The seasonality of phytoplankton density and species diversity of Lake Siliserh was studied together with physico-chemical factors for one year on the basis of monthly sampling. Correlation between various physico-chemical parameters, phytoplankton groups were also calculated according to Karl Pearson's formula. The phytoplankton community of the lake belonged to three major groups and according to their density these groups ranked Chlorophyceae > Cynophyceae > Bacillariophyceae. Ulothrix and Closterium were the main contributors in density of Chlorophyceae. Group Cynophyceae was dominated by Coelospharium while Nitzschia and Synedra were the most common genera of Bacillariophyceae. These dominant forms also exhibited a clear seasonal succession. The phytoplankton number represented by 27 species. The lake was found to be slightly eutrophic during monsoon months. Algal bloom was observed during September. At the same time biological oxygen demand (8.98 mgL^-1) and nutrients (Nitrate 5.78 mgL^-1) were also found higher. These results suggest that temperature and nutrients play a crucial role in the phytoplankton dynamics of Lake Siliserh. The value of Shannon diversity index shows that the water quality can be classified as moderately polluted to clean water.

Keywords: Lake Siliserh, Phytoplankton, Nutrients, Seasonal succession, Algal Bloom.