Abstract: To evaluate the restoration from the contaminative water condition of four rivers with different ecological types, which are Wuchaogang river, Henggang river, Chaoyang river and Caoyanghuanbang river in the urban unit of Shanghai, the investigations of the spatial variations of phytoplankton community structure, and chemical and physical parameters were undertaken from November 2007 to January 2008, and the relationship between the phytoplankton diversity and water pollution was also analyzed comprehensively. There are 150 species of 7 phylums of planktonic algae, including 61 species of Chlorophyceae, 27 of Cyanobacteria, 24 of Acillariophyta, 23 of Euglenophyta, 11 of Xanthophyta, 3 of Cryptophyta, and 1 of Pyrrophyta. Dominant species are Chlorophyceae, Cyanobacteria and Acillariophyta. Judging from the whole winter, the four rivers have almost no differences in the distribution of species composition such as Chaoyang river (78 species)>Wuchaogang river (76 species)> Henggang river (74 species)> Caoyanghuanbang river (72 Species). The distribution of the four rivers in cell density and biomass are as follows respectively: Chaoyang river>Caoyanghuanbang river>Wuchaogang river>Henggang river, and Caoyanghuanbang river>Chaoyang river>Wuchaogang river>Henggang river. Chlorophyceae is the main phytoplankton, then Bacillariophyta and Cyanophyta. The contents of N and P in the four rivers are all surplus and have an obvious correlation with both the cell density and the biomass, which are negatively correlated with P, and positively with N. The four rivers were analyzed and valued with diversity indexes of nutritional status and the Caoyanghuanbang river is the best one, followed by Wuchaogang river and Henggang river, and Chaoyang river is the worst. It is found the three parameters of the diversity indexes are too ideal in assessing small urban rivers water quality, and they are inconsistent with that evaluation criteria of the species, cell density and biomass of phytoplankton. Therefore, it should be cautious to apply the biological diversity indexes to valuate urban rivers environment.

Key words: phytoplankton, community, eutrophication, environmenal assessment , urban rivers, phytoplankton, community, eutrophication, environmenal assessment