Abstract: In order to figure out the current eutrophic state in east-half of lake chaohu -the tendency of eutrophication has aggravates gradually in recent years. The community structure of phytoplankton and water quality in east-half of lake chaohu was investigated. 119 speices of phytoplankton belonging to 8 phyla were identified, the most abundant was bacillariophyta, with 41 species, accounting for 34.45% of the total species; followed successively was chlorophyta, with 40 species, accounting for 33.61% of the whole species; the average density was about 47.36×10⁵ cells/L, cyanobacteria, which was the richest phyla in the lake, aproximately 40.08×10⁵ cells/L, accounting for 84.62% of the total phytoplankton. Result shows, the peak of phytoplankton density appear in summer, and relatively low in winter. The chlorophyll a content is height, and obvious change follow temperature. The waters of the eastern half of the chaohu lake total phosphorus and total nitrogen content obviously change with the seasons, after a year of monitoring, change of TP in the range of between 0.016-0.045mg/L, the overall average of 0.059mg /L, TN change in the range of between 0.164 -1.432mg/L, the overall average of 0.803 mg/L, the COD content is not high in the eastern half of the chaohu lake, the variation in the range of between 1.26-4.33mg/L, with an average of 2.795mg/L, transparency in the 2014 range between 0.100-0.280m. According to the nutritional status of each evaluation index calculation formula parameters and related weights and nutritional status were calculated in accordance with the comprehensive trophic state index grading standards, the eastern half of the chaohu lake eutrophication evaluation, from the evaluation results, the eastern half of the chaohu lake full nutritional type reached mesotrophic, and gradually worsening.

Key words: Lake Chaohu, Phytoplankton, Chlorophyll a, Eutrophication