UC Davis

Mercury (Hg) is a ubiquitous environmental contaminant that impacts aquatic environments and human health globally. Methylmercury (MeHg), the neurotoxic form of Hg that bioaccumulates and biomagnifies up aquatic food webs, is prominent in the Everglades’ waters and organisms. Biogeochemical drivers controlling MeHg formation in the freshwater portion of the Florida Everglades include dissolved organic matter (DOM), which complexes inorganic divalent Hg (Hg(II)) and enhances lability for microbial methylation; sulfate (SO42-) from agricultural inputs, which influence the presence and activity of organisms that impact Hg(II) methylation; and abundant inorganic Hg delivered to the Everglades by rainfall. The Everglades are particularly susceptible to conditions that increase SO42- delivery and possibly Hg(II) methylation due to increasing magnitude, frequency, and duration of salinity spikes in Everglades National Park caused by sea level rise. In the Everglades, and globally, there is an urgent need to assess how Hg cycling will respond in coastal environments in light of rapidly rising sea level.To investigate the effect of sea level rise on Hg(II) methylation, peat cores were collected from the freshwater Everglades and incubated in the lab for 0-20 days with water at 5 relevant salinity levels from 0.16 – 6.0 parts-per-thousand, representing SO42- amendments from 0.2 to 450 mg L-1, which represent a freshwater to brackish transition in coastal wetlands. Treatment waters were spiked with enriched stable 201Hg(II) isotope to track the transformation of 201Hg(II) to Me201Hg through time at each salinity treatment, accounting for speciation of native Hg in peat cores at time of core flooding. At 8 time points from 0-20 days, porewater was sampled from peat cores and analyzed for relevant geochemical constituents and the transformation of inorganic 201Hg(II) to Me201Hg. Peat was also measured for concentrations of inorganic 201Hg(II) and Me201Hg. In all five salinity treatments, shortly after inundation, the porewater became anoxic and with increased incubation time the DOM composition shifted to more aromatic in composition, as evidenced by DOM SUVA254 across all five salinities. In the four highest salinity treatments, SO42- concentrations decreased and sulfide concentrations increased with incubation time due to microbial dissimilatory SO42- reduction. The shifts to microbial SO42- reduction temporally aligned with observed conversion of 201Hg(II) to Me201Hg at each of the elevated salinity. Although there were no statistical differences in total Me201Hg production between the five salinity treatments (porewaters plus peat), Me201Hg formed at elevated salinity was present at higher concentrations in the porewaters (as opposed to associated with the peat). This result is interpreted to be due to porewater MeHg stabilization by aqueous complexation of MeHg by ligands such as aromatic DOM and sulfide at higher salinity treatments, inhibiting binding of MeHg to the peat sediment substrate. Overall, the experiments demonstrate that salinity intrusion can result in pronounced MeHg production in porewaters. The produced MeHg is expected to be highly mobile for coastal export to coastal marine ecosystems due to via rising and falling tidal cycles which flush estuarine peat regularly, having implications for MeHg uptake in coastal aquatic food webs.