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Toxic Methylmercury and Atmospheric Deposition


Fish and other species are negatively impacted by mercury. Photo courtesy of Sharon Grant.

Mercury is found naturally in emissions from volcanoes and evaporation from soil and oceans. It also is produced by human activities such as coal-fired utilities, metals mining and municipal- and medical-waste incineration. Atmospheric mercury may be carried thousands of miles before entering lakes and streams through deposition.

What Are the Impacts of Mercury Pollution?

Mercury concentrations can accumulate and magnify through the food chain in fish, humans and other animals. Non-organic forms of mercury are converted to methylmercury by sulfur reducing bacteria (SRB) in aquatic sediments. Methylmercury is a potent neurotoxin, and has been shown to have detrimental health effects in human populations as well as behavioral and reproductive impacts to wildlife. Almost every state has consumption advisories for certain lakes and streams, warning of mercury-contaminated fish and shellfish. High concentrations of mercury have been measured in sediments and fish tissue, even in remote areas of the Arctic. Recently, elevated methylmercury loads have been detected in upland bird species, calling into question the traditional wisdom that methylmercury contamination is solely linked to aquatic systems. The link between SRB and biotic mercury concentrations has led researchers to establish that decreases in sulfur dioxide emissions with a resulting decrease in sulfate deposition will lessen mercury concentrations in wildlife. Consequently, as the level of sulfates is lowered in aquatic systems, SRB will reduce less sulfur leading, in turn to less inorganic mercury being methylated. For more information, on mercury deposition read the USFS briefing paper by Chuck Sams (2006).

Monitoring Mercury

Mercury deposition is monitored through the Mercury Deposition Network (MDN), an arm of the National Acid Deposition Program (NADP). Mercury accumulation can be measured directly in any part of the ecosystem although because methylmercury bioaccumulates in organisms levels of mercury in fish tissue can be orders of magnitude higher than mercury concentrations found in water and sediments. Mercury monitoring is accomplished in each region of the Forest Service as needed using methods appropriate for that area. In some cases the Forest Service has partnered with state and other federal agencies to determine the rate of mercury deposition and its effects on the ecosystem.

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