Abstract: Qingshan Reservoir is an important utilization large reservoir which was built in 1964. Its eco-environment and water management have a great significance on sustainable development of this area. With the fast economical development, the water quality has been declined as an amount of pollutants discharged. To explore the characteristics of the phytoplankton community structure and the response of water quality, a survey was carried out quarterly in Qingshan Reservoir in 2009. Based on the analysis of phytoplankton and water samples from inflow, center and dam in Qingshan Reservoir, the characteristics of community structure and abundance distribution were studied, and the water situation was evaluated. Cell number and species identification were performed by light microscope. The dominant population, species diversity and community structure of phytoplankton were determined as well. A total of 89 species in 40 genera and 7 divisions were identified, which the most abundant group was Chlorophyta, with 33 species that account for 37% of total species, followed by Bacillariophyta (27 species), Cyanophyta (17 species), Euglenophyta (6 species), other alges (6 species). The abundance of phytoplankton cells varied from 6.20×10⁵ to 1.18×10⁷ cells/L (averaging 4.88×10⁷ cells/L). The main physical and chemical indicators results showed that the water quality of Qingshan Reservoir was in the light-middle eutrophic state. The trophic state index was higher in inflow than in center and dam sites. The concentrations of total nitrogen and total phosphorus were higher than the surface water quality standards Ⅳ value. The indices of Shannon-Wiener, Margalef and Pielou Evenness were 1.37-2.23 (averaging 1.79), 0.56-1.52 (averaging 1.05) and 0.45-0.77 (averaging 0.64), respectively. The dominant species of each month were identified and their contribution to the total abundance was analyzed. The seasonal variation of phytoplankton community structure was significant. In spring Bacillariophyta, Cryptophyta and Pyrrophyta dominated, in summer Cyanophyta and Chlorophyta dominated, in autumn Cyanophyta, Bacillariophyta and Cryptophyta, and in winter Bacillariophyta and Cryptophyta. The relationship between phytoplankton and environmental factors were discussed. Correlation analysis showed that there was significant negative correlation between phytoplankton density and transparency, inflow, outflow, and significant positive correlation between phytoplankton density and TP. Canonical correspondence analysis (CCA) showed that the inflow, outflow, water temperature, dissolved oxygen and nutrients were important environmental factors affecting the distribution pattern of phytoplankton. Qingshan Reservoir is located in a typical subtropical monsoon climate area, seasonal changes in the density of phytoplankton communities significantly affected by seasonal changes in watershed hydrology, and in particular, rainfall, temperature and hydraulic retention time are the main factors affecting phytoplankton community.

Key words: phytoplankton, eutrophication, canonical correspondence analysis(CCA), enviromental factors