Mercury/Toxics Effects
Persistent, bioaccumulating toxic chemicals (PBTs) can be transporting air currents, from sources as far away as Europe and Asia, and as close as local agricultural fields. These compounds last a long time, in the environment, are human-made, toxic in small amounts, and can travel long distances via air currents. They are often carried towards polar or high elevation environments where, in cold conditions, they condense and are deposited. This is believed to account for the surprisingly high concentrations present in arctic environments, and in the indigenous peoples who live there.
The National Park Service (NPS) is concerned about airborne contaminants because they can pose serious health threats to wildlife and humans, as some of these compounds tend to "biomagnify" in the food chain. Biological effects of airborne contaminants can include impacts on reproductive success, growth, behavior, disease, and survival of fish, birds, wildlife, and humans, if these compounds accumulate to toxic levels. Toxic compounds that have been documented in National Parks include industrial byproducts such as PCBs; agricultural herbicides and insecticides such as DDT, Dieldrin, Chlordanes and endosulfans; and mercury.
Mercury deposition and effects are being monitored in many national parks. Mercury in aquatic systems is of particular concern because, mercury can be readily transformed into methylmercury, which can reach toxic levels in organisms at the top of the food web, like fish and birds that eat fish or aquatic insects. Fish consumption is the most important pathway for human and wildlife exposure to methylmercury. Toxic effects of methylmercury can include reduced reproductive success, impaired growth and development, behavioral abnormalities, reduced immune response and decreased survival.
Anthropogenic sources of mercury in the atmosphere include the burning of sulfur-containing coal in coal-fired power plants, utility and industrial boilers, smelting, chlor-alkali plants, and gold extraction, as well as from uses of fungicides containing Hg in latex paints and the paper and pulp industry. The NPS monitors mercury in wet deposition at several parks across the country through the National Atmospheric Deposition Program (NADP)/Mercury Deposition Network (MDN), which includes 95 sites nationwide. However, the atmospheric deposition of mercury is not directly related to methylmercury in ecosystems because conversion to methylmercury is controlled by microorganisms that thrive under varying conditions.
States currently monitor mercury in fish tissue in certain surface waters in order to develop public fish consumption advisories. In the U.S. in 2004, 35% of the Nation's lake acres (excepting the Great Lakes) and 24% of the Nation's river miles were under a fish advisory. While mercury is the leading cause of fish advisories, other contaminants such as pesticides may also invoke additional restrictions. The National Park Service has compiled data on impact thresholds of mercury and PBT compounds published for fish, birds and wildlife. A summary of the effects thresholds for concentrations in fish tissue that impact fish, birds and wildlife is in Selected PBT Toxicity Thresholds for Fish Consuming Birds and Wildlife. A compliation of published thresholds for mercury is in Selected Wildlife Toxicity Thresholds for Mercury.