Seasonal characteristics of new production in the coastal area of Hokkaido, the Okhotsk Sea during the ice-free period

Abstract

Primary production is divided into new or regenerated production based on nitrate or ammonium. New production leads to a net increase in the phytoplankton abundance. Here, we investigated seasonal variations in phytoplankton uptake rates of NO3 and NH4 to estimate the contribution of new production (f-ratio) in the coastal region of Hokkaido, Okhotsk Sea, during ice-free period. High f-ratios (> 0.7) were observed during the spring bloom period (April and May). Relatively high contributions of over 0.4, which have not been previously reported in oligotrophic regions, were frequently observed in the summer and autumn (June to October), despite oligotrophic conditions. In winter (November and December), contributions were below 0.4, despite the relatively high nitrate concentrations. Contributions of new production to that of primary were high during the spring bloom period, summer, and autumn, while being low in the winter. Despite being a relatively eutrophic season, the low contribution in the winter is important because of a significant increase of phytoplankton in the spring bloom and its increase consumption by associated organisms. This seasonal cycle of new production likely plays an important role in the high productivity of fisheries in the study area.