Picophytoplankton abundance and distribution in three contrasting periods in the Pearl River Estuary, South China

Abundance and distribution characteristics of three picophytoplankton groups (Synechococcus, Prochlorococcus and picoeukaryotes), identified by flow cytometer, and two types of picocyanobacteria (phycoerythrin and phycocyanin-rich strains), determined by epifluorescence microscope, were assessed in three contrasting periods in the Pearl River Estuary, South China. The average abundance of picophytoplankon and picocyanobacteria was 10(4) cells mL(-1) in the two summer observations and 10(3) cells mL(-1) in winter. Low cell density in the cold season was probably due to high turbidity and low water temperature. Prochlorococcus was detected within the estuarine plume in the summer investigations and was undetectable in the winter. Higher abundance and the further upstream occurrence of Prochlorococcus in the summer of 2011 were mainly associated with extreme low river flows. We presumed that Prochlorococcus abundance and distribution range were balanced by river discharge and saline-water intrusion. Synechococcus was the dominant group in the inner estuary. Similar to Prochlorococcus, Synechococcus distribution was related with freshwater inflow. The river inflow exerted strong limitation on Synechococcus and Prochlorococcus, with this effect weakening along the salinity gradient. Picoeukaryotes were the least abundant category among picophytoplankton and showed a different distribution pattern from that of Synechococcus and Prochlorococcus. For picocyanobacteria, there was a clear spatial gradient with phycocyanin-rich strains dominant in the upper estuary, and phycocyanin-rich and phycoerythrin-rich cells dominant downstream. A significant negative correlation was observed between phycocyanin-rich cells to phycoerythrin-rich cells ratio and salinity.