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ES&T News
Mercury in western U.S. fish
In a broad swath of landscape with few monitoring stations for mercury in air, researchers have collected a sizable database of information on mercury content in a variety of fish species. Their data, published in this issue of ES&T (pp 58–65), fill in a gap for fish mercury levels in the western half of the U.S. The researchers also use their analyses to suggest that atmospheric deposition might correspond directly to mercury levels in fish, which is a contentious claim.
The new data come from 2700-plus fish of varying species and sizes, whose tissues were measured for their mercury content. The U.S. EPA and Oregon State University’s fish and wildlife team, led by Spencer Peterson of EPA’s Western Ecology Division in Corvallis, Ore., randomly picked streams from digitized 1:100,000-scale maps. They then randomly selected stream and river sites, characterized them chemically and physically, and collected fish from the sites for tissue analysis. From those data, the team used selection probabilities to estimate the total length of a stream or river that would have fish with various mercury concentrations.
Measured using EPA protocols, the levels of mercury were highest in larger fish-eating fish (or piscivores); this result supports previously known information about bioaccumulation of the neurotoxin in fish. Even though Peterson emphasizes that no “clear and present danger” exists, he says, “we found no fish below the method detection limit used in our analyses” of 0.0024 micrograms per gram (µg/g).
Because the fish were all sampled consistently, the study allows comparisons to be made across varying geographic regions, says David Krabbenhoft, an investigator on the METAALICUS project (Mercury Experiment To Assess Atmospheric Loading in Canada and the United States) and a U.S. Geological Survey researcher. Krabbenhoft says he likes this study because it will fill “a general paucity of data in the western U.S. with regard to the East in terms of fish [mercury] levels.”
The data from this study match commonly observed trends, in which piscivores have mercury levels about three times higher than those of nonpiscivorous fish. Krabbenhoft also notes that the piscivores’ levels, “even though not higher than most action levels as a rule, [had] a mean value of 0.26 [µg/g], [which] is approaching EPA standards” for suggested human consumption advisories.
However, Krabbenhoft also says that “it’s a complex signal” and one that is “not necessarily true across all species.” The data could indicate that pH and population factors have an impact on mercury levels, he says. Other recently published studies, such as a smaller-scale investigation in Voyageurs National Park (Environ. Sci. Technol. 2006, 40, 6261–6268), have illustrated how basin-level characteristics can control mercury levels in fish. But these results do not necessarily conflict with the continent-wide view, Krabbenhoft and others say. “Overall, the patterns in this paper are reflective of what the West is all about: complex, changing characteristics from basin to basin,” he says.
Taking into account all the parameters detailed in their statistical analyses, Peterson’s team found no consistent correlations between the 26 measured environmental variables and fish mercury concentrations over the entire western U.S. Because of that absence and in light of previous research, they suggest that atmospheric deposition of mercury could be playing a key role in determining mercury levels in fish.
Chad Hammerschmidt of the Woods Hole Oceanographic Institution comments that Peterson and colleagues’ paper “makes a very important point: fish mercury did not correlate with all these factors everybody is saying are important,” such as sulfate loading and pH, “especially with their most robust data sets” of three larger fish-eating species. But the data are “not conclusive” and make a “weak case for atmospheric [contributions], because they don’t have mercury deposition data.”
Hammerschmidt and his colleague Bill Fitzgerald recently published a study of actual atmospheric mercury deposition measurements in the eastern U.S. that correlated to fish levels. This work, published in ES&T (2006, 40, 7764–7770), is also controversial, other researchers say.
These results from two very different parts of the U.S. are still too preliminary for any conclusions to be drawn, researchers say, although evidence may be building to implicate atmospheric mercury deposition as a key factor. With regard to regulatory efforts to protect fish, humans, and other animals from mercury, the question is still up in the air, for example, as to whether adequate protection could come from controlling only sulfur emissions from power plants: sulfur deposition enhances the microbial methylation of mercury, turning the metal into a more bioavailable form, no matter what its source.
