Bacterial abundance and production during the unique spring phytoplankton bloom in the central Yellow Sea

Abstract

To elucidate the factors responsible for the resulting bacterial properties during the spring phytoplankton bloom in the central Yellow Sea, we investigated bacterial abundance and pro- duction together with physico-chemical and biological parameters. In spring, the central Yellow Sea is characterized by the horizontal intrusion of high-temperature and high-salinity Yellow Sea Warm Current (YSWC), which forms a thermohaline front between the central Yellow Sea and cold coastal waters, and by the vertical mixing of high-nutrient Yellow Sea Cold Water (YSCW) over the previous winter. The combination of high nutrient conditions with weak vertical density gradients because of increased irradiance, which increases the residence time of phytoplankton within the euphotic layer, seems to trigger the spring phytoplankton bloom. Enhanced bacterial biomass, production, and turnover rates were observed in the bloom area. Bacterial variables were significantly correlated with chl a concentration, but not with temperature. These results indicated that resource supply from phytoplankton primarily stimulated bacterial growth. Despite the enhanced bacterial growth in the bloom area, abundant heterotrophic nanoflagellates and their grazing on bacteria were responsible for the relatively smaller increase in bacterial biomass in the bloom area. Higher bacterial growth but smaller increases in bacterial biomass indicated that bacterial growth and biomass are independently controlled during the spring bloom in the central Yellow Sea, in which bacterial growth is primarily stimulated by organic material produced from the phytoplankton bloom, but the enhanced biomass is more tightly controlled by grazing.