| The Estuary of Changjiang River and its adjacent sea areas is the important area of fishery and breed aquacultures, while there has been high frequency of red tide in this area. Diatoms play an important role in the food chain, which are important component of phytoplankton, and some of them are red tide causative species. This paper focuses on the study of diatom biodiversity, community structure, ecological distribution and community dynamics in the main HAB area of East China Sea, which will provide basic and scientific data for red tide studies in the area. According to the data from the eight cruises during May 2002 to June 2005, the distribution and ecology of planktonic diatoms were investigated. The diatom community dynamics and yearly variation were studied in the investigated area in four successive spring seasons. At the same time, the relationship between environmental factors and diatom community was analyzed by regression statistic analysis methods. The first record of Thalassiosira curviseriata Takano,H.,1981 and its bloom were also reported.Major results were summarized as follows:1. In spring(from the end of April to middle or late of May from 2002 to 2005,4 cruises), average yearly quantity of planktonic diatoms in the investigated area was 2.10×103cells/L,ranged from 0 to 23.3×103cells/L. The average quantity of phytoplankton in different years fluctuated in a small range for the same quantity level.2. The horizontal distributions of higher planktonic diatom cell density were occurred near 123°E,30.0～31.0°N in every water layer,that is the area of Hangzhou Bay and the east of Zhoushan fisheries. The vertical distribution was different in different cruises,the average diatom abundance was reduced successively from surface to bottom in spring for 4 cruises.
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