Eutrophic lakes are hot spots of CH₄ and volatile sulfur compound (VSC) emissions, especially during algal blooms and decay. However, the response of CH₄ and VSC emissions to lake eutrophication and algae growth as well as the underlying mechanisms remain unclear. In this study, the emissions of CH₄ 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 CH₄ emissions ranged from 20.4 to 126.9 mg m⁻² d⁻¹ in the investigated sites and increased with eutrophic levels and temperature. H₂S and CS₂ were the dominant volatile sulfur compounds (VSCs) emitted from the lake. The CH₄ oxidation potential of water ranged from 2.1 to 14.9 μg h⁻¹ L⁻¹, which had positive correlations with trophic level index and the environmental variables except for the NH₄⁺-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, SO₄²⁻-S, and total sulfur. Partial least squares path modelling revealed that the algal growth could promote VSC emissions, which had a positive correlation with CH₄ oxidation potential, likely due to the positive correlation between the CH₄ and VSC emissions from lakes. These findings indicate that water eutrophication and algae growth could increase the emissions of CH₄ and VSCs from lakes. Controlling algae growth might be an effective way to mitigate the emissions of CH₄ and VSCs from freshwater lakes.

Freshwater lakes, especially eutrophic ones, have become a hotspot of methanogenesis. However, the effects of eutrophication and seasonality on methanogenesis activity and methanogenic microbial community remain unclear. In the current study, for two adjacent lakes at different trophic status, their methanogenesis potential in different seasons was evaluated using incubation experiments. The density, diversity, and community structure of methanogens were analyzed based on the mcrA gene. Correlation analysis and redundancy analysis were carried out to identify the environmental factors driving the variations of methanogenesis potential and methanogen community. The results showed that eutrophication could result in active methanogenesis with relatively high seasonal variance. The methanogenesis variation could be well explained by carbon input in association with algal growth, as well as the change of methanogen population density. With the dominance of Methanomicrobiales in both lakes, the hydrogenotrophic pathway had a major contribution to total methane production. The considerable proportion of Methanomassiliicocales in eutrophic lake implied that methylotrophic methanogenesis might be previously underestimated. These results added new insights towards methanogenesis process in eutrophic freshwater lakes.