Photosynthetic features and primary productivity of phytoplankton in the Oyashio and Kuroshio-Oyashio transition regions of the northwest Pacific

Diel and seasonal changes in the photosynthetic physiology of phytoplankton and primary productivity were investigated during 2005, together with community composition in the Oyashio (OY) and Kuroshio – Oyashio transition (TR) regions of the northwest Pacific. In both regions, diel changes in the maximum photosynthetic rate (P* max) and the light saturation index (Ek) in the photosynthesis – irradiance (P –E) curve were observed, due to diel differences in photo-physiology. In the OY region, the highest values of chlorophyll a concentration, depth-integrated primary production and the maximum quantum yield of carbon fixation in photosynthesis (Fc max) were observed in May when diatom blooms occurred. Furthermore, a higher water-column light utilization efficiency (C) of photosynthesis for phytoplankton was found in the OY region in both May and September. In contrast, in the TR and warm-core ring regions, Fc max was nearly constant, while depth-integrated primary production in May was significantly lower than in the OY region. These results show that the spring phytoplankton assemblages in OY waters had a higher light utilization ability during photosynthesis. Such a high photosynthetic property would contribute to the highest seasonal biological drawdown of surface pCO2 among the world’s oceans (Takahashi, T., Sutherland, S. C., Sweeney, C. et al. (2002) Global sea – air CO2 flux based on climatological surface ocean pCO2, and seasonal biological and temperature effects. Deep-Sea Res. II, 49, 1601– 1622).