he photosynthetic processes and the rate equation of photosynthesis are discussed in this paper. The seasonal variation of phytoplankton,zooplankton and nutrients at a fixed sta-tion and in the Extended Gulf Stream System are studied by using a vertically averaged plankton model,respectively.The results show that entrainment processes bring the interme-diate water with rich nutrients into the mixed layer to cause the growth of phytoplankton.The model produces the spring bloom in the EGSS well and basically ...