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A survey was conducted in Dianshan Lake to study the eutrophication indexes including total phosphorus (TP), total nitrogen (TN), pH, temperature, diaphaneity and chlorophyll-a level and dominant algae in seasons. The impacts of temperature, light, nitrogen and phosphorus on growth of and microcystin LR production by Microcystis aeraginosa strain under laboratory conditions were studied. Relationship between algal cell density and concentration of microcystin LR were studied. Results suggest that water in Dianshan Lake was eutrophicated. The suitable seasons for algae growth are the end of spring and summer. The annual average of TP and TN were 1.93 mg/L and 0.18 mg/L respectively. And 93.5 and 92.2 percent of TP and TN were higher than the criteria for the third class water body. Significant impact from agriculture was indicated since the peak of algae laged one month after the maxium use of fertilizer. The dominant algae in Dianshan Lake were cyanobacteria, bacillariophyta, cryptophyta and euglenophyta. Microcystis, anabaena and synedra, which excrete toxins and indicate water pollution, and are dominant algae species in summer. M. aeraginosa strain had a biggest growth rate at temperature of 25 degrees C and light intensity of 3 0001x, while microcystin LR production contents reached maximum at 20 degrees C and 5000lx respectively. The optimum TP and TN concentrations for growth of and toxin production by M. aeraginosa were found to be 650 micromol/L and 6.5 micromol/L respectively. TP is suspected to be the limiting factor for the growth of algae both in field and laboratory conditions. Positive correlations between total microcystin LR concentrations and algae cell density or M. aeraginosa cell densities are found. The algae cell density can be used to predict the level of algal toxins in water.