Response of picophytoplankton to a warm eddy in the northern South China Sea

We investigated the distribution of several picophytoplankton groups (Prochlorococcus, Synechococcus, and picoeukaryotes) in relation to a warm eddy in the northern South China Sea in summer 2012. An anticyclonic eddy centered on 117°E longitude was identified during the sampling period using satellite data and hydrologic mapping. The layer of maximum Prochlorococcus and Synechococcus abundance within the eddy core dropped from 50 to 75 m, which was consistent with the subsurface chlorophyll a maximum. The water-column integrated abundance of Prochlorococcus, Synechococcus, and picoeukaryotes in the eddy core (9.67±0.23, 1.56±0.04, and 0.28±0.01 × 10³ cells ml^-1, respectively) was significantly lower (P<0.05) than that of the reference stations (25.10±2.32, 2.71±0.63, and 0.92±0.15 × 10³ cells ml^-1, respectively), and the abundance of Prochlorococcus in the core was also significantly lower than that at eddy edges (15.75±1.78 × 10³ cells ml^-1). However, there were no differences in the water-column integrated Chl a between the eddy core and edge. Our findings show that the warm eddy led to the reduced picophytoplankton abundance, especially of Prochlorococcus and picoeukaryotes, causing the layer of maximum picophytoplankton abundance to differ from that of the subsurface Chl a maximum.