The Effect of Atmospheric Sulfate Deposition on Mercury Biogeochemistry in an Experimental Peatland: Impacts, Recovery, and Natural Variability

University of Minnesota Ph.D. dissertation. July 2014. Major: Water Resources Science. Advisor: Daniel Engstrom. 1 computer file (PDF); xv, 140 pages.Elevated mercury deposition resulting from human activities has caused wide-spread mercury contamination of aquatic systems around the world. Peatlands are generally considered to be sinks for mercury deposited to the landscape, but also act as biogeochemical reactors wherein inorganic mercury is transformed into bioaccumulative, organic methylmercury (MeHg). Recent, short-term investigations have demonstrated that sulfate deposition alone can increase MeHg production in, and flux from, peatlands through the stimulation of sulfate-reducing bacteria, a group of known mercury methylators. However, over longer periods of time the interaction between the biogeochemical cycles of mercury and sulfur is complicated by variability in climate, hydrology, and sulfur and mercury deposition rates. These complexities were addressed by experimentally altering sulfate-loading to a 2.5-ha peatland in northern Minnesota over eight years. The peatland was initially divided into control and experimental treatments and sulfate was added to the latter three times each field season in simulated rainfall events. Porewaters were sampled before and after each sulfate addition and peat samples were collected five times from sites located within the raised central bog and along the peatland margins. The lagg margin is generally considered to be the primary site of mercury methylation in peatlands. However, sulfate addition caused more pronounced and persistent increases in MeHg in the central bog sites, relative to the margin sites, demonstrating that sulfate delivery to the central bog can greatly expand the areal extent of mercury methylation in peatlands. MeHg production also responded to sulfate release following severe summer drought. The increase was much higher in experimental-treatment sites than in control sites suggesting that the experimental treatment was "primed" to quickly respond to new sulfate inputs. In early 2006 sulfate addition was halted to the upgradient one-third of the original experimental treatment in order to monitor how MeHg production changed as sulfate deposition declined. Although drought appeared to slow the recovery process by increasing sulfate availability and mobilizing MeHg, three years after sulfate additions ceased MeHg in the recovery treatment was significantly lower than in the experimental treatment. This indicates that MeHg production in peatlands formerly affected by elevated sulfate deposition may return to background conditions and highlights the potential benefits that further controls on atmospheric sulfur emissions may have on MeHg production in peatlands and consequent mercury burdens in aquatic foodwebs. The long-term nature of this study allowed for an in-depth exploration of the effects that hydrologic flucutations on mercury cycling in peatlands and calls attention to the potential negative consequences that changing precipitation patterns and evapotranspirative demands may have on MeHg production in these systems.Coleman Wasik, Jill. (2014). The Effect of Atmospheric Sulfate Deposition on Mercury Biogeochemistry in an Experimental Peatland: Impacts, Recovery, and Natural Variability. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/174845

Methylmercury production in a chronically sulfate-impacted sub-boreal wetland

This is an Open Access version of the article that was published on May 16, 2016.Increased deposition of atmospheric sulfate exacerbates methylmercury (MeHg) production in freshwater wetlands by stimulating methylating bacteria, but it is unclear how methylation in sub-boreal wetlands is impacted by chronically elevated sulfate inputs, such as through mine discharges. The purpose of our study is to determine how sulfate discharges to wetlands from iron mining activities impact MeHg production. In this study, we compare spatial and temporal patterns in MeHg and associated geochemistry in two wetlands receiving contrasting loads of sulfate. Two orders of magnitude less sulfate in the un-impacted wetland create significant differences in acid-volatile sulfide and porewater sulfide; however, dissolved and solid-phase MeHg concentrations and methylation rate potentials (Kmeth) are statistically similar in both wetlands. Permitted mine pumping events flood the sulfate-impacted wetland with very high sulfate waters during the fall. In contrast to observations in sulfate-limited systems, this large input of sulfate to a chronically sulfate-impacted system led to significantly lower potential relative methylation rates, suggesting a predominance of demethylation processes over methylation processes during the sulfate loading. Overall, short-term measurements of methylation and demethylation potential are unrelated to gross measures of long-term MeHg accumulation, indicating a decoupling of short- and long-term process measurements and an overall disequilibrium in the systems. High sulfide accumulation, above ∼600-800 μg l(-1) sulfide, in the sulfate-impacted system lowers long-term MeHg accumulation, perhaps as a result of less bioavailable Hg-S complexes. Although continued research is required to determine how sulfate-limited freshwater wetlands might respond to new, large inputs of high-sulfate runoff from mining operations, chronically impacted wetlands do not appear to continually accumulate or produce MeHg at rates different from wetlands unimpacted by mining.Funding for this work was provided by the Minnesota Department of Natural Resources

The effects of hydrologic fluctuation and sulfate regeneration on mercury cycling in an experimental peatland

The AGU allows final articles to be placed in an institutional repository 6 months after publication. For an html copy of the article or additional information please refer publisher's website http://dx.doi.org/10.1002/2015JG002993A series of severe droughts during the course of a long-term, atmospheric sulfate-deposition experiment in a boreal peatland in northern Minnesota created a unique opportunity to study how methylmercury (MeHg) production responds to drying and rewetting events in peatlands under variable levels of sulfate loading. Peat oxidation during extended dry periods mobilized sulfate, MeHg, and total mercury (HgT) to peatland pore waters during rewetting events. Pore water sulfate concentrations were inversely related to antecedent moisture conditions and proportional to past and current levels of atmospheric sulfate deposition. Severe drying events caused oxidative release of MeHg to pore waters and resulted in increased net MeHg production likely because available sulfate stimulated the activity of sulfate-reducing bacteria, an important group of Hg-methylating bacteria in peatlands. Rewetting events led to increased MeHg concentrations across the peatland, but concentrations were highest in peat receiving elevated atmospheric sulfate deposition. Dissolved HgT concentrations also increased in peatland pore waters following drought but were not affected by sulfate loading and did not appear to be directly controlled by dissolved organic carbon mobilization to peatland pore waters. Peatlands are often considered to be sinks for sulfate and HgT in the landscape and sources of MeHg. Hydrologic fluctuations not only serve to release previously sequestered sulfate and HgT from peatlands but may also increase the strength of peatlands as sources of MeHg to downstream aquatic systems, particularly in regions that have experienced elevated levels of atmospheric sulfate deposition.Funding for this long-term project came from the U.S. EPA–Science to Achieve Results Program grant R827630, the Great Lakes Commission, Great Lakes Air Deposition program, and the Minnesota Pollution Control Agency. The USDA Forest Service's Northern Research Station provided access to the study site as well as substantial in-kind support

Long-Term Experimental Manipulation of Atmospheric Sulfate Deposition to a Peatland: Response of Methylmercury and Related Solute Export in Streamwater

Changes in sulfate (SO42–) deposition have been linked to changes in mercury (Hg) methylation in peatlands and water quality in freshwater catchments. There is little empirical evidence, however, of how quickly methyl-Hg (MeHg, a bioaccumulative neurotoxin) export from catchments might change with declining SO42– deposition. Here, we present responses in total Hg (THg), MeHg, total organic carbon, pH, and SO42– export from a peatland-dominated catchment as a function of changing SO42– deposition in a long-term (1998–2011), whole-ecosystem, control-impact experiment. Annual SO42– deposition to half of a 2-ha peatland was experimentally increased 6-fold over natural levels and then returned to ambient levels in two phases. Sulfate additions led to a 5-fold increase in monthly flow-weighted MeHg concentrations and yields relative to a reference catchment. Once SO42– additions ceased, MeHg concentrations in the outflow streamwater returned to pre-SO42– addition levels within 2 years. The decline in streamwater MeHg was proportional to the change in the peatland area no longer receiving experimental SO42– inputs. Importantly, net demethylation and increased sorption to peat hastened the return of MeHg to baseline levels beyond purely hydrological flushing. Overall, we present clear empirical evidence of rapid and proportionate declines in MeHg export from a peatland-dominated catchment when SO42– deposition declines

Long-Term Experimental Manipulation of Atmospheric Sulfate Deposition to a Peatland: Response of Methylmercury and Related Solute Export in Streamwater

Changes in sulfate (SO42–) deposition have been linked to changes in mercury (Hg) methylation in peatlands and water quality in freshwater catchments. There is little empirical evidence, however, of how quickly methyl-Hg (MeHg, a bioaccumulative neurotoxin) export from catchments might change with declining SO42– deposition. Here, we present responses in total Hg (THg), MeHg, total organic carbon, pH, and SO42– export from a peatland-dominated catchment as a function of changing SO42– deposition in a long-term (1998–2011), whole-ecosystem, control-impact experiment. Annual SO42– deposition to half of a 2-ha peatland was experimentally increased 6-fold over natural levels and then returned to ambient levels in two phases. Sulfate additions led to a 5-fold increase in monthly flow-weighted MeHg concentrations and yields relative to a reference catchment. Once SO42– additions ceased, MeHg concentrations in the outflow streamwater returned to pre-SO42– addition levels within 2 years. The decline in streamwater MeHg was proportional to the change in the peatland area no longer receiving experimental SO42– inputs. Importantly, net demethylation and increased sorption to peat hastened the return of MeHg to baseline levels beyond purely hydrological flushing. Overall, we present clear empirical evidence of rapid and proportionate declines in MeHg export from a peatland-dominated catchment when SO42– deposition declines