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Abstract
en
During the research of "WEST-COSMIC (Western Pacific Environment Assessment Study on C02 Ocean Sequestration for Mitigation of Climate Change)", vertical distribution of plankton community structure was studied at 39°N, 147°E (down to 2000 m) and 44°N, 155°E (down to 4000 m) in the western North Pacific Ocean. Plankton community was largely divided into four major groups (bacteria, phytoplankton, protozooplankton, and metazooplankton) and their biomass were expressed carbon unit by measuring volume and using volume-carbon equations. Total plankton biomass in the water column was 13,873 mgC m^-2 at 39°N, 147°E and 29,772 (daytime) to 32,780 (nighttime) mgC m^-2 at 44°N, 155°E. The water column was divided into three strata: epipelagic (0-200 m); mesopelagic (200-1000 m); bathypelagic (>1000 m), and their contribution to total water-column plankton biomass was 33-38% (epipelagic), 33-37% (mesopelagic), and 25-34% (bathypelagic), respectively. In the epipelagic, the contribution of four major taxa in plankton biomass was nearly equal (ratios of four taxa; bacteria, phytoplankton, protozooplankton, and metazooplankton to the total biomass were 18-25%, 29-38%, 6-25%, and 17-40%, respectively). Below the epipelagic zone, phytoplankton biomass was decreased rapidly (1-2% of total biomass). In the mesopelagic zone, metazooplankton was the most dominated (60-85%) and bacteria was the second (12-28%). These two taxa dominated also in the bathypelagic zone，but the contribution of metazooplankton was decreased (43-47%) while that of bacteria was increased (41-47%). Constantly，the biomass of each taxon (mgC m^-3) was decreased with depth, and the relationship between carbon biomass and depth was expressed by regression model: log10Y = log10a＋b×log10X (or Y=a×X^b), where Y is the biomass (mgC m^-3), X is the depth (m), and a and b are fitting constants. Most of the taxa fitted to the regression well, and the slope (b) of regression was different between taxa: -0.56 for bacteria, -1.32 to -1.34 for phytoplankton, -0.95 for protozooplankton and -0.96 for metazooplankton. Analyzing the relationship between plankton biomass and depth from previous literatures, the appropriate slope (b) value of the regression for each taxon in the western North Pacific was considered as: -0.45 for bacteria, -1.33 for phytoplankton, -0.95 for protozooplankton, and -1.24 for metazooplankton.
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