Wetland Methylmercury declines rapidly following decreases in sulfate deposition

Although many studies have demonstrated the stimulatory effect of sulfate addition on mercury methylation in wetlands, none have directly measured the recovery of such systems once sulfate addition ceases. Sulfate loads were experimentally elevated through simulated rainfall to half of a 2.5-ha peatland in northern Minnesota between 2001 and 2008. In 2006 sulfate addition was halted to a portion of the experimental treatment, and methylmercury concentrations were monitored. Porewater methylmercury concentrations in this recovery treatment declined to control levels by summer-2008 and in the solid phase by spring-2009, but remained significantly elevated in the experimental treatment. Total mercury concentrations in predaceous diving-beetle larvae collected during spring- 2009 followed the same trends observed for methylmercury concentrations in wetland soils. Preliminary data suggest that sequestration of added sulfate in an increasingly recalcitrant organic pool contributed to declining methylmercury concentrations in the recovery treatment. Overall these results indicate that controls on atmospheric sulfate emissions and deposition could lead to rapid reductions in wetland methylmercury pools with possible consequences for mercury accumulation in wetland foodchains.