| Summary: | The abundance, biomass and size structure of mesozooplankton samples collected with Norpac nets from 0-150 m at 5-l3 latitudinal stations (35°N to 44°N) on 155°E in the western North Pacific annually during May-June (1993-2004) were analyzed using an optical plankton counter. Zooplankton counts of 4096 size units (size range: 0.25 to 20 mm equivalent spherical diameter [ESD]) were converted to biomass and summed as the community biomass. Data of each size class were combined with in situ water temperature data to estimate production potential. Based on the latitude, the study region was classified into subarctic front (SF:>42°N), transition domain (TR: 40-42°N), subarctic boundary (SB: 38-40°N) and subtropic current system (ST: <38°N). Throughout the study period, regional variations were seen in most size fractions in abundance (52,754-86,926 inds. m^-2), biomass (2,656-10,183 mg dry mass m^-2), and production (134-219 mg C m^-2d^-1). The TR was characterized by high biomass and production, but it had the lowest abundance. Interannual variations in abundance, biomass and production were largely due to variation in the 2-3 mm ESD fraction. Thus, the 2-3 mm ESD fraction (composed of Neocalanus spp.) was the most important one affecting not only the regional but interannual variation patterns of mesozooplankton. Apart from the consistent importance of Neocalanus spp., gelatinous zooplankton such as appendicularians, doliolids and salps were observed to form irregular peaks in the ST and SB regions in some years. It is suggested that the outbreaks of gelatinous zooplankton were related to the development of the thermocline or halocline in the top 40 m of the water column, which may stop the nutrient supply to the surface layer and increase the food supply via the microbial loop.
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