Abstract: Tangbahu Reservoir is located in Xinjiang. It is one of the many reservoirs in Altay. The Tangbahu Reservoir stored up the thawed snow, and the snow water is the principal source of the reservoir. In recent years, the global climate become warming and the influence of human activity increased significantly, the glacier’s snow line has flinched 500 meters. It will make an impact in aquatic ecosystem. The phytoplankton which have an important impact on the plankton community, are major component groups of aquatic plankton. Phytoplankton community structure of Tangbahu Reservoir was investigated and analyzed in July 2008 （the flow period）, October 2008 （the dry period） and May 2009 （the flood period）. Results showed that a total of 97 phytoplankton species, belonging to 8 phyla were identified, of which 43 species were Bacillariophyta, 28 species were Chlorophyta, 12 species were Cyanophyta, 5 species were Euglenophyta, 3 species were Cryptophyta, 3 species were Xanthophyta, 2 species were Pyrrophyta and 1 species were Chrysophyta. The number of species for Bacillariophyta, Chlorophyta, Cyanophyta, Euglenophyta, Cryptophyta, Xanthophyta, Pyrrophyta Chrysophyta accounted for 45%，29%，12%，5%，3%，3%，2% and 1% of the total species respectively. The trends of species changing showed: the flood period （56 species）> the flow period （39 species）> the dry period （19 species）, the flood period and the flow period was made up by 8 phyla, but the dry period was made up by 6 phylathe. The dominance analysis showed that the phytoplankton community structure was typical type of Chlorella-diatoms in this reservoir. A. oscillarioides, M. aeruginisa,C. minor,C. bodanica,S. bijuga,W. botryoides, C. vulgaris and T. oblonga were the dominant species in the normalwater season. In the dry season, dominant species were M. flos-marginata, C. minor, M. granulata, C. debaryana, S. bijuga, C. vulgaris and C. erosaerosa. But in the wet season M. incerta Lemm, M. granulate, A. formosa,S. armatus, C. vulgaris, C. minima, C. erosaovata and G. aeruginosum were the dominant species. The phytoplankton in Tangbahu Reservoir showed larger alternation of hydroperiod with the highest density （9.42±2.20×106 ind/L） in dry period, followed by the flow period （7.80±1.41×106 ind/L）, the flood period was the lowest for 1.11±2.32×106 ind/L, it also had a very significant differences （p<0.01）, the most obvious difference was in flood period. The cell density was the greatest in the dry season, and the wet season has the smallest cell density. The paper analyzed the ShannonWiener and evenness indexes of phytoplankton community during three periods in Tangbahu Reservoir. The diversity index showed as follows: the ShannonWiener indices ranged between 2.03 and 2.45, the Pielou indices ranged between 0.41 and 0.47, and the Margalef indices ranged between 0.78 and 2.74.Moreover, the phytoplankton composition, cell density as well as the diversity were closely related to the temperature and the water volume of Tangbahu Reservoir was also discovered.

Key words: phytoplankton, community composition, biodiversity, Xinjiang