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The present study investigated the dynamics of Microcystis population and microcystin (MC) production in relation to the environmental factors in the Dau Tieng Reservoir (DTR), with results indicating that Microcystis dominated the total cyanobacterial abundance at all times. M. aeruginosa was the most predominant of the bloom-forming species in the DTR. The average biomass of Microcystis was relatively high from February to August, which ranged from 9.1 to 39.7mg Intracellular MC concentrations in the water ranged from 1.3-17.2 mu g L-1 and varied on spatial and temporal scales, and the MC concentration was positively correlated with M. aeruginosa biomass. The Pearson correlation analysis reveals that water temperature and nutrients are the most important variables accounting for the occurrence of M. aeruginosa, suggesting that the increases in temperature together with the nutrient concentrations may promote more frequent toxic Microcystis blooms and higher concentrations of MC. In contrast, turbidity was negatively correlated with the total biomass of Microcystis and MC concentrations, indicating that rising turbidity may suppress toxic Microcystis abundance and reduce the MC concentrations. Therefore, these results highlight the fact that future eutrophication and global climate change could promote toxic Microcystis blooms and hence increase the MC levels in surface water.