which were possibly released by phytoplankton during spring blooms.
• RESIDENCE TIMES FOR CD AND PO4 IN FUNKA BAY, JAPAN

  K ABE, K KUMA, KUDO, I, K MATSUNAGA MARINE CHEMISTRY 28 (4) 325 -331 1990年01月 [査読無し][通常論文]
  
• BEHAVIOR OF NUTRIENTS AND HEAVY-METALS IN A HIGH PRODUCTIVE ESTUARY

  KUDO, I, K MATSUNAGA NIPPON SUISAN GAKKAISHI 55 (6) 957 -962 1989年06月 [査読無し][通常論文]
  
• Behavior of Dissolved Manganese in Lake Ohnuma

  Katsuhiko Matsunaga, Kohtarou Tani, Isao Kudo, Kazuo Abe, Kenji Toya japanese journal of limnology (rikusuigaku zasshi) 48 (4) 243 -247 1987年 [査読無し][通常論文]
   

  Dissolved manganese concentration in lake and river waters was monitored monthly for a one-year period and found to show significant seasonal changes. In April, the manganese concentration increased about two-fold compared to that in river water and decreased almost to zero during the summer months. Laboratory studies revealed that such a seasonal change was due to redissolution of manganese from suspended matter (primary source from lake sediments) generated through wind mixing of the lake water in April and manganese re-adsorption on suspended matter (primarily from dead plankton) during the summer months. Manganese adsorbed on suspended matter is slightly oxidized to Mn (III) or Mn (IV). © 1987, The Japanese Society of Limnology. All rights reserved.
• DIFFERENTIAL-PULSE ANODIC VOLTAMMETRIC DETERMINATION OF DISSOLVED AND ADSORBED PHOSPHATE IN TURBID NATURAL-WATERS

  K MATSUNAGA, KUDO, I, M YANADA, K HASEBE ANALYTICA CHIMICA ACTA 185 355 -358 1986年07月 [査読無し][通常論文]
  
• Distribution of cadmium in Lake Tohya

  The Japanese Journal of Limnology 48 1986年 [査読無し][通常論文]
  

受賞

• 2001年 日本海洋学会日高論文賞
  

共同研究・競争的資金等の研究課題

• 海洋物質循環の基準値であるレッドフィールド比を再考する

  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。
• 陸奥湾における持続的な二枚貝養殖のための貧栄養化のメカニズム解明と対策提言

  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。
• 二枚貝養殖を行っている内湾で顕在化した貧栄養化のメカニズム解明

  科研費 基盤（C)

  研究期間 : 2012年 -2014年 

  代表者 : 工藤 勲
• 亜寒帯汽水域の二次生産は何故高いのか?ー底生系の卓越とその維持機構の解明ー

  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 : 門谷 茂, 工藤 勳

   

  アサリは全生活史を通して、二次生産は温度に依存していることを明らかにした。密度および生物量が安定し、個体群が多くのコホートによって形成されるため、高い二次生産量を記録した。そのため、生産が活発化する夏季においては、餌となる底生微細藻の現存量を大きく減少させることが確認された。その一方で、生物活性の上昇に伴う、栄養塩類の再生産によって、底生微細藻増殖速度を増加させる可能性が示された。定量的推定によると、アサリにより再生産された栄養塩の多くが底生微細藻によって利用されている可能性が示され、二枚貝による栄養塩の再生産過程と底生微細藻による栄養塩取り込み過程を浮き彫りにした。
• ホタテガイの成長に及ぼす付着生物の影響

  研究期間 : 2010年 -2011年 

  代表者 : 工藤 勲
• ホタテ増養殖漁業における餌生物生産と餌要求量の関係

  研究期間 : 2007年 -2008年 

  代表者 : 工藤 勲
• 亜寒帯沿岸生態系の低次生産構造に与える河川起源栄養塩の影響と歴史的変遷

  科研費 基盤（C)

  研究期間 : 2006年 -2007年 

  代表者 : 工藤 勲
• オホーツクの豊かな漁業生産を支える海洋環境と低次生産過程

  研究期間 : 2005年 -2006年 

  代表者 : 工藤 勲
• 干潟域の低次生産過程における親生物元素循環の定量化と生態系モデルの構築

  日本学術振興会：科学研究費助成事業

  研究期間 : 2003年 -2005年 

  代表者 : 門谷 茂, 工藤 勲, 水田 浩之, 多田 邦尚, 柳 哲雄, 堤 裕昭

   

  本研究では、地球上で最も単位面積当たりの動物生産量が高い場であるとされながらも、研究上のさまざまな隘路により、これまで包括的な知見が極めて限られていた干潟域における基礎生産過程や、それに続く食物網のダイナミクスについて、詳細かつ長期の調査研究を基礎として、さまざまな素過程の定量化を進めるとともに、将来の環境管理に必要不可欠である干潟域の生態系モデルの構築を同時に実施する事を計画して実施した。我が国のように南北に細長く温度環境等が大きく異なる環境場に点在する干潟群の汎用的なモデル化を、1カ所の干潟での観測例のみで構築することは不可能である。そこで、我々の研究チームは、北海道から九州までの代表的な3カ所の干潟を選定し、それぞれの場所で同質・同種の観測・実験結果を得て、従来知見の乏しかった底生生物の親生物元素動態にはたす役割を定量化するとこで、我が国では初めてのケースとなる基礎生産から高次生産まで含めた包括的な生態系モデルを構築することを目的とした。高松の河口干潟において実施された研究により、底生珪藻の基礎生産量を正確に見積もることができた。また、そのレベルは、瀬戸内海の海水中の基礎生産量を超える値であることを明らかにした。さらに、底生動物と環境因子の関係について詳細に把握した。その結果、生物は環境因子により生物量やその組成に影響を与えられるが、逆に生物自自身が環境を作り替えていることも明らかにした。最終的に、干潟域での包括的な生態系モデルを作成することに成功した。
• 大河川の影響を受ける亜寒帯沿岸域における基礎生産過程のダイナミクス

  科研費 基盤（B)

  研究期間 : 2003年 -2005年 

  代表者 : 工藤 勲
• 海鳥類の排泄物が海洋表層の低次生産過程に及ぼす影響

  日本学術振興会：科学研究費助成事業

  研究期間 : 2003年 -2004年 

  代表者 : 門谷 茂, 工藤 勲

   

  北海道大学水産学部所属の練習船うしお丸等を使用して、日本海の北海道北部に位置する世界最大の海鳥(ウトウ)繁殖地である天売島付近の海域において、2004年5月および7月に停泊観測を実施した。観測定点では、時系列セヂメントトラップを使用して海鳥の糞由来の沈降粒子を捕捉するとともに、計器および採水観測を実施した。その結果、観測船近傍に海鳥群集が蝟集していた時間帯に、懸濁態有機物や栄養塩濃度が上昇する傾向が認められた。 同海域では、対馬暖流の影響下にあるため周年に渡って、栄養塩濃度が低く植物プランクトン量も少ないことが知られているが、海鳥群集が蝟集する海域では、その糞塊由来の栄養塩によって局部的な植物プランクトンの増殖が期待できることが明らかとなった。 海鳥が排糞した直後の糞塊の動態を実際に目視観測するために、各種の海鳥が実際に海域において摂餌していると考えられる魚種を選び、その魚種を餌として与え、飼育水槽内において新鮮な糞塊の採取とその化学組成の定量を実施するとともに、水槽内における糞塊の沈降・拡散過程を観測モニターする事を試みた。さらに、水槽内の時系列の採水試料を得て、栄養塩類の溶出過程を調べた。栄養塩の溶出・拡散は糞塊の形状が大きく関与していることを認めた。また、この海水を使用して植物プランクトンの増殖ポテンシャルを測定したところ、天塩海域から採取した対照海水に比べて有意に大きな増殖があることが分かった。
• 亜寒帯循環海洋生態系の気候変化への応答-北太平洋における東西比較

  日本学術振興会：科学研究費助成事業

  研究期間 : 2002年 -2004年 

  代表者 : 齊藤 誠一, 飯田 浩二, 桜井 泰憲, 志賀 直信, 工藤 勲, 久万 健志

   

  本研究の目的は、北東太平洋のアラスカ湾循環域では、サケ・マス類、シマガツオ、アカイカを高次の鍵種とし、南東ベーリング海大陸棚周辺海域ではスケトウダラを鍵種として、生態系の低次から高次までの各種生物個体群が、生息域の物理・生物環境の変化にどのように応答しているかを解明することにあった。また、東西の循環生態系における基礎生産システムの違い(鉄による制限)がどのようにそれぞれの生態系を特徴づけているか、船舶による観測と衛星観測を統合して解明することであった。さらに、海洋生物の分布と資源量評価のための新たなサンプリングギアーの開発と調査への利用を図り、同時に水中ロボットカメラと音響資源計測技術によって海中の生物の分布・資源量、行動の解明を行った。 本研究では、2002年、2003年、2004年の3年間、練習船「おしょろ丸」を用いて観測を行い、特に2003年および2004年は、アラスカ湾循環生態系と西部亜寒帯循環生態系の東西比較のために、東経165度および西経165度の2観測線を設定して観測を実施した。2003年に「おしょろ丸」がハワイに入港した機会を利用して"Marine Ecological Studies in the Bering Sea and the North Pacific-Commemorative symposium for UH-HU academic exchange agreement and to mark the 2003 cruise of the T/S Oshoro Maru"というミニシンポジウムをハワイ大学で開催し、本研究の途中成果の公表および議論をおこなった。
• 鉄濃度調節が植物生理・生産に及ぼす影響に関する研究

  研究期間 : 2001年 -2003年 

  代表者 : 工藤 勲
• 海洋生態系における珪藻ブルームの役割

  日本学術振興会：科学研究費助成事業

  研究期間 : 2000年 -2001年 

  代表者 : 伴 修平, 磯田 豊, 工藤 勲, 丸尾 雅啓

   

  北海道沿岸に位置する噴火湾湾奥部において、春季ブルーム直前から夏季に渡って、動・植物プランクトンおよびバクテリア現存量、一次生産およびバクテリア生産、メソ及びマイクロ動物プランクトンの摂食速度を測定した。珪藻の春季ブルームは、水温3-4℃下で2月中旬から始まり、3月中旬まで続いた。この間、メソ(中型)動物プランクトンバイオマスは常に低く、これらによる摂食ロスも極めて低い。マイクロ(小型)動物プランクトンバイオマスはブルームと共に増加したが、これの珪藻類に対する摂食ロスはほとんどなかった。一方、バクテリア生産はブルームと共に増加する傾向を示すものの、バイオマスの増加は見られなかった。これらの観測結果に従って噴火湾におけるエネルギーフローを描き、生態系モデルを構築し、春季ブルーム前後における食物網とエネルギーフローの違いを明らかにした。春季ブルーム期には、メソ動物プランクトン現存量が著しく低いため、珪藻の一次生産からメソ動物プランクトンに流れるエネルギーは極めて少なく、珪藻が排泄する溶存有機物からバクテリア-原生動物-メソ動物プランクトンという「微生物ループ」が卓越した。これに対して、ブルーム期以降は、メソ動物プランクトンが排泄する無機栄養塩を使って渦鞭毛藻が増殖しこれをメソ動物プランクトンが摂食する経路と、メソ動物プランクトンが排泄した溶存有機物を使ってバクテリアが増殖し、これをマイクロ動物プランクトンが摂食し、さらにこれをメソ動物プランクトンが利用する経路の2経路が存在することが分かった。すなわち、珪藻の春季ブルームは溶存有機物を介して間接的にメソ動物プランクトンを支えているが、ブルーム以降はメソ動物プランクトンによって再生される無機及び有機の栄養素が食物網を循環している事を示すことができた。
• 食物連鎖系を通してホタテガイに濃縮されるカドミウムの濃縮機構の解明

  日本学術振興会：科学研究費助成事業

  研究期間 : 1997年 -1998年 

  代表者 : 工藤 勲

   

  ホタテガイの中腸腺は、肥料・飼料としての有効利用の可能性があるにもかかわらず、カドミウムが高濃度に含まれていることが問題となっている。そこで、本研究では、ホタテガイに濃縮されるカドミウムを例にとり、魚介類に濃縮される金属の濃縮機構を解明することを目的とした。 調査は、前年に引き続き北海道噴火湾森沖のホタテ養殖施設内と養殖の行われていない湾中央部の定点において5月から11月にかけて計10回の調査を行った。毎回、水温、塩分、栄養塩、基礎生産量、懸濁態有機炭素・窒素、植物プランクトン量とカドミウム含量、それとホタテ一年貝の成長速度、中腸腺量とそのカドミウム含量を測定した。 【結果】ホタテガイの餌となる植物プランクトン現存量は、5月から8月にかけて横ばいであったが、プランクトン中のカドミウム含量は、8月に他の3倍程度と高濃度を示し、その後減少した。ホタテ1年貝の成長速度は、平均で0.35g/月でほぼ直線的に重量は増加した。殻長もほぼ同様に増加した。中腸腺中のカドミウムは今回の測定値とこれまでの報告値を総合すると春から夏にかけ増加し、その後、冬にかけ減少する傾向がある。これらは、ホタテガイが餌として主に植物プランクトンなどの懸濁態有機物を摂取していることを考えあわせると調和的な結果である。植物プランクトン中のカドミウム含量の変化の原因について、これまでの知見より、噴火湾では、植物プランクトンの種組成が、春から夏にかけて珪藻類から渦べん毛藻類へ遷移することが知られており、この植物プランクトン種の変化がホタテ中腸腺中へのカドミウムの蓄積に影響を与えている可能性が示唆された。また、海水中のカドミウムの濃度は、他の海域と比較して特に汚染されているわけではなく、また春から夏にかけて減少傾向にあるため海水中の濃度がこの濃縮に与えている影響は少ないと考えられる。
• 生化学的手法を用いた海洋における一次生産過程・特に窒素代謝に及ぼす鉄の影響評価

  日本学術振興会：科学研究費助成事業

  研究期間 : 1996年 -1996年 

  代表者 : 工藤 勲

   

  海洋において植物プランクトンの増殖に不可欠な鉄の重要性は、近年、世界的に注目されている。それは、その濃度が極めて低濃度であることと、人類起源による二酸化炭素の増大がもたらした地球温暖化に際して、海洋がどの程度二酸化炭素を吸収できるかということに密接に係わっているからであると思われる。特に植物プランクトンが硝酸塩を利用する際に必要な硝酸還元酵素が鉄を必須とすることから、鉄欠乏時にはどの程度硝酸塩利用効率が変化するかを知るためには、この硝酸還元酵素を測定することは、有効な手段であると思われる。そこで本補助金によって鉄と植物プランクトンの生長と硝酸還元酵素の出現との間の関係を解明した。解明された主な点は、クリーン技術を導入し、培養液中の鉄濃度を除去し海水中と同程度(1nM)にまで減少させた。この無鉄培地と鉄を十分量添加した培地中で植物プランクトンを培養した結果、生長速度に差異が認められ鉄不足によって生長制限を受けた細胞を得た。同時に硝酸還元酵素の測定を行った結果、鉄制限細胞では、ほとんど酵素活性が現れなかった。この活性は、培地中の鉄濃度の減少に伴う生長速度の減少に伴い徐々に減少した。このことは、鉄欠乏がこの酵素の出現に関与していることを強く示唆するものと思われる。また窒素源としてアンモニウム塩のみを添加した場合、硝酸還元酵素活性は、認められなかった。これはアンモニウムは、すでに還元された形でありこの酵素を必要としないためと思われる。つまり、植物プランクトンが硝酸塩かアンモニウム塩を窒素源として利用する場合、アンモニウム塩利用型のほうが鉄の要求量は、小さいことが示唆された。
• 海洋の湧昇域における生物生産規模と鉄の関係に関する研究

  研究期間 : 1993年 

  代表者 : 工藤 勲
• 沿岸海域の生産力を高めるための鉄の役割に関する研究

  日本学術振興会：科学研究費助成事業

  研究期間 : 1988年 -1989年 

  代表者 : 松永 勝彦, 工藤 勲, 久万 健志, 梶原 昌弘

   

  1.粒状鉄の溶解度ならびに溶解速度を求めた。溶解度は20℃で0。048μg/1で、その溶存形はFe(OH)^+_2と考えられる。溶解速度は、粒状鉄1、3、5μ/1において、それぞれ0。0030、0。010、0。017μg/1・dayであった。 2.噴火湾におけるスプリングブル-ム時に、数μg/1のFe^<2+>が検出された。この還元は室内実験でヒドロキシカルボン酸、特にグルコン酸、グルカル酸、グルコン酸1,4ラクトンと紫外線で還元が生じることがわかった。プランクトン培養液に紫外線を照射し、HPLCでヒドロキシカルボン酸の定量を行ったが、およそ100μmol/1検出された。但し、グルカル、グルコン酸等の分離はまだ解決していないし、天然海水中の有機酸の定量には至っていない。 3.マコンブによる鉄の取り込み実験から、コンブ1g、1時間当たり、0.003μg/1の溶存鉄を取り込むこたがわかった。1の実験で求めた粒状鉄の溶解速度から、粒状鉄と平衡で溶存するFe(OH)^+_2では不十分であることが明らかとなった。 4.鋼鉄製増殖礁を沈設し、それから拡散するFe^<2+>を測定したところ、鉄礁の周り40mの範囲に渡ってFe^<2+>を拡散させていることがわかった。鉄礁と天然岩盤で生育するコンブ中の鉄濃度を比較したところ、前者は後者よりも2倍高かった。この要因は鉄礁からのFe^<2+>によるものである。ウニについても同様に比較したところ、前者の実入り(可食部)は後者よりも2-3割よかった。この要因はFe^<2+>によって摂餌効果が高まるためなのか、現在研究を続けている。
• 食物連鎖系を通した重金属の濃縮機構の解明
• 酵素法を用いた植物プランクトンの窒素代謝に及ぼす鉄の影響
• 海洋における生物生産機構と微量重金属の相互作用に関する研究
• Mechanism of bioaccumulation of heavy metals through marine food web
• Effect of Fe on nitrogen metabolism using enzyme assay approach
• Studies on interaction between biological productivity and trace metals in the ocean

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