Eutrophic levels and algae growth increase emissions of methane and volatile sulfur compounds from lakes.

Eutrophic lakes are hot spots of CH and volatile sulfur compound (VSC) emissions, especially during algal blooms and decay. However, the response of CH and VSC emissions to lake eutrophication and algae growth as well as the underlying mechanisms remain unclear. In this study, the emissions of CH and VSCs from four regions of Lake Taihu with different eutrophic levels were investigated in four months (i.e., March, May, August and December). The CH emissions ranged from 20.4 to 126.9 mg m d in the investigated sites and increased with eutrophic levels and temperature. HS and CS were the dominant volatile sulfur compounds (VSCs) emitted from the lake. The CH oxidation potential of water ranged from 2.1 to 14.9 μg h L, which had positive correlations with trophic level index and the environmental variables except for the NH-N concentration. Eutrophic levels could increase the abundances of bacteria and methanotrophs in lake water. α-Proteobacteria methanotroph Methylocystis was more abundant than γ-Proteobacteria methanotrophs in March and May, while the latter was more abundant in August and November. The relative abundance of Cyanobacteria, including Microcystis, A. granulata var. angustissima and Cyanobium had significantly positive correlations with temperature, turbidity, SO-S, and total sulfur. Partial least squares path modelling revealed that the algal growth could promote VSC emissions, which had a positive correlation with CH oxidation potential, likely due to the positive correlation between the CH and VSC emissions from lakes. These findings indicate that water eutrophication and algae growth could increase the emissions of CH and VSCs from lakes. Controlling algae growth might be an effective way to mitigate the emissions of CH and VSCs from freshwater lakes.