A half-mile of PVC pipe stretched across a spongy bog in remote northern Minnesota, pumping water to a sprinkler system set up to water half of a small peatland. For nearly ten years, the plumbing project raised and lowered the amount of sulfate falling on the bog, helping explain how precipitation chemistry and climate affect how much toxic mercury flows out of wetlands.
The study took creative engineering, careful analysis, and innumerable workdays in buggy swamps. It paid off with insights about how chemistry and drought cycles conspire to contaminate fish in northern lakes – and potentially the people who eat them.
Manipulating and measuring the experimental bog, the researchers teased out complicated connections between sulfate, mercury, water, and weather. Together, these variables affect the formation of methylmercury.
Methylmercury is a form of mercury that is strongly absorbed when eaten and travels easily up the food chain, concentrating in the top predators: the biggest fish, fish-eating wildlife, and people. It is formed by certain types of bacteria that “breathe” sulfate as part of their metabolism. These bacteria live in low-oxygen environments like the mud on the bottom of lakes and in water-saturated peat. Peatlands in particular provide optimal conditions for mercury methylation, so that lakes with large areas of peatland in their watershed often have high levels of mercury in their fish.
The Marcell Experimental Forest, a U.S. Forest Service research facility located in the Chippewa National Forest north of Grand Rapids, was the perfect place to play with peatlands. Three times each summer for nine years, the scientists sprinkled sulfate-rich water over half of the four-acre test site, increasing annual deposition about four times the local rate.
While the researchers, led by Dr. Dan Engstrom, director of the St. Croix Watershed Research Station, knew that there was a connection between sulfate and methylmercury, the project showed it is surprisingly strong. Each time the sprinklers ran and extra sulfate rained down on the peatland, methylmercury levels spiked upward. Nothing like that was observed in the half of the peatland not receiving the sulfate addition.
Sulfate and mercury both get into surface water from the air, often carried long distances from industrial emission sources, especially coal-fired power plants. If sulfate magnifies mercury methylation, as the study showed, it means that both pollutants have contributed to the contamination of Minnesota’s waters and wildlife. Mercury in rainfall has increased by three or four times in the industrial era, but the increase in our aquatic food chains could be far greater because of the multiplier effect of the sulfate.
The research team also made a discovery about the role of drought. They were “fortunate” that during the course of the study, the bog experienced one of its biggest dry spells in 58 years of record-keeping. And the resulting large swings in water levels produced spikes in the amount of methylmercury in the bog and the amount flowing out.
Through careful measurement, the researchers determined that as the top layers of the bog dried out, they slowly decomposed, releasing sulfate as well as previously-formed methylmercury. When the water rose again, re-wetting the peat, the sulfate-breathing, mercury-methylating microbes thrived, and more methylmercury was sent downstream.
Key results of this study were recently published in the Journal of Geophysical Research, a prominent earth-science journal, and also featured online in Earth and Space News by the American Geophysical Union.
Lead author of the paper, Jill Coleman Wasik – formerly of the St. Croix Research Station and now at the University of Wisconsin, River Falls – notes that as the world warms as predicted in the decades ahead, the volatility of northern Minnesota’s weather is expected to increase. That could be bad news for anyone who likes to eat the fish they catch.
“As climate change alters rainfall patterns and makes droughts more common, it could increase the amount of mercury and sulfate released from peatlands, which would have significant implications for water resources and downstream ecosystems,” Coleman Wasik says. “And many of the efforts society has undertaken to reduce sulfur and mercury emissions could be counteracted by increased climatic variability.”
The study was a highly collaborative effort involving scientists from the Minnesota Pollution Control Agency, the Metropolitan Council Environmental Services, the University of Toronto Scarborough, the University of Western Ontario, Gustavus Adolphus College, and the Northern Research Station of the U.S. Forest Service. Funding was provided by the U.S. Environmental Protection Agency, Science to Achieve Results Program grant R827630, the Great Lakes Commission, Great Lakes Air Deposition program, and the Minnesota Pollution Control Agency.
Drought Changes How Peat Bogs Cycle Mercury and Sulfur. Eos. Earth and Space Science News. https://eos.org/research-spotlights/drought-changes-how-peat-bogs-cycle-mercury-and-sulfur.
Coleman Wasik, J.K., D.R. Engstrom, C.P.J. Mitchell, E.B. Swain, B.A. Monson, S.J. Balogh, J.D. Jeremiason, B.A. Branfireun, R.K. Kolka, and J.E. Almendinger. 2015. The effects of hydrologic fluctuation and sulfate regeneration on mercury cycling in an experimental peatland. Journal of Geophysical Research: Biogeosciences 120: doi:10.1002/2015JG002993.