Methylmercury explodes after acid rains
The first ecosystem-scale experiment confirms that the sulfate in acid rain speeds up the production of the methylmercury that bioaccumulates in fish.
Researchers have long worked under the premise that acid rain triggers the production of methylmercury, the bioavailable and toxic form of mercury that can accumulate in fish living downstream of freshwater wetlands and make them unsafe for human consumption. Now, the first large-scale field experiment on the interactions of mercury and sulfate in wetlands, described in a paper published today on ES&T’s Research ASAP website (DOI: 10.1021/es0524144), provides the most persuasive evidence yet that acid rain increases production of methylmercury.
The “cause-and-effect relationship” between sulfur and mercury deposition from the atmosphere has been “demonstrated in the lab and in the field in small-scale experiments,” points out Charles Driscoll of Syracuse University. Because this is the first such large-scale ecosystem experiment, Driscoll and other scientists studying the methylation of mercury have been paying close attention to the trial as it has progressed. “It’s exciting work,” he says.
Jeff Jeremiason of Gustavus Adolphus College and colleagues took an acre-sized patch of a wetland in Minnesota, part of a larger parcel scientists have been studying since the 1950s, and sunk new sampling wells across their test site. They peppered half of the site with sprinklers that could simulate rainfall with sulfate loads four times as high as annual background levels. The sulfate levels used in the test are equivalent to historic levels of sulfate deposition from acid rain in the northeastern U.S., the authors note.
The team applied the first 6-hour sulfate “rainstorm” to the wetland in May 2002, followed by two more “rains” in July and September of that year. They measured the sulfate and methylmercury content of the wetland on subsequent days, with matching measurements in a control section in an upstream and untouched part of the same wetland.
The team found a jump in sulfate levels followed by a surge of methylmercury after the first application, just as the theory behind mercury’s methylation predicts. The sulfur apparently stimulates certain microbes that transform other forms of mercury to methylmercury as they respire, although the exact biological mechanism remains unknown. They could also track the concentrations of methylmercury exported out of the swampy wetland, which increased threefold.
Methylmercury surge
Then things got muddy. The researchers still measured methylmercury flowing out of the marsh after the two applications later in the summer, but the levels did not increase dramatically. They could no longer find evidence of sulfate in the marsh, either.
“We might have missed all the action,” says Jeremiason, by waiting a day after the extra sulfate was applied before beginning to take staggered measurements. He says his group is also split on other explanations for the conundrum, including a change in marsh chemistry and biology because of higher temperatures as the summer waxed. Another possibility is that the microbial community may have fluctuated as the seasons changed, affecting how much sulfur the bugs could use up and in turn potentially affecting the amount of methylmercury that they produced.
Mark Hines, a microbiologist at the University of Massachusetts Lowell, says that he, too, would have expected bursts of activity following later applications similar to the first event. “You’d think [the microbes] would be raring to go the second time around,” creating a surge of methylmercury. He suggests that because the team only measured marsh waters, they may have missed both sulfate and methylmercury that ended up bound to dead organic matter. In fact, the continued output of dissolved methylmercury that the researchers reported could be explained by the slow release of the bound pool, Hines conjectures.
Downstream effects?
The freshwater wetlands setting of this newest study complements observations elsewhere, such as the Everglades, where experiments also substantiate the link between sulfate and methylmercury. The new study makes the connection that much clearer, says Brian Branfireun, a hydrologist at the University of Toronto. The Everglades, for example, start out at higher sulfate concentrations; Branfireun says that these Minnesota peatlands lack sulfate to begin with, and that makes this experiment even more important. He notes, “adding sulfate alone increased the methylmercury in this system and increased the output into receiving waters” downstream. “That’s an important message. There aren’t any fish living in the peat... . The question is: Is that the [same] methylmercury that gets into the fish [downstream]?”
“Implications of this work are interesting from a policy perspective,” Driscoll says. For example, he explains, the research implies that acid-rain regulations, such as the 2005 Clean Air Interstate Rule, could control methylmercury and thus tempt policymakers to relax controls on mercury itself. Still, “mercury is an extremely dangerous substance,” Driscoll says, and any evidence that acid rain controls alone could reduce mercury concentrations in fish “will play out for next 15 to 20 years.”
