A comprehensive sulfate and DOM framework to assess methylmercury formation and risk in subtropical wetlands

Wetlands play a vital role in contaminant cycling and uptake. Understanding how sulfate (SO₄^2‒) influences the conversion of inorganic mercury (Hg(II)) to toxic methylmercury (MeHg) is critical for predicting wetland responses to land use and climate change. Here, we sampled surface and pore waters across SO₄^2‒ gradients in three freshwater Everglades wetlands to assess linkages between SO₄^2‒, MeHg, dissolved organic matter (DOM), and inorganic sulfide (S(‒II)). Increasing SO₄^2‒ concentrations increase S(‒II) and DOM concentrations and DOM aromaticity. MeHg concentration show a unimodal response to surface water SO₄^2‒, which reflect high Hg(II) methylation at low-to-intermediate SO₄^2‒concentration (2-12 mg/L) and low Hg(II) methylation at higher SO₄^2‒concentrations ( > 12 mg/L). MeHg concentrations in surface waters correlate positively with MeHg concentrations in prey fish. The coherent biogeochemical relationships between SO₄^2‒ and MeHg concentrations and biologic uptake improve MeHg risk assessment for aquatic food webs and are globally relevant due to anthropogenic and climate-driven increases in SO₄^2‒.