Unexpected increase of sulfate concentrations and potential impact on CH₄ budgets in freshwater lakes

The continuous increase in sulfate (SO₄^2-) concentrations discharged by anthropogenic activities lacks insights into their dynamics and potential impact on CH₄ budgets in freshwater lakes. Here we conducted a field investigation in the lakes along the highly developed Yangtze River basin, China, additionally, we analyzed long-term data (1950-2020) from Lake Taihu, a typical eutrophic lake worldwide. We observed a gradual increase in SO₄^2- concentrations up to 100 mg/L, which showed a positive correlation with the trophic state of the lakes. The annual variations indicated that eutrophication intensified the fluctuation of SO₄^2- concentrations. A random forest model was applied to assess the impact of SO₄^2- concentrations on CH₄ emissions, revealing a significant negative effect. Synchronously, a series of microcosms with added SO₄^2- were established to simulate cyanobacteria decomposition processes and explore the coupling mechanism between sulfate reduction and CH₄ production. The results showed a strong negative correlation between CH₄ concentrations and initial SO₄^2- levels (R² = 0.83), indicating that higher initial SO₄^2- concentrations led to lower final CH₄ concentrations. This was attributed to the competition for cyanobacteria-supplied substrates between sulfate reduction bacteria (SRB) and methane production archaea (MPA). Our study highlights the importance of considering the unexpectedly increasing SO₄^2- concentrations in eutrophic lakes when estimating global CH₄ emission budgets.