Mercury methylation in Laurentian Great Lakes tributaries across a gradient in stream water sulphate

Agricultural sulphur use is increasing, yet implications for mercury (Hg) cycling in receiving waters are poorly constrained. Here, we combine field surveys of Laurentian Great Lakes tributary streams with enriched mercury isotope incubation and sulphate addition experiments to evaluate sulphate controls on methylmercury (MeHg) production in streambed sediment across a gradient of sulphate exposure. In the field, stream water MeHg concentrations exhibit a non-linear relationship with sulphate, with elevated concentrations at intermediate sulphate levels (∼14–26 mg l −1 ), consistent with observations in other aquatic ecosystems. Streambed sediments actively methylate Hg across all sites, with potential methylation rate constants (K meth = 0.01–0.08 d −1 ) comparable to habitats with relatively elevated Hg methylation capabilities. Experimental sulphate additions further stimulate MeHg production only in relatively low sulphate stream sediments, whereas K meth remains stable in sediment with elevated ambient sulphate despite similar sulphide concentrations. Across all treatments, K meth increases with porewater sulphide production, indicating limitations from sulphate reduction activity rather than sulphide inhibition of Hg bioavailability. Agricultural streams function as active biogeochemical reactors, with sulphate limitation being an important prerequisite for sulphate loading effects on Hg methylation. Given the changing state of sulphate sources, reinstating routine sulphate monitoring in Laurentian Great Lakes tributaries would help to identify watersheds vulnerable to sulphate-driven MeHg increases.