Precontrol of algae-induced black blooms through sediment dredging at appropriate depth in a typical eutrophic shallow lake

Algae-induced black blooms, which trigger both the collapse of lake ecosystems and crises in urban water supplies, have become a serious ecological problem in numerous eutrophic shallow lakes. Therefore, preventing and suppressing the outbreak of black blooms is critical for maintaining the health of lake ecosystems. In this study, sediment dredging was employed as a black bloom precontrol method, and the effects of different dredging depths (0 cm, 7.5 cm, 12.5 cm, and 22.5 cm) were compared regarding the prevention of black bloom formation. Drained algae (mostly cyanobacteria) were added to these treatments at a density of 5000 g/m2. The main physical and chemical characteristics of the water and sediment important for black blooms were analyzed during the study. The results showed that dredging was unable to suppress the offensive odor of the algal bloom. High concentrations of volatile organic sulfur compounds due to algal decomposition were detected in all of the added-algae treatments. Black blooms occurred in the undredged (UDR), 7.5 cm dredged (7.5DR), and 12.5 cm dredged (12.5DR) treatments but did not occur in the 22.5 cm dredged (22.5DR) treatment. Therefore, black blooms can be suppressed by sediment dredging at an appropriate depth (22.5 cm in the current study). The ∑H2S concentrations in the water samples of the 22.5DR treatment were remarkably lower than in the other added-algae treatments during the experiment. Hence, ∑H2S, as compared to DO, Eh, pH, and Fe2+, was considered as the most important limiting factor in the overlying water for black blooms. Two major reasons may have caused the low ∑H2S concentrations and suppressed the black blooms: (a) low levels of acid-volatile sulfide in the sediment after dredging (22.5 cm) may have reduced the release of hydrogen sulfide to the overlying water, and (b) the low porosity (≤60.5%) observed after dredging is unconducive to pollutant diffusion and migration in sediments, which may have suppressed the release of Fe2+ and ∑H2S into the overlying water.