| According to the phytoplankton data from the five investigations during 2003—2005 in Hangzhou Bay, compare with the histroy data, the species, distribution, abundance, Shannon Weaver index, Pielou's index, Margalef's index, and the changes of these indexs was analyzed and discussed.These results revealed that 107 species of phytoplankton in Hangzhou Bay from 2003 to 2005, and the most of them were diatom , the secondly were Dinophyceae species. The dominant species were Skeletonema costatus, Coscinodiscus jonesianus , Coscinodiscus oculns-iridis, Nitzschia lorenziana, Coscinodiscus radiatus, Diploneis brightwellii. As a result of fresh water from Qiantang River, Changjiang River, Yongjiang River,as well as influence of open sea high salt water, there was all kinds of ecology type in the investigated aera, such as fresh water class group, river mouth half salty water class group, nearshore low salinity class group, open sea high salinity class group, sea Guang Buxing class group and so on.The quantity of phytoplankton was 3.7×103～6.6×107ind./m3, the highest value appeared in Auguest, the second value appeared in March and the lowest value appeared in May. The quantity of phytoplankton in different years fluctuated: compare the data in 2004 with 80's and 2000-2003 from WangYunlong et al. in Spring, the quantity reduced; And in Summer, the quantity of phytoplankton reduced too.The Shannon Weaver index was 0.75-3.61, Pielou's index was 0.23-0.96, Margalef's index was 0.16-1.31. The index of phytoplankton in different years fluctuated: In spring, the Shannon Weaver index and Pielou's index increased from 2000 to 2004, the Margalef's index was lowest in 2004; In summer, the Shannon Weaver index and Pielou's index highest in 2004 and lowest in 2003, the Margalef's index was d2003>d2005>d2004.
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