Inter-oceanic differences in macrozooplankton biomass and community structure in four regions around Hokkaido Island, Japan : consequences for marine ecosystem structure

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  • Inter-oceanic differences in macrozooplankton biomas and community structure in four regions around Hokkaido Island, Japan : consequences for marine ecosystem structure

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Inter-oceanic differences in macrozooplankton(euphausiids, amphipods, cnidarians and chaetognaths) biomass and community structure were studied in four regions(northern Japan Sea, southern Okhotsk Sea, Donan and Doto areas of North Pacific) around Hokkaido Island. Whole zooplankton biomass ranged between 25 and 315 g WM m-2 and was significantly higher in the southern Okhotsk Sea. Macrozooplankton composed 5-15% of the whole zooplankton biomass. Across all macrozooplankton taxa, the number of species varied with region : the fewest species(2-4 species) were in the southern Okhotsk Sea, followed by the northern Japan Sea(3-5 species), and the most species(4-9 species) were in the Donan and Doto areas of the North Pacific. Depth topography and thermal conditions are considered possible causes of the differences in species richness across regions. Through inter-oceanic comparison, marine ecosystem characteristics of each region were evaluated. The zooplankton community in the northern Japan Sea was characterized by a predominance of carnivorous macrozooplankton taxa(amphipods, cnidarians and chaetognaths) and fewer herbivorous taxa(euphausiids and mesozooplankton), and it was determined to be as a top-down controlled ecosystem. In contrast, in the southern Okhotsk Sea, the zooplankton community was predominated by herbivorous mesozooplankton and euphausiids, and the marine ecosystem was determined to be a bottom-up control region. In the North Pacific, zooplankton community in the Doto area was predominated by chaetognaths. The zooplankton community in the Donan area showed mixed characteristics of the Doto area and the northern Japan Sea because of the seasonal water mass exchange from these two regions.

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