Seasonal variations of size-fractionated chlorophyll a and primary production in the coastal area of Hokkaido in the Okhotsk Sea

The size structure of phytoplankton has a significant influence on the marine ecosystem and fishery production. To elucidate which size of phytoplankton make up the majority of the primary producer community, size-fractionated chlorophyll a and primary production were investigated in the coastal area of Hokkaido in the Okhotsk Sea, which has high fishery production. Large (>10 mu m) phytoplankton accounted for approximately 80% of biomass and production in the spring bloom, approximately 40-75% in summer and autumn, but only approximately 20% in winter. Nutrient limitation possibly led to the lower contributions of large phytoplankton to biomass and production in summer and autumn. The size structure of chlorophyll a reflected that of primary production. Large phytoplankton were considered the main producers throughout the year, except winter. However, there was no significant difference in the chlorophyll a-specific primary production among the three sizes (large:>10 mu m; medium: 2-10 mu m; small:<2 mu m) in any season, implying the same growth rates in the three sizes. The lower contribution of large phytoplankton in winter may has been due to an intense copepod grazing. Large phytoplankton are likely to be the main primary producers throughout the year, leading to high fishery production in the study area.