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

The dramatical increase of sulfur concentration in eutrophic lakes, especially sulfate (SO), has brought attention to the impact on the lake ecosystem; however, the mechanisms driving the intensification of eutrophication and the role of SO₄ concentrations remain poorly understood. To assess the impact of eutrophication on SO dynamics in lakes, this study monitored SO 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 SO concentrations. Exogenous sulfur input is the primary driver of increased SO concentrations in lakes, the highest SO 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 SO concentration on the spatio-temporal scale, especially in sediment, and trophic lake index (TLI) showed a positive correlation with the SO in sediments (R = 0.99; 0.88). The SO concentration in water and TLI showed a nonlinear correlation on the temporal scale (R = 0.44), and showed a positive correlation on the spatial scale (R = 0.49). Microscopic experiments demonstrate that the anaerobic environment created by cyanobacteria decomposition induced sulfate reduction and significantly reduces SO 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 SO concentrations in overlying water, which is helpful to accurately evaluate and predict the change of SO concentrations in lakes.