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Project Title: Enhanced Rates of Mercury Methylation from Sulfate Deposition:
A Whole Wetland Experiment
Chemicals Studied: Total and methyl mercury was monitored
in the experimental wetland. In addition, a suite of general water chemistry
monitored, including cations, anions, dissolved organics, nitrogen and sulfate.
Geographic Areas: The experiment took place at the Marcell
Experimental Forest in Northern Minnesota. The results are applicable
to similar ecosystems,
which are common across the Great Lakes basin.
Project Start Date: April 2004
Project Status: Completed
Synopsis: This project involved 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. This study was conducted in 2005 and 2006 and focusedin particular on the possible chronic effects and recovery of wetlands previously impacted by elevated sulfate deposition.
In both 2005 and 2006 (and 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 2005 summer (July) application, because dry conditions prevented sulfate from reaching the water-table. The methylation response to the 2005 fall (October) addition 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 as compared to 2005. The actual fall-2005 sulfate addition raised MeHg concentrations in the treated section only modestly, and it may be that SRB activity and methylation were limited at this time, not by sulfate, but by carbon quality/quantity or inorganic mercury bioavailability. Severe drought returned to the MEF in 2006, drying the peat surface to such an extent that pore-waters could not be collected from most sample sites after late June. Throughout the experiment, the response of SRB Hg-methylation to sulfate addition was more pronounced in the central raised bog than in the marginal lagg, presumably because the bog is more sulfate-limited than the lagg; the bog is fed solely by atmospheric deposition whereas the lagg receives periodic inputs of sulfate from upland runoff.
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 addition. However, the recovery section remained elevated in sulfate and Hg relative to the control for this first year of the new treatment, indicating a persistent influence of the added sulfate. Chronic effects of sulfate addition are evident in the divergence in MeHg concentrations (and %MeHg) between experimental and control treatments over the five years of the study. However, absolute MeHg concentrations vary from year to year in both treatments based on antecedent hydrological conditions and do not show a directional increase. 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.
Reports and Presentations:
at 2006 Mercury Conference in Madison, WI (PDF, 3MB)
Daniel R. Engstrom, Ph.D.
St. Croix Watershed Research Station
Science Museum of Minnesota
Marine on St. Croix, MN 55047