In the present study, we evaluated the effects of different environments on the filtering rate (FR), mortality, and biodeposition (BD) of the freshwater mussel Unio douglasiae in bloom waters containing the toxic cyanobacterium Microcystis aeruginosa. The mean FR of 19 selected individuals (shell length, 5.0-9.8 cm) was 0.30 ± 0.03 L g-1 h-1 (range = 0.24-0.35 L g-1 h-1). Shell length was strongly correlated with both net and gross BD of mussels (P < 0.0001). The mean FR was higher in river water (0.405 ± 0.052 L g-1 h-1) than in lake water (0.304 ± 0.051 L g-1 h-1). In contrast, the BD of mussels was higher in RW (0.671 ± 0.609 mg g-1 h-1) than in LW (0.275 ± 0.027 mg g-1 h-1). For algal species, the FR of mussels ranged from 0.114 ± 0.024 to 0.553 ± 0.019 L g-1 h-1. The FR of U. douglasiae was higher in river water (mainly diatoms), whereas BD was higher in lake water (mainly Microcystis). U. douglasiae did not prefer toxic M. aeruginosa, which was significantly accumulated in pseudofaeces and faeces. The maximum FR of U. douglasiae in algal bloom water was recorded at a water temperature of 25°C and water depth of 50 cm. Moreover, the in situ mortality of U. douglasiae was strongly affected by water temperature and nitrogen concentration. Overall, U. douglasiae can enhance water quality in eutrophic areas by removing dominant cyanobacteria, although its removal efficiency depends on environmental parameters and site of introduction.