The multidisciplinary oceanographic survey and in situ nutrient enrichment experiments were conducted in the northern South China Sea during summer of 2004, aimed at better understanding of the responses of phytoplankton growth on nutrient enrichments. Elucidated the effects of the coastal upwelling resulted from Ekman transportation formed by southwest monsoon and coastal current toward northeast and middle scale eddies on the distribution of nutrients, and effected ultimately on the phytoplankton growth and standing stock, and the responses of the latter to the former, the physical-chemical coupling processes. The water mass with the properties of high temperature, low salinity, high dissolved oxygen concentration, low nutrient concentration and low phytoplankton biomass was found in anticyclonic eddies, resulted from the downwelling induced by water convergence. Results of nutrient enrichment experiments showed that only multi-nutrients adding could stimulate the phytoplankton growth. At the station of S1008, for example, Chla obviously increased in NP and NPSi experimental groups from 0.28 mg•m－３ to 1.07 mg•m－３ and 1.19 mg•m－３ respectively after 60 hours incubation since nutrients adding. Furthermore, we also found that the nutrient enrichments resulted in the successions of not only size-structure but also species structure of phytoplankton communities, i.e. the shifts from picophytoplankton to nanophytoplankton and netphytoplankton, and from dinoflagellates to diatoms.