Abstract: The annual cycle of phy to plankton dynamics in the southern Huanghai Sea is studied by a three-dimensional physical-biological model.The physical model is Princeton ocean model(POM),the biological model involves interactions between the inorganic nutrients(nitrogen,phosphate,silicate),phytoplankton and herbivorous zooplankton biomass,and detritus.Given a know ledge of physical forcing,the model simulates main observed seasonal characteristic features,in particular,the annual cycle of phytoplankton with the spring and fall blooms,and the subsurface phytoplankton maximum layer in summer.Initiation of the spring bloom is shown to be critically dependent on the water column stability.It commences as soon as the convective mixing process weakens and before the seasonal stratification of surface begins to develop(in earlier April).At the time of establishment of the seasonal thermocline from May to October,the nutrient concentrations in the mixed layer are low enough to limit production in the center of the South Huanghai Sea,the layer between the thermocline and the base of the euphotic zone provides sufficient light and nutrients to support subsurface phytoplankton development.The fall bloom takes place sometime between September and November depending on environmental conditions.