Response of sulfate concentration to eutrophication on spatio-temporal scale in freshwater lakes

The dramatical increase of sulfur concentration in eutrophic lakes, especially sulfate (SO₄^2-), has brought attention to the impact on the lake ecosystem; however, the mechanisms driving the intensification of eutrophication and the role of SO₄^2- concentrations remain poorly understood. To assess the impact of eutrophication on SO₄^2- dynamics in lakes, this study monitored SO₄^2- concentrations in water and sediments across seven lakes with varying trophic statuses on a spatial scale, and in the eutrophic Lake Taihu over one year on a temporal scale, as well as a series of microcosms with different initial SO₄^2- concentrations. Exogenous sulfur input is the primary driver of increased SO₄^2- concentrations in lakes, the highest SO₄^2- concentration in overlying water was 100 mg/L, as well as which reached 310.9 mg/L in sediment. The concurrent input of nutrients such as nitrogen and phosphorus exacerbated eutrophication, resulting in the destabilization of the sulfur cycle. Eutrophication promoted the SO₄^2- concentration on the spatio-temporal scale, especially in sediment, and trophic lake index (TLI) showed a positive correlation with the SO₄^2- in sediments (R² = 0.99; 0.88). The SO₄^2- concentration in water and TLI showed a nonlinear correlation on the temporal scale (R² = 0.44), and showed a positive correlation on the spatial scale (R² = 0.49). Microscopic experiments demonstrate that the anaerobic environment created by cyanobacteria decomposition induced sulfate reduction and significantly reduces SO₄^2- concentrations. Concurrently, the anaerobic environment facilitates the coupling of iron reduction with sulfate reduction, leading to a substantial increase in Acid Volatile Sulfides (AVS) in the sediment. These findings reveal that eutrophication has a dual effect on the dynamic change of SO₄^2- concentrations in overlying water, which is helpful to accurately evaluate and predict the change of SO₄^2- concentrations in lakes.