Chronic Enhancement of Mercury Methylation in a Boreal Wetland from Elevated Sulfate Deposition and Climate Change

Edward Swain
Minnesota Pollution Control Agency,
St. Paul, MN USA

We present here the results of 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 (Hg) on production and export of the bioaccumulative neurotoxin, methylmercury (MeHg). A whole-wetland irrigation system for amending sulfate deposition through simulated rainfall was established in the fall of 2001 at the USFS Marcell Experimental Forest (MEF) in northern Minnesota. The 2-ha wetland was divided into control and experimental halves, and the experimental half was treated with a sulfate solution meant to increase annual sulfate loading by four times the current rate of atmospheric deposition. We focus here on the study-years 2005 - 2007 and address in particular possible chronic effects and recovery of wetlands previously impacted by elevated sulfate deposition.

Similar to previous years, peat porewater MeHg concentrations and %MeHg in the experimental treatment spiked three days after the spring (May) sulfate application, while control levels remained constant. The increase in MeHg coincided with declining sulfate concentrations, indicating Hg methylation by sulfate-reducing bacteria (SRB) as the likely process. A similar spike was not observed following the summer (July) applications, because dry conditions prevented sulfate from reaching the water-table. The methylation response to fall (October) additions in 2005 and 2007 was complicated by a prolonged summer drought and late-season water-table rise which regenerated sulfate from within the peat, stimulating SRB activity and Hg methylation just prior to the addition date. This response was evident in both the experimental and control sections, but was much more pronounced in the former. Drought-induced sulfate regeneration was also evident in the higher sulfate and MeHg concentrations prior to the spring addition in 2006 and 2007, as compared to 2005.

Beginning with the 2006 field season a recovery treatment, in which sulfate addition was suspended to a portion of the experimental section of the wetland, was incorporated into the project design to simulate the effects of reduced sulfate deposition. Sulfate and MeHg concentrations and %MeHg declined in the recovery treatment over the spring period, while the experimental sites continued to respond to sulfate loading following the spring additions. However, the recovery section remained elevated in sulfate and Hg relative to the control, indicating a persistent influence of the added sulfate. Chronic effects of sulfate addition are also evident in the divergence in MeHg concentrations (and %MeHg) between experimental and control treatments over the six years of the study. Results of this long-term experiment imply that boreal wetlands affected for decades by high sulfate deposition will continue to experience lingering, elevated mercury methylation, especially when the effects of climate remobilize previously sequestered sulfate.