While some people are focusing attention on fish advisories and loons, other scientists have been looking at the mountaintops of New England, following the Bicknell’s thrush and the yellow-rumped warbler, testing leaf litter and insects for the presence of mercury. And, they’re finding it.
A study recently released in Ecotoxicology(1) shows that Bicknell’s thrushes and other perching songbirds have elevated methylmercury levels in their blood and feathers. It also shows the levels of methylmercury in these birds correlate with the amount of acid deposition in their environment.
Knowing that mercury is released into the atmosphere by the same coal-fired power plants that release acid rain pollutants, we shouldn’t be surprised that where there’s acid rain, there’s mercury. The EPA estimates that coal-fired power plants release about: 40% of U.S. mercury emissions(2), 67% of US sulfur dioxide emissions, and 25% of US nitrogen oxide emissions.(3)
Mercury is transported the same way acid rain pollutants are—by the atmosphere. Despite mercury and acid deposition’s shared origination sites and transportation method, the unique and problematic relationship between the two is often not mentioned.
Studies have shown that acid rain makes mercury more bioavailable, meaning it is easier for organisms to bioaccumulate the toxin. As acidity increases in a body of water, the levels of methylmercury in fish and fish-eating wildlife have also been shown to increase.(4) This pattern of higher acid deposition correlating with higher methylmercury levels in wildlife is also proving true on mountains.
Significantly higher blood and feather mercury levels were found in Bicknell’s thrushes on Stratton Mountain (southwestern Vermont) versus Mount Mansfield (north-central Vermont), a pattern that mirrors acid deposition in Vermont. While the process of mercury availability is less understood for montane environments, the Bicknell’s thrush, a terrestrial bird restricted to coniferous forests, often above 900m, offers proof that mercury isn’t just an aquatic issue anymore.
Mercury is working its way up the terrestrial food chain. Found in tree leaves and needles, dragonflies and caterpillars, litterfall and now, insectivorous birds, it’s evident that mercury is available for uptake by plants and organisms, even in the absence of standing water. Yellow-rumped warblers, blackpoll warblers, white-throated sparrows and Bicknell’s thrushes were studied on Mount Mansfield. All four species of birds showed the same ratio of methylmercury to total mercury blood levels as fish-eating birds despite their insect based diets.
Whether considering the impacts of acid rain on wetlands or mountains, forests or lakes, there’s evidence that mercury is also there—often methylating, bioaccumulating, and potentially threatening many species. The common loon, the northern crayfish, the two-lined salamander, and the Bicknell’s thrush are all subject to exposure. It doesn’t matter how one looks at it—fish to loon, leaf to caterpillar, black fly to yellow-rumped warbler, or mayfly to trout—the mercury is moving through the system and is made possible with the help of acid deposition.
Footnotes:
(1) Rimmer, Christopher C., Kent P. McFarland, David C. Evers, Eric K. Miller, Yves Aubry, Daniel Busby, and Robert J. Taylor. “Mercury Concentrations in Bicknell’s Thrush and Other Insectivorous Passerine in Montane Forests of Northeastern North America.” Ecotoxicology 14 (2005): 223-240. On-line. Internet. 10 March 2005. Available http://briloon.org/mercury/FINAL%20PAPERS/5386270.pdf.
(2)United States. Environmental Protection Agency. “Mercury: Basic Information.” On-line. Internet. 3 April 2005. Available http://www.epa.gov/mercury/about.htm.
(3) United States. Environmental Protection Agency. “What is Acid Rain and What Causes It?” On-line. Internet. 3 April 2005. Available http://www.epa.gov/airmarkets/acidrain/index.html#what.
(4)United States. U.S. Department of the Interior—U.S. Geological Survey. Mercury Contamination of Aquatic Ecosystems: Fact Sheet FS-216-95. On-line. Internet. 3 April 2005. Available http://water.usgs.gov/wid/FS_216-95/FS_216-95.pdf.
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