The impact of enhanced sulfate deposition and recovery on mercury dynamics and water quality in a peatland catchment

Abstract:

Atmospheric deposition of sulfate has been linked to enhanced mercury methylation in peatlands and more broadly decreased water quality in catchments. However, following legislative amendments in 1977 and 1990 in the U.S. and Canada, sulfate deposition has significantly declined, leading to, at times, differential changes in water quality. There remains little information on methylmercury (a known bioaccumulating neurotoxin) export from peatland-dominated watersheds as sulfate deposition declines. Here, we present results from a multi-year (1998-2011), ecosystem-scale control-impact experiment at the USDA Forest Service Marcell Experimental Forest in northern Minnesota, where annual sulfate deposition was experimentally increased by 4X and then returned to ambient levels in two phases. There was no impact on common acid rain indicators (e.g., pH, dissolved organic carbon, or sulfate) in outflowing surface waters due to the experimental manipulation. No changes of these indicators suggest that they were more strongly governed by overarching processes, such as inter-annual climate variability, rather than enhanced sulfate deposition. Methylmercury increased 5-fold during the sulfate additions but returned to pre-impacted levels within two years after additions ceased in the outflowing surface waters. The lack of change in the common acid rain indicators and rapid recovery of Methylmercury imply that once sulfate deposition ceases the long-term effects may be much shorter than previously assumed in peatland dominated catchments.