Following mercury in a pristine park
Researchers track mercury from air to fish in a relatively pristine national park in northern Minnesota.
Evidence that links atmospheric mercury to methylmercury in fish is mounting in the scientific literature. In a paper published today on ES&T’s Research ASAP website (DOI: 10.1021/es060822h), researchers strongly identify atmospheric deposition as the source of mercury contaminating fish living in Voyageurs National Park. The study comes just weeks after another ES&T paper reported similar conclusions from the Experimental Lakes Area in northern Ontario, Canada.
Voyageurs National Park, in northern Minnesota, is a relatively pristine region with no point sources for mercury. Two of the park’s lakes, however, contain fish with some of the highest mercury concentrations in the state, and the federal government posted fish consumption advisories in the mid-1990s.
To determine the source of mercury in Voyageurs National Park and how it ends up in fish, a team of researchers led by Jim Wiener of the University of Wisconsin–La Crosse measured mercury in bedrock and soils, and they statistically modeled lake and wetland functions. To study the food chain, the team analyzed 1-year-old yellow perch, which are eaten by loons and larger fish. One of the perch’s predators, the northern pike, was also tested; those data will be published later.
The team found negligible geologic sources of mercury in 17 of the area’s lakes and in the stream or groundwater catchments that feed them; however, sediment cores record that airborne mercury accounted for two-thirds of total mercury accumulated after 1900. The researchers concluded that atmospheric deposition from anthropogenic activities dominates the contemporary source of mercury pollution.
Atmospheric mercury deposition appears to be the only factor in the ecosystem that doesn’t vary spatially, Wiener points out, referring to data from the Mercury Deposition Network. Between lakes, mercury levels in the perch varied 5-fold, whereas mercury levels in the pike varied 10-fold. Such variations, the team says, are caused by differences in wetland and lake behavior.
For example, total organic carbon served as an indicator of wetland output into the lakes. The team documented wetlands exporting methylmercury—the form of mercury that bioaccumulates in fish—along with the organic matter. That export rate differed among disparate watersheds but was highly correlated with the methylmercury concentrations in lakes and fish.
“My overall impression,” says Ed Swain of the Minnesota Pollution Control Agency, which provided financial support for the research, is that the new work “confirms in a single study a lot of our modern understanding of mercury.” Although many of the team’s conclusions have long been suspected and some have been published in bits and pieces (e.g., as results from METAALICUS), “no one had put it all together before,” Swain says.
“It’s a neat demonstration of the fact that a lot of our mercury problems in North America are in areas that are pretty pristine,” adds Drew Bodaly, project leader of the Penobscot River Mercury Study in Maine and a former researcher with Fisheries and Oceans Canada.
The results solidly point to atmospheric mercury as the source of mercury in fish, Bodaly says. In the past, variability in mercury levels in both lakes and fish across a region has been used as an argument against atmospheric sources for mercury. Swain suggests that the variation among watersheds shows that each aquatic system processes mercury from the atmosphere in a unique way but with direct relationships to changes in mercury levels. “The implication is that if you lowered the mercury inputs... in each lake, the contamination levels would go down,” he says.
Bodaly also points to the continued effects of acid precipitation, which influences pH and sulfate levels. In the team’s models, the lower-pH, higher-sulfate lakes tended to have fish with higher mercury levels, he notes. “As a factor, [acid rain] hasn’t gone away.”
