TERRAPUB Journal of Oceanography
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Journal of Oceanography, Vol. 60 (No. 4), pp. 773-787, 2004

Latitudinal Differences in the Planktonic Biomass and Community Structure Down to the Greater Depths in the Western North Pacific

Atsushi Yamaguchi1*, Yuji Watanabe2, Hiroshi Ishida2, Takashi Harimoto2, Kazushi Furusawa3, Shinya Suzuki3, Joji Ishizaka4, Tsutomu Ikeda1 and Masayuki Mac Takahashi5

1Biological Oceanography Laboratory, Faculty of Fisheries, Hokkaido University, Minato-cho, Hakodate, Hokkaido 041-0821, Japan
2Ocean Environmental Survey Team, Environmental Chemistry Department, Kansai Environmental Engineering Center Co., Ltd., Azuchimachi, Chuo-ku, Osaka 541-0052, Japan
3Marine Biological Research Institute of Japan Co., Ltd., Yutakamachi, Shinagawa-ku, Tokyo 142-0042, Japan
4Faculty of Fisheries, Nagasaki University, Bunkyo, Nagasaki 852-8521, Japan
5Department of Systems Science, Graduate School of Arts and Science, University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8902, Japan

(Received 7 January 2004; in revised form 25 April 2004; accepted 5 May 2004)

Abstract: As part of the research program WEST-COSMIC Phase I (1997-2001), vertical profiles down to the greater depths (0-2000 m or 5800 m) of the plankton community structure composed of heterotrophic bacteria, phytoplankton, protozooplankton and metazooplankton were studied at one station in each subarctic (44°N) and in transitional region (39°N), and two stations in subtropical region (30°N and 25°N); all in 137-155°E in the western North Pacific Ocean. The biomass of all four taxonomic groups decreased rapidly with increasing depths at all stations, although the magnitude of depth-related decrease differed among the groups. As plankton community structure, metazooplankton biomass and bacterial biomass occupied >50% of the total in 0-2000 and 2000-4000 or 5000 m strata, respectively, at subarctic and transitional stations, while bacterial biomass contributed to >50% of the total consistently from 0 through 4800 or 5800 m at subtropical stations. Metazooplankton biomass integrated over the greater depths exhibited a clear latitudinal pattern (high north and low south), but this was not the case for those of the other taxonomic groups. As a component of metazooplankton, an appreciable contribution of diapausing copepods to the metazooplankton was noted at subarctic and transitional stations, but they were few or nil at subtropical stations. As protozooplankton assemblages, heterotrophic microflagellates (HMF) and dinoflagellates were two major components at subarctic and transitional stations, but were only HMF predominated at subtropical stations. From biomass ratios between heterotrophic bacteria, HMF and dinoflagellates, "sinking POC-DOC-heterotrophic bacteria-HMF-heterotrophic dinoflagellates" link was proposed as a microbial food chain operative in the deep layer of the western North Pacific. All results are discussed in the light of latitudinal differences in the structure and functioning of plankton community contributing to the 'biological pump' in the western North Pacific Ocean.


*Corresponding author E-mail: a-yama@fish.hokudai.ac.jp


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