Zooplankton community structure and its long-term variation in the western subarctic Pacific Ocean

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Mesozooplankton biomass (wet weight), species composition and abundance were determined seasonally and interannually in the western subarctic Pacific Ocean, using NORPAC net samples (45 cm diameter, 0.33 mm mesh, 0〜150 m vertical haul) from early 1980's to early 2000's. Zooplankton community was always predominated by copepods both in term of biomass and abundance, with their peak in early summer Large copepods, Neocalanus cristatus, N.plumchrus, N.flemingeri and Eucalanus bungii were accounted for a large part of the copepod peak. This increase is interpreted as that these large copepods move up to the surface layer by early spring, in which they actively feed and rapidly growth until early summer. Following with copepods, chaetognaths and euphausiids were abaundnt, with sporadic occurence of salpids and doliolids seasonally and locally. Summer copepod biomass showed interannual variation with about one order difference, but they showed no clear relationship with hydrographic conditions, especially with surface layer temperature. Species diversity of zooplankton community in the western subarctic Pacific, where are temporally affected by the Kuroshio Extention, was implied to be higher than in the eastern subarctic Pacific (Gulf of Alaska).