Interannual and latitudinal changes in zooplankton abundance, biomass and size composition along a central North Pacific transect during summer : analyses with an Optical Plankton Counter

To evaluate zooplankton interannual and latitudinal changes, Optical Plankton Counter analyses were made on preserved net zooplankton samples collected by NORPAC net from 0–150 m at 35°N–51°N stations along 180° in the central North Pacific during early–mid June 1981–2000. The mean numerical abundance of total zooplankton for the 20 years varied latitudinally from 19,200 to 84,300 ind. m^－2 but the differences between the three oceanic domains were not significant. However, highly significant latitudinal changes were observed in the mean zooplankton biomass, which ranged from 1.44 to 13.2 mg dry mass m^－2 with higher values in the Transitional Domain (TR) than in the Subarctic and Subtropical Domains. The high biomass in the TR was caused by the dominance of large-sized zooplankton with equivalent spherical diameters (ESD) of 2–4 mm, regarded to consist mainly of Neocalanus spp. C5. Both the slope and intercept of the Normalized Biomass Size Spectrum also showed significant latitudinal changes with a moderate slope and low intercept in the TR due to the dominance of large zooplankton with 2–4 mm ESD in biomass. In contrast to these large latitudinal changes, only limited interannual variations were observed for zooplankton abundance and biomass in the central North Pacific during the study period