Researcher Database

KEIRI IMAI
School of Fisheries Sciences Training Ship “Oshoro-maru”
Assistant Professor

Researcher Profile and Settings

Affiliation

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

Job Title

  • Assistant Professor

Degree

  • fishery(Hokkaido Univ.)

Research funding number

  • 40725983

J-Global ID

Association Memberships

  • THE OCEANOGRAPHIC SOCIETY OF JAPAN   

Research Activities

Published Papers

  • Upward transport of iron at the west shelf edge–slope of the Okinawa Trough in the East China Sea
    Sasayama R, Hioki N, Morita Y, Isoda Y, Imai K, Ooki A, Kuma K
    Journal of Oceanography 74 (4) 367 - 379 2018 [Refereed][Not invited]
  • Yuichiro Kumamoto, Michio Aoyama, Yasunori Hamajima, Hisao Nagai, Takeyasu Yamagata, Yoshimi Kawai, Eitarou Oka, Atsushi Yamaguchi, Keiri Imai, Akihiko Murata
    JOURNAL OF RADIOANALYTICAL AND NUCLEAR CHEMISTRY 311 (2) 1209 - 1217 0236-5731 2017/02 [Refereed][Not invited]
     
    In 2014, we measured activity concentration of radiocesium in the western North Pacific Ocean. In the north of Kuroshio Front high activity concentration of Fukushima-derived radiocesium in surface mixed layer in 2012 had been transported eastward by 2014. In the south of the front we found a radiocesium subsurface maximum in 200-600 m depth, which was similar to that observed in 2012. The subsurface maximum spread southward from 18A degrees N to 15A degrees N between 2012 and 2014, which suggests spreading of Fukushima-derived radiocesium into the whole western subtropical area by 2014 due to formation and subduction of the subtropical mode water.
  • fN-Oscillation trapped in the benthic front in the Abyssal Japan Sea
    Yamauchi Y, Shoji K, Isoda Y, Arita S, Kawano K, Fujiwara S, Fang X, Asahi K, Ida S, Kuma K, Tateno M, Imai K, Owada M
    Oceanography in japan 24 (4) 147 - 169 2015 [Refereed][Not invited]
  • Time series of radioactive cesium concentration in the North Pacific surface water after the TEPCO Fukushima Dai-ichi Nuclear Power Plant accident by using T/S “Oshoro-Maru” cruses
    Keiri Imai, Yutaka W, Watanabe
    北海道大学水産科学彙報 65 (2) 111 - 116 2015 [Not refereed][Not invited]
  • Kenshi Kuma, Ryohei Sasayama, Nanako Hioki, Yuichiroh Morita, Yutaka Isoda, Tohru Hirawake, Keiri Imai, Takafumi Aramaki, Tomohiro Nakamura, Jun Nishioka, Naoto Ebuchi
    JOURNAL OF OCEANOGRAPHY 70 (4) 377 - 387 0916-8370 2014/08 [Refereed][Not invited]
     
    In the southwestern Okhotsk Sea, the cold water belt (CWB) is frequently observed on satellite images offshore of the Soya Warm Current flowing along the northeastern coast of Hokkaido, Japan, during summertime. It has been speculated that the CWB is upwelling cold water that originates from either subsurface water of the Japan Sea off Sakhalin or bottom water of the Okhotsk Sea. Hydrographic and chemical observations (nutrients, humic-type fluorescence intensity, and iron) were conducted in the northern Japan Sea and southwestern Okhotsk Sea in early summer 2011 to clarify the origin of the CWB. Temperature-salinity relationships, vertical distributions of chemical components, profiles of chemical components against density, and the (NO3 + NO2)/PO4 relationship confirm that water in the CWB predominantly originates from Japan Sea subsurface water.
  • 鶴島修夫, 今井圭理
    海洋理工学会誌 20 (1-2) 29 - 36 2014 [Refereed][Not invited]
  • Takeshi Yoshimura, Hiroshi Ogawa, Keiri Imai, Takafumi Aramaki, Yukihiro Nojiri, Jun Nishioka, Atsushi Tsuda
    DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 56 (26) 2863 - 2874 0967-0645 2009/12 [Refereed][Not invited]
     
    The dynamics of organic carbon (C), nitrogen (N), and phosphorus (P) were examined during an in situ mesoscale iron-enrichment experiment in the western North Pacific in the summer of 2004. We separately determined the production of particulate organic matter (POM) and dissolved organic matter (DOM) and their subsequent removal during the bloom decline. As the iron-induced phytoplankton bloom progressed (days 0-14), POM increased in the surface mixed layer, while DOM did not increase significantly. The molar ratios for C:N, C:P, and N:P of the newly produced POM were estimated to be 4.9,190, and 37 in the surface mixed layer, whereas the dissolved inorganic nitrogen to soluble reactive phosphorus drawdown ratio was 17. Preferential remineralization of P over C and N from the POM was postulated during the developing phytoplankton bloom. During the bloom decline (days 16-25), surface POM decreased with a similar C:N of 5.2. The N:P ratio of surface DOM increased during the bloom decline. Below the surface mixed layer, DOC and DON increased moderately after the peak of the bloom. The time-series variation of DOC and DON was not identical. The C, N, and P dynamics through the accumulation and removal of POM and DOM were complex. Grazing by mesozooplankton during the experiment may have played a significant role in the uncoupling of the dynamics of C, N, and P. (C) 2009 Elsevier Ltd. All rights reserved.
  • Hiroaki Saito, Atsushi Tsuda, Yukihiro Nojiri, Takafumi Aramaki, Hiroshi Ogawa, Takeshi Yoshimura, Keiri Imai, Isao Kudo, Jun Nishioka, Tsuneo Ono, Koji Suzuki, Shigenobu Takeda
    DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 56 (26) 2852 - 2862 0967-0645 2009/12 [Refereed][Not invited]
     
    Biogeochemical cycles of N and Si were examined in the surface mixed layer during the mesoscale iron-enrichment (IE) experiment in the high-nutrient low-chlorophyll (HNLC) western subarctic Pacific (SEEDS-II). Although the IEs increased nitrate uptake, silicic acid utilization was not stimulated. The nitrate drawdown in the iron-patch (IN-patch, 140.3 mmol m(-2) in the surface mixed layer, 0-30 in) was only 25% of the initial inventory, which was 1/3-2/5 of the previous IE experiments in the subarctic Pacific. This relatively weak response of nutrient drawdown to IEs was due to the high biomass of mesozooplankton (MZ) dominated by copepod Neocalanus; plumchrus. Feeding of MZ (247.2 mmol m(-2) during Day 0-21 from the first IE) in the IN-patch was higher than the nitrate drawdown and prevented further development of the phytoplankton bloom. In the later period of the experiment (Day 14-21) the increase in the feeding activity and resultant decrease in phytoplankton biomass induced the accumulation of dissolved organic nitrogen (DON) and ammonium. Among total growth of MZ (81.6 mmol N m(-2)), 89% (72.8 mmol N m(-2)) was transported to the depth by the ontogenetic downward migration of N. plumchrus. Although silicic acid drawdown was not increased by the IEs, Si export flux increased by 2.7 times. The increase in Si export was also due to the increase in MZ, which egested faecal pellets with higher Si:N ratio and faster sinking speed than diatoms. The export efficiency (78% of new production) and total amount of export flux (143.8 mmol N m(-2) 1392 mmol C m(-2)) were highest records within the IE experiments despite weak responses of nutrient drawdown to the IE. During SEEDS-II, the high biomass of MZ reduced the phytoplankton response and nutrient drawdown to the IEs but via grazing and ontogenetic vertical migration accelerated the export flux as well as accumulations of dissolved forms of N. Results of the present and previous IE experiments indicate that the ecosystem and biogeochemical responses to IEs in the HNLC region are quite sensitive to the ecosystem components, especially for grazers of diatoms such as copepods and heterotrophic dinoflagellates. More attention needs to be paid to the ecosystem components and their biogeochemical functions as well as physical and chemical properties of the ecosystems in order to hindcast or forecast the impacts of changes in atmospheric iron deposition. (C) 2009 Elsevier Ltd. All rights reserved.
  • Takafumi Aramaki, Yukihiro Nojiri, Keiri Imai
    DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 56 (26) 2875 - 2888 0967-0645 2009/12 [Refereed][Not invited]
     
    During two mesoscale iron-enrichment studies in the northwestern subarctic Pacific (SEEDS in 2001 summer and SEEDS II in 2004 summer), particulate materials from the iron-induced phytoplankton bloom in the upper water column were monitored to analyze the export processes beneath the upper mixed layer, mainly with drifting sediment traps. We could not observe the total downward export process of the high accumulation of particulate organic carbon from the mixed layer induced by the large diatom bloom of SEEDS [e.g., Tsuda, A., Takeda, S., Saito, H., Nishioka, J., Nojiri, Y., Kudo, L, Kiyosawa, H., Shiomoto, A., Imai, K., Ono, T., Shimamoto, A., Tsumune, D., Yoshimura, T., Aono, T., Hinuma, A., Kinugasa, M., Suzuki, K., Sohrin, Y., Noiri, Y., Tani, H, Deguchi, Y., Tsurushima, N., Ogawa, H., Fukami, K., Kuma, K., Saino, T., 2003. A mesoscale iron enrichment in the western subarctic Pacific induces large centric diatom bloom. Science 300, 958-961] because the 2-week observation period was too short to examine the decline phase of the bloom. In contrast, in SEEDS II, the particulate organic carbon and particulate organic nitrogen were accumulated 123 and 23 mmol m(-2), respectively, in the mixed layer until day-15 (days from iron-enrichment), and then ca. 90% were removed from the mixed layer by day-25. The sediment traps at 40 rn depth between day-15 and day-25 accounted for at least more than 35% of these particles. There was no large variation in chemical composition in settling particles above 100m depth throughout the experimental periods both in SEEDS and SEEDS II. The content of biogenic opal remained more than 50% of all settling particles during SEEDS, while the content of biogenic calcium carbonate was relatively high, with a low biogenic opal content of consistently less than 30% during SEEDS II. These results suggest that high standing stock of seed population of diatoms before the iron fertilization, indicated by low C/Si ratio of particulate matter, is an important factor to induce the large diatom bloom in SEEDS. (C) 2009 Elsevier Ltd. All rights reserved.
  • Atsushi Tsuda, Shigenobu Takeda, Hiroaki Saito, Jun Nishioka, Isao Kudo, Yukihiro Nojiri, Koji Suzuki, Mitsuo Uematsu, Mark L. Wells, Daisuke Tsumune, Takeshi Yoshimura, Tatsuo Aono, Takafumi Aramaki, William P. Cochlan, Maki Hayakawa, Keiri Imai, Tomoshi Isada, Yoko Iwamoto, William K. Johnson, Sohiko Kameyama, Shungo Kato, Hiroshi Kiyosawa, Yoshiko Kondo, Maurice Levasseur, Ryuji J. Machida, Ippei Nagao, Fumiko Nakagawa, Takahiro Nakanish, Seiji Nakatsuka, Akira Narita, Yoshifumi Noiri, Hajime Obata, Hiroshi Ogawa, Kenji Oguma, Tsuneo Ono, Tomofumi Sakuragi, Motoki Sasakawa, Mitsuhide Sato, Akifumi Shimamoto, Hyoe Takata, Charles G. Trick, Yutaka W. Watanabe, Chi Shing Wong, Naoki Yoshie
    JOURNAL OF OCEANOGRAPHY 63 (6) 983 - 994 0916-8370 2007/12 [Refereed][Not invited]
     
    mesoscale iron-enrichment study (SEEDS II) was carried out in the western subarctic Pacific in the summer of 2004. The iron patch was traced for 26 days, which included observations of the development and the decline of the bloom by mapping with sulfur hexafluoride. The experiment was conducted at almost the same location and the same season as SEEDS (previous iron-enrichment experiment). However, the results were very different between SEEDS and SEEDS II. A high accumulation of phytoplankton biomass (similar to 18 mg chl m(-3)) was characteristic of SEEDS. In contrast, in SEEDS II, the surface chlorophyll-a accumulation was lower, 0.8 to 2.48 mg m(-3), with no prominent diatom bloom. Photosynthetic competence in terms of F-v/F-m for the total phytoplankton community in the surface waters increased after the iron enrichments and returned to the ambient level by day 20. These results suggest that the photosynthetic physiology of the phytoplankton assemblage was improved by the iron enrichments and returned to an iron-stressed condition during the declining phase of the bloom. Pico-phytoplankton (< 2 mu m) became dominant in the chlorophyll-a size distribution after the bloom. We observed a nitrate drawdown of 3.8 mu M in the patch (day 21), but there was no difference in silicic acid concentration between inside and outside the patch. Mesozooplankton (copepod) biomass was three to five times higher during the bloom-development phase in SEEDS II than in SEEDS. The copepod biomass increased exponentially. The grazing rate estimation indicates that the copepod grazing prevented the formation of an extensive diatom bloom, which was observed in SEEDS, and led to the change to a pico-phytoplankton dominated community towards the end of the experiment.
  • N Ramaiah, S Takeda, K Furuya, T Yoshimura, J Nishioka, T Aono, Y Nojiri, K Imai, Kudo, I, H Saito, A Tsuda
    PROGRESS IN OCEANOGRAPHY 64 (2-4) 253 - 261 0079-6611 2005 [Refereed][Not invited]
     
    Dynamics of transparent exopolymer particles (TEP) was studied during the first in situ iron-enrichment experiment conducted in the western subarctic Pacific in July-August 2001, with the goal of evaluating the contribution of TEP to vertical flux as a result of increased primary production following iron enrichment in open ocean ecosystems. Subsequent to the enhancement of phytoplankton production, we observed increase in TEP concentration in the surface layer and sedimentation of organic matter beneath it. Vertical profiles of TEP, chlorophyll a (Chl a) and particulate organic carbon (POC) were obtained from six depths between 5 and 70 m, from a station each located inside and outside the enriched patch. TEP and total mass flux were estimated from the floating sediment traps deployed at 200 m depth. Chl a and TEP concentrations outside the patch varied from 0.2 to 1.9 μ g L-1 and 40-60 μ g XG equiv. L-1, respectively. Inside the patch, Chl a increased drastically from day 7 reaching the peak of 19.2 μ g L-1 on day 13, which coincided with the TEP peak of 189 μ g XG equiv. L-1. TEP flux in the sediment trap increased from 41 to 88 mg XG equiv. m(-2) d(-1), with 8-14% contribution of TEP to total mass flux. This forms the basic data set on ambient concentrations of TEP in the western subarctic Pacific, and evaluation of the effect of iron enrichment on TEP. © 2005 Elsevier Ltd. All rights reserved.
  • Export fluxes of POC estimated from 234Th /238U disequilibrium in the subartic Pacific during the iron-enrichment experiment
    Aono, T, Yamada, M, Kudo, I, Imai, K, Nojiri, Y, Tsuda, A
    Progress in Oceanography 64 (2-4) 263 - 282 2005 [Refereed][Not invited]
  • A Tsuda, H Kiyosawa, A Kuwata, M Mochizuki, N Shiga, H Saito, S Chiba, K Imai, J Nishioka, T Ono
    PROGRESS IN OCEANOGRAPHY 64 (2-4) 189 - 205 0079-6611 2005 [Refereed][Not invited]
     
    Phytoplankton species composition was analyzed inside and outside of the iron-enriched patch during the SEEDS experiment. Before the iron-enrichment, the phytoplankton community consisted of similar proportions of pico-, nano- and micro-sized phytoplankton, and the micro-phytoplankton was dominated by the pennate diatom Pseudo-nitzschia turgidula. Although all the diatoms, except the nano-sized Fragilariopsis sp., increased during the two weeks of the observation period, the flora in the patch dramatically changed with the increase of phytoplankton biomass to a centric diatom-dominated community. Neritic diatoms, especially Chaetoceros debilis, showed higher growth rates than other diatoms, without any delay in the initiation of growth after the enrichment, and accounted for 90% of the micro-phytoplankton after day 9. In contrast, the oceanic diatoms showed distinct delays in the initiation of growth. We conclude that the responses of the diatoms to the manipulation of iron concentration were different by species, and the fast and intensive response of the phytoplankton to iron-enrichment resulted from the presence of a small amount of neritic diatoms at the study site. The important factors that determine the dominant species in the bloom are the potential growth rates under an iron-replete condition and the growth lag. Abundant species in the patch are widely distributed in the North Pacific and their relative contributions in the Oyashio area and at Stn KNOT are high from spring to summer. However, a characteristic difference of species composition between the SEEDS bloom and natural blooms was the lack of Thalassiosira and Coscinodiscus species in the patch, which usually account for a major part of the phytoplankton community under blooming conditions in the western North Pacific. © 2005 Elsevier Ltd. All rights reserved.
  • Kudo, I, Noiri, Y, Imai, K, Nojiri Y, Tsuda, A
    Progress in Oceanography 64 (2-4) 207 - 222 2005 [Refereed][Not invited]
  • C Komuro, H Narita, K Imai, Y Nojiri, RW Jordan
    DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 52 (16-18) 2206 - 2217 0967-0645 2005 [Refereed][Not invited]
     
    The microplankton communities on filter samples from three shallow (0-300 m) hydrocasts, conducted in August 1999, January 2000, and May 2000 at Station KNOT in the Western Subarctic Gyre of the NW Pacific, have been analyzed by scanning electron microscopy (SEM). The results clearly show that small (<10 mu m) phytoplankton, such as coccolithophorids, Parmales, and the pennate diatom Fragilariopsis pseudonana, play important roles in seasonal productivity. The Parmales were the most dominant mineralized phytoplankton group in May and August, and their year-round presence in scaly form suggests they are not cysts as previously thought. As the Parmales are rare or absent in the underlying sediment traps or surface sediments, their siliceous scales are undoubtably being efficiently recycled in the euphotic zone where silica levels are never depleted. The phytoplankton community structure observed by SEM is different to those reported previously, in which more traditional light microscopical techniques were used, with the consequence of missing the smaller phytoplankton. This study, regardless of its biogeographic location, highlights the potential problem of enumerating marine phytoplankton assemblages purely by light microscopy. An inability to determine the contribution made by these small phytoplankton can lead to an overestimation of the importance of large phytoplankton, and an underestimation of total productivity. (c) 2005 Elsevier Ltd. All rights reserved.
  • T Odate, K Imai
    JOURNAL OF PLANKTON RESEARCH 25 (12) 1497 - 1505 0142-7873 2003/12 [Refereed][Not invited]
     
    Chlorophyll-specific growth (mu) and grazing mortality rates (g) due to microzooplankton (grazers of <200 mum) were estimated, based on the dilution method, in surface waters collected at four stations in Funka Bay, Japan, from December 1995 to March 1997. From May to October, the mean mu and g were 0.66 +/- 0.44 day(-1) (n = 21) and 0.60 +/- 0.31 day(-1) (n = 21), respectively. The estimated chlorophyll variation was 0.02 +/- 0.31 mug l(-1) day(-1) (n = 21), which is consistent with field observations in this bay. On the other hand, mu was 0.51 +/- 0.37 day(-1) (n = 4) during the spring bloom (February and March). There was no significant difference between the mean mu in summer and in the spring bloom (P > 0.05). However, g during the bloom (0.15 +/- 0.09 day(-1)) (n = 4) was significantly lower than in summer (P < 0.01). These results suggest that the low g was one of the factors contributing to the development of the spring bloom. On a seasonal basis, it was estimated that 83 and 39% of phytoplankton production were grazed by microzooplankton in summer and winter-spring seasons, respectively. The seasonal variation in the microzooplankton grazing pressure seems to result from the dominant size class of the phytoplankton community of this bay, i.e. picophytoplankton (<2 mum) predominate in summer, while phytoplankton >10 mum prevail in winter-spring.
  • A Tsuda, S Takeda, H Saito, J Nishioka, Y Nojiri, Kudo, I, H Kiyosawa, A Shiomoto, K Imai, T Ono, A Shimamoto, D Tsumune, T Yoshimura, T Aono, A Hinuma, M Kinugasa, K Suzuki, Y Sohrin, Y Noiri, H Tani, Y Deguchi, N Tsurushima, H Ogawa, K Fukami, K Kuma, T Saino
    SCIENCE 300 (5621) 958 - 961 0036-8075 2003/05 [Refereed][Not invited]
     
    We have performed an in situ test of the iron limitation hypothesis in the subarctic North Pacific Ocean. A single enrichment of dissolved iron caused a large increase in phytoplankton standing stock and decreases in macronutrients and dissolved carbon dioxide. The dominant phytoplankton species shifted after the iron addition from pennate diatoms to a centric diatom, Chaetoceros debilis, that showed a very high growth rate, 2.6 doublings per day. We conclude that the bioavailability of iron regulates the magnitude of the phytoplankton biomass and the key phytoplankton species that determine the biogeochemical sensitivity to iron supply of high-nitrate, low-chlorophyll waters.
  • KNOT: Ocean time-series program in western North Pacific completes first phase
    Tsurushima N, Imai K, Nojiri Y, Murphy P.P
    U. S. JGOFS NEWS 11 (4) 11 - 13 2002 [Not refereed][Not invited]
  • K Mizobata, SI Saitoh, A Shiomoto, T Miyamura, N Shiga, K Imai, M Toratani, Y Kajiwara, K Sasaoka
    PROGRESS IN OCEANOGRAPHY 55 (1-2) 65 - 75 0079-6611 2002 [Refereed][Not invited]
     
    Using satellite altimeter and ship data, Bering Sea cyclonic and anticyclonic eddies were observed in summer 2000 and 2001 to examine their biological, chemical and physical structures. Results from the ship transect revealed the interactions between the physical and biological conditions of Bering Sea eddies. At the center of a cyclonic (anticlockwise) eddy, upwelling was transporting nutrient (NO3 + NO2) rich water (>25 muM) to the surface, which resulted in relatively high chlorophyll a concentrations (>1.0 mg m(-3)) developing under the pycnocline. In contrast, in the center of an anticyclonic (clockwise) eddy there was downwelling. This downwelling of surface warm water was destroying a cold layer (at about 150 m depth) caused by winter convection. However, around the periphery of the anticyclonic eddy the isopycnals were tilted up and nutrient-rich water was being transported along with them up into the euphotic zone, so that high chlorophyll a concentrations were being developed above the pycnocline inside the anticyclonic eddy. (C) 2002 Elsevier Science Ltd. All rights reserved.
  • A Shiomoto, S Saitoh, K Imai, M Toratani, Y Ishida, K Sasaoka
    PROGRESS IN OCEANOGRAPHY 55 (1-2) 147 - 163 0079-6611 2002 [Refereed][Not invited]
     
    Surface chlorophyll a concentrations were measured at seven stations located at 1degrees latitude intervals between 52degrees30' N and 58degrees30' N along longitude 179degrees30' W, in late June and early July from 1991 through 1999. Surface chlorophyll a concentrations at the same locations were estimated from Sea-viewing Wide Field-of-view Sensor (SeaWiFS) data from mid June through mid July 2000. Shipboard data collected from 1993-95 showed that the surface chlorophyll a concentration was correlated with chlorophyll a standing stock integrated in the upper 100 m of the water column. Based on the 8-day time series of mean chlorophyll a concentrations at stations on 179degrees30' W in 1998, 1999, and 2000 from SeaWiFS data, high concentration and great variability were observed in chlorophyll a in June. When examining interannual differences in phytoplankton biomass, it is preferable to use a month when high chlorophyll a concentrations are high, and show great variability than one when its concentrations are low with little variability. Thus a comparison of surface data from shipboard and satellite observations in June, was considered best to represent the interannual variation in phytoplankton biomass in the Bering Sea basin in the 1990s. There were no significant differences in chlorophyll a concentrations among years (1991-2000) or among stations, though high chlorophyll a concentrations were observed occasionally around the shelf edge. Chlorophyll a concentrations rarely exceeded 2 mug 1(-1) in the Bering Sea basin and the values were usually < 1 mug 1(-1). The results suggest that phytoplankton biomass in the basin is relatively stable and rather low; in other words, the Bering Sea basin may be considered to be a high nitrate, low chlorophyll (HNLC) region. (C) 2002 Elsevier Science Ltd. All rights reserved.
  • K Sasaoka, S Saitoh, Asanuma, I, K Imai, M Honda, Y Nojiri, T Saino
    DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 49 (24-25) 5557 - 5576 0967-0645 2002 [Refereed][Not invited]
     
    The objectives of this study are to describe and understand the processes controlling the temporal and spatial variability of chlorophyll-a (chl-a) in the subarctic northwestern Pacific Ocean from 1997 to 1999. Remotely sensed data from multi sensors, including ocean color (OCTS and SeaWiFS), sea-surface temperature (SST, AVHRR), and wind (SSM/I) datasets were utilized for the purpose of this study. Ocean-color imagery clearly showed seasonal and interannual variability in the spatial abundance and distribution of chl-a in the study area. Chl-a concentrations were generally low (0.53 +/- 0.24 mg m(-3)) for most part of the year, except for a few peaks (1.0-2.0 mg m(-3)) observed in the spring and fall bloom seasons (May, June, September, and October). Chl-a concentrations (> 10 mg m(-3)) were consistently high along the Kuril Islands and in the coastal waters around the Kamchatka Peninsula, and in 1998 they were clearly higher than in 1999. The Western Subarctic Gyre (WSG) was characterized by positive sea-surface temperature anomalies (SSTA) during the summer to fall of 1998. These anomalies appear to be a high-latitude response to 1997/1998 ENSO event. High concentrations of chl-a appeared in WSG in September-November only in 1998. We suggest that the high chl-a around the WSG from summer to fall in 1998 was facilitated by (1) negative SSTA in winter, in spite of the negative wind anomaly, which provided larger amount of nutrients to the sea surface, (2) positive wind anomaly from April to June, which might cause light limitation of the phytoplankton growth, due to deepening of the surface-mixed layer in summer, and (3) positive SSTA, even with the slightly positive wind anomaly, which was accompanied with the water-column stabilization and hence recovered light limitation of the phytoplankton. Warmer SST also might have enhanced the phytoplankton growth. During summer to fall in 1999, the phytoplankton biomass between 42degreesN and 43degreesN along 165degreesE was greater than in other years. The enhanced chl-a concentration coincided with a distinct frontal temperature gradient located between 40degreesN and 45degreesN in September 1998 and 1999. The peak chl-a concentration in September 1999 (> 2.0 mg m(-3)) within the temperature front was much larger than in L998 (about 0.7 mg m(-3)). The position of this front in September 1999 (41degreesN) was south of its position in 1998 (43degreesN). Convergence of chl-a is linked to the temperature gradient and southward expansion of well-mixed subarctic water that is cold, low in salinity, and nutrient-rich. Our results showed significant interannual biological variability, and illustrate how remote sensing with multi sensors can aid in monitoring relationships between local ecosystems and global climate change. (C) 2002 Elsevier Science Ltd. All rights reserved.
  • Honda, M.C, Imai, K, Nojiri, Y, Hoshi, F, Sugawara, T, Kusakabe, M
    Deep Sea Research Part II 49 (24/25) 5595 - 5625 2002 [Refereed][Not invited]
  • M Mochizuki, N Shiga, M Saito, K Imai, Y Nojiri
    DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 49 (24-25) 5421 - 5439 0967-0645 2002 [Refereed][Not invited]
     
    The standing stock and, species composition of phytoplankton (> 10 mum) were studied using monthly water samples collected at Stn KNOT (Kyodo North Pacific Ocean Time Series; 44degreesN, 155degreesE) in the western subarctic gyre in the Pacific Ocean through June 1998 to June 1999 (except for January-April 1999) and January-February 2000. One-liter water samples were preserved in 1% neutrally buffered formalin. Identification and enumeration of phytoplankton were made with an inverted microscope. Nutrients did not appear to be depleted for phytoplakton growth during any season. The vertical distribution of phytoplankton was primarily restricted by the pycnocline, and the bulk of phytoplankton assemblage existed within the surface mixed layer. In July, however, some senescent cells were observed at 200-m. Phytoplankton abundance clearly showed a spring maximum (i.e. spring bloom) in May. The seasonal change in cell numbers, however, did not coincide closely with the change in chlorophyll a concentration. Centric diatoms, which were composed of Thalassiosira, Chaetoceros, and Coscinodiscus, dominated all year round, and showed temporal succession. Pennate diatoms (mostly Neodenticula seminae and Fragilariopsis) increased only during the spring bloom. Dinoflagellates (mostly Gymnodinium and Prorocentrum) were low in abundance, although they increased in summer when the phytoplankton standing stock was low. Silicotlagellate abundance was extremely low. Comparing the annual species composition of phytoplankton between Stn KNOT and Stu P (50degreesN, 145degreesW) in the Alaskan Gyre, there was a remarkable difference between the two sites. The phytoplankton assemblage at Stu P is characterized by a high abundance of Rhizosolenia alata and low abundance of Thalassiosira. In contrast, Thalassiosira dominates at Stu KNOT during most seasons. (C) 2002 Elsevier Science Ltd. All rights reserved.
  • HB Liu, K Imai, K Suzuki, Y Nojiri, N Tsurushima, T Saino
    DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY 49 (24-25) 5409 - 5420 0967-0645 2002 [Refereed][Not invited]
     
    Picoplankton community structure in the upper 200 m at Station KNOT in the western subarctic North Pacific was studied by flow cytometric analysis of the time-series samples collected from June 1998 through June 2000. Phototrophic picoplankton, Synechococcus and picoeukaryotes, displayed a clear seasonal cycle in population abundance during the 2-yr period, where chlorophyll a concentration remained relatively low and constant throughout the year, except in spring when the diatom bloom occurred. High picoplankton abundance occurred after the spring diatom bloom when the water column became more stratified. The maximum abundance of Synechococcus occurred in late June to early August, whereas the peak in picoeukaryotes abundance appeared to take place earlier in May and remained high through the summer. Heterotrophic bacteria abundance also displayed a seasonal variation pattern that was closely related to the picoplankton biomass, but not total chlorophyll biomass. (C) 2002 Elsevier Science Ltd. All rights reserved.
  • Tsurushima, N, Nojiri, Y, Imai, K, Watanabe, S
    Deep Sea Research Part II 49 (24/25) 5409 - 5420 2002 [Refereed][Not invited]
  • Imai, K, Nojiri, Y, Tsurushima, N, Saino, T
    Deep Sea Research Part II 49 (24/25) 5377 - 394 2002 [Refereed][Not invited]
  • A possible explanation for the dominance of chlorophyll in pico and nano-size fractions in the waters around the south Shetland Islands
    Kawaguchi S, Shiomoto A, Imai K, Tsuruga Y, Yamaguchi H, Noiri Y, Iguch N, Kameda T
    Ocean and Polar Research 23 (4) 379 - 388 2001 [Refereed][Not invited]
  • 高緯度域におけるノルパックネット標準採集の特性-その2.荒天下に起こる過剰曳網とプランクトン現存量評価
    小達恒夫, 坂岡桂一郎, 梶原善之, 今井圭理, 小林直人, 目黒敏美, 福地光男
    日本プランクトン学会報 46 (2) 134 - 142 1999 [Refereed][Not invited]
  • A Shiomoto, S Kawaguchi, K Imai, Y Tsuruga
    POLAR BIOLOGY 19 (5) 361 - 364 0722-4060 1998/05 [Refereed][Not invited]
     
    Size-fractionated chlorophyll a (Chla)-specific productivity (mu gC mu gChla(-1) h(-1)) was measured at 11 stations off the northern coast of the South Shetland Islands during summer. The Chla-specific productivity of the 2- to 10 or 10- to 330-mu m fraction was highest at 100% and 23% light depths. The Chla-specific productivity of the 2- to 10-mu m fraction was generally highest, and that of the < 2 or 10- to 330-mu m fraction was sometimes highest at 12% and 1% light depths. Temperature was less than 3 degrees C within the euphotic zone at all stations. The hypothesis of Shiomoto et al., according to which Chin-specific productivity of picophytoplankton (< 2 mu m) is not significantly higher than that of larger phytoplankton (> 2 mu m) in water colder than 10 degrees C, was supported on condition that light is not limited for larger phytoplankton.
  • Fluctuations of Nutrients and Primary Production Structure during Winter and Spring in Funka Bay
    Iinuma, T, Imai, K, Odate, T, Maita., Y
    Memoirs of the Faculty of Fisheries Hokkaido University 45 (1) 30 - 35 1998 [Not refereed][Not invited]
  • Relationship Between Water Mass and Chlorophyll a Concentration in Bottom Water on the Northern Bering Shelf in Summer
    Imai, K, Yanada, M, Maita. Y
    Bulletin of the Faculty of Fisheries Hokkaido University 49 (1) 41 - 49 1998 [Not refereed][Not invited]
  • 夏季噴火湾における物理・化学環境の変化に対する植物プランクトン群集の応答
    今井圭理, 小達恒夫, 簗田満, 米田義昭
    水産海洋研究 62 (2) 89 - 96 1998 [Refereed][Not invited]
  • 高緯度域におけるノルパックネット標準採集の特性
    小達恒夫, 今井圭理, 福地光男
    日本プランクトン学会報 45 (1) 1 - 8 1998 [Refereed][Not invited]


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