Seasonal variability of zooplankton size spectra at Mombetsu Harbour in the southern Okhotsk Sea during 2011: An analysis using an optical plankton counter

Abstract To evaluate the temporal changes in zooplankton size spectra, optical plankton counter (OPC) measurements were made of high-frequency time-series zooplankton samples collected at approximately 3.5-day intervals in Mombetsu Harbour, which is located in the southern Okhotsk Sea, from January to December 2011. Based on biomasses of 47 equivalent spherical diameter (ESD) size classes binned at 0.1 mm intervals across 0.35–5 mm, the Bray–Curtis similarity index separated the zooplankton community into six groups (A–F). The occurrence of each group was separated seasonally. Thus, groups A and B were observed during the ice-covered season and summer season, respectively. During March and June, groups C–F were observed. Their occurrence varied in the short term in relation to the exchange of water masses. Groups A and C, which were observed from January to April, showed flatter normalized biomass size spectra (NBSS) slopes (−0.85 to −1.1), which indicate low productivity. In contrast, the other groups showed steeper slopes (−1.31 to −1.52) from May to December, with high productivity. Throughout the year, the frequency of highly productive groups occurred at a high level (95.2%). Although the seasonal variability in zooplankton size and productivity in Mombetsu Harbour was mainly governed by water mass exchanges, the productivity was continuously high throughout nearly all of the one-year study period.