Dynamics of autotrophic picoplankton and heterotrophic bacteria in the East China Sea

Dynamics of Synechococcus, Prochlorococcus, picoeukaryotes and heterotrophic bacteria in the East China Sea, a marginal sea of the Northwest Pacific, were investigated by flow cytometry in winter 1997 and summer 1998. Temporally, Prochlorococcus were always more abundant in the summer than in the winter, the same was true to Synechococcus except for the oceanic region. In contrast, picoeukaryotes were more abundant in the winter than in the summer. Heterotrophic bacteria were the least variable component among the four groups of picoplankton. Spatially, Prochlorococcus were extremely variable in the sea. They were largely confined to the warm water current areas and absent in the coastal areas in the winter, but present at most locations in the shelf water in the summer. Synechococcus were more abundant in the coastal areas than in the open waters in the summer but inverse in the winter. Compared with other picoplankters, picoeukaryotes showed more responses to water fronts on the shelf. In surface water, the lower boundary temperature for Prochlorococcus to present was around 15.6 °C in the winter but 26.4 °C in the summer, while it could also be found in the stratified deep water where temperature was as low as 14.3 °C in the summer. The higher boundary concentrations of total inorganic nitrogen for Prochlorococcus were about 6.5 μmol L−1 in the winter and about 3.0 μmol L−1 in the summer in the surface layer. The lower boundary salinities for Prochlorococcus were 33.5 psu in the winter and 29.1 psu in the summer. The key limiting factor for the coastward distribution of Prochlorococcus in the East China Sea were considered to be the movements of the Kuroshio Current and the Taiwan Warm Water Currents year around, temperature in winter and freshwater input in summer. Synechococcus correlated to temperature positively and nitrogen negatively in the winter when the high phycourobilin (PUB) to phycoerythrobilin (PEB) ratio strain dominated, and were independent from temperature but were associated with nutrients in the summer when the low PUB to PEB ratio strain dominated. Heterotrophic bacteria were not significantly affected by temperature but showed associations with nutrients in the summer. The big seasonal difference in the abundance of picoeukaryotes was most likely due to species succession.