Long-Term Experimental Manipulation of Atmospheric Sulfate Deposition to a Peatland: Response of Methylmercury and Related Solute Export in Streamwater.

Changes in sulfate (SO 4 2- ) deposition have been linked to changes in mercury (Hg) methylation in peatlands and water quality in freshwater catchments. There is little empirical evidence, however, of how quickly methyl-Hg (MeHg, a bioaccumulative neurotoxin) export from catchments might change with declining SO 4 2- deposition. Here, we present responses in total Hg (THg), MeHg, total organic carbon, pH, and SO 4 2- export from a peatland-dominated catchment as a function of changing SO 4 2- deposition in a long-term (1998-2011), whole-ecosystem, control-impact experiment. Annual SO 4 2- deposition to half of a 2-ha peatland was experimentally increased 6-fold over natural levels and then returned to ambient levels in two phases. Sulfate additions led to a 5-fold increase in monthly flow-weighted MeHg concentrations and yields relative to a reference catchment. Once SO 4 2- additions ceased, MeHg concentrations in the outflow streamwater returned to pre-SO 4 2- addition levels within 2 years. The decline in streamwater MeHg was proportional to the change in the peatland area no longer receiving experimental SO 4 2- inputs. Importantly, net demethylation and increased sorption to peat hastened the return of MeHg to baseline levels beyond purely hydrological flushing. Overall, we present clear empirical evidence of rapid and proportionate declines in MeHg export from a peatland-dominated catchment when SO 4 2- deposition declines.