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Project Title: Changes in Mercury Methylation in a Boreal Wetland Previously Enriched in Sulfate: Synergistic Effects of Atmospheric Deposition and Water-level Fluctuations
Chemicals Studied: Total and Methyl Mercury
Geographic Areas: The Marcell Experimental Forest in Northern Minnesota and Similar Ecosystems
Project Start Date: April 2007
Project Status: Completed
Synopsis: This project builds on a long-term ecosystem-scale experiment in which sulfate deposition to a boreal wetland was increased in order to examine the synergistic effects of atmospheric sulfate and mercury on methylmercury production. This previous work, funded by the EPA STAR and GLAD programs, established a whole-wetland irrigation system for amending sulfate deposition through simulated rainfall. Results from the first four years of the experiment showed a consistent increase in mercury concentration and export from the wetland following sulfate addition, although the response differed markedly depending on the season of application. The focus of the project was on possible chronic effects and recovery (i.e., reduction in mercury methylation) of this sulfur-enriched wetland with the cessation of sulfate addition and to address the effect of water-table fluctuations on mercury methylation and the recovery of wetlands previously impacted by elevated sulfate deposition.
Pore-water mercury and sulfate results for 2007 and 2008 show very clear trends that reflect both recovery of the S6 peatland from chronic sulfate enrichment and the added effect of water-table drawdown and peat oxidation during several years of recurring mid-summer droughts at Marcell Experimental Forest (MEF). Extended periods of pre-addition sampling in spring and fall 2007 provide strong evidence for sulfate regeneration by oxidation of organic-sulfur stores in the peat during periods of water-table drawdown and subsequent release to pore-waters during rebound. The released sulfate stimulates mercury methylation, raising pore-water concentrations in a manner similar to that following experimental sulfate addition. This natural methylation response was observed in control and recovery treatments as well as in the experimental section. Experimental sulfate additions following prolonged water-table fluctuations, especially those in fall, produced only a muted response in MeHg levels, presumably because sulfate was not limiting microbial methylation at these times. The role of hydrologic fluctuations in stimulating sulfate regeneration and mercury methylation was confirmed by water-table manipulations in experimental mesocosms.
Results from the 2007 and 2008 field seasons indicate a relatively rapid re-establishment of baseline sulfate and MeHg concentrations in peat pore-waters following the cessation of experimental sulfate addition. Although seasonal hydrologic fluctuations induced sulfate regeneration and mercury methylation, the intensity of these events lessened dramatically over time. In 2007 MeHg and sulfate concentrations in the recovery section tended to be intermediate between control and experimental concentrations, but by fall of 2008 there were few, if any significant differences between control and recovery concentrations despite a water-table rise that elevated experimental MeHg and sulfate. It thus appears that new sulfate inputs are more available to drive methylation events than pre-existing stores and that sulfur reactivity diminishes over time. These results suggest that reductions in atmospheric sulfate loading should be met with a relatively rapid decline in MeHg production in boreal wetlands.
Preliminary project results have been presented during the 2010-11 GLAD webinar series.
Daniel R. Engstrom, Ph.D.
St. Croix Watershed Research Station
16910 152nd St. North
Marine on St. Croix, MN 55047
Phone: (651) 433-5953