Simulated Sea Level Rise in Coastal Peat Soils Stimulates Mercury Methylation

Coastal wetlands are vulnerable to sea level rise with unknown consequences for mercury (Hg) cycling, particularly the potential for exacerbating neurotoxic methylmercury (MeHg) production and bioaccumulation in food webs. Here, the effect of sea level rise on MeHg formation in the Florida Everglades was evaluated by incubating peat cores from a freshwater wetland for 0-20 days in the laboratory at five salinity conditions (0.16-6.0 parts-per-thousand; 0.20-454 mg L-1 sulfate (SO42-)) to simulate the onset of sea level rise within coastal margins. Isotopically enriched inorganic mercury (Hg-201(II)) was used to track MeHg formation and peat-porewater partitioning. In all five salinity treatments, porewaters became anoxic within 1 day and became progressively enriched in dissolved organic matter (DOM) of greater aromatic composition over the 20 days compared to ambient conditions. In the four highest salinity treatments, SO42- concentrations decreased and sulfide concentrations increased over time due to microbial dissimilatory SO42- reduction that was concurrent with Hg-201(II) methylation. Importantly, elevated salinity resulted in a greater proportion of produced (MeHg)-Hg-201 observed in porewaters as opposed to bound to peat, interpreted to be due to the complexation of MeHg with aromatic DOM released from peat. The findings highlight the potential for enhanced production and mobilization of MeHg in coastal wetlands of the Florida Everglades due to the onset of saltwater intrusion.