Microplankton with cell size less than 20µm are susceptive to environmental alteration,and often employed to study the ecological effect of anthropogenic disturbance in various environment.In this study,we investigated the temporal and spatial patterns of microplankton including bacteria,Synechococcus,microeukaryote and virus by flow cytometry in Sanmen Bay,and explored the driving physicochemical factors.The results showed that the abundance of bacteria,Synechococcus,microeukaryote and virus were 6.98X105-9.84X106 cells/mL,1.10X103-3.71X104 cells/mL,1.04X103-3.75X104 cells/mL and 1.01X106-3.47X107 mL-1,respectively.The abundance of microplankton in surface water was higher than that in bottom both in summer and autumn.Bacteria,Synechococcus and virus were more abundant in summer,but the abundance of microeukaryote had higher abundances in autumn.Among environmental variables detected here,water temperature was the main factor leading to the seasonal variation of microplankton.As to the horizontal distributio