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: As a part of Korea Deep Ocean Study program, we investigated the distribution of planktonic protists in the upper 200 m of the northeast Pacific from 5oN to 17oN, along 131o30'W. Area of divergence was formed at 9oN which is boundaries of the north equatorial counter current (NECC) and the north equatorial current (NEC) during this cruise. Chlorophyll-a concentration was higher in NECC than in NEC area. Pico chl-a(<2 mm) to total chl-a accounted for average 89% in the study area. The contribution of pico chl-a to total chl-a was relatively high in NEC area than in NECC area. Biomass of planktonic protists, ranging from 635.3 to 1077.3 mgC m-2(average 810 mgC m-2), was most enhanced in NECC area and showed distinct latitudinal variation. Biomass of HNF ranged from 88.7 to 208.3 mgC m-2 and comprised 15% of planktonic protists. Biomass of ciliates ranged from 123.6 to 393.0 mgC m-2 and comprised 25% of planktonic protists. Biomass of HDF ranged from 407.2 to 607.8 mgC m-2 and comprised 60% of planktonic protists. HDF was the most dominant component in both NECC and NEC areas. Nano-protist biomass accounted for more than 50% of total protists in the both areas. The contribution of nanoprotist to total protists biomass was relatively higher in NEC area than in NECC. The biomass of planktonic protists was significantly correlated with phytoplankton biomass in this study area. The size structure of phytoplankton biomass coincided with that of planktonic protists. This suggested that the structure of the planktonic protists community and the microbial food web were dependent on the size structure of the phytoplankton biomass. However, biomass and size structure of planktonic protist communities might be significantly influenced by physical characteristics of the water column and food concentration in this study area.