Assessing the Factors that Control Mercury Distribution in Aquatic Ecosystems of Northeastern North America
From 2001 to 2005, BRI and Environment Canada led a comprehensive effort to compile mercury data from across the northeastern U.S. and eastern Canada. This groundbreaking project produced a database of over 300,000 measurements, mostly from freshwater environments. The results highlight the broad extent and serious effects of mercury across the landscape, the need to expand the view of the problem to include forest ecosystems, the occurrence of biological hotspots in sensitive environments, and the demand for enhanced mercury monitoring.
This BRI effort produced a series of 21 scholarly papers published in aspecial issue of the journal Ecotoxicology. These papers present the most comprehensive understanding of mercury pollution in freshwater ecosystems in northeastern North America. A second phase of analysis produced several additional synthesis papers, including one in BioSciencein 2007 by Evers et al. (Biological mercury hotspots in the northeastern Unites States and southeastern Canada). These 23 papers were later condensed into the Mercury Connections report, which highlights and translates the key findings of these papers for policy makers and the public.
KEY RESULTS OF THE PROJECT
1. Mercury levels are high and pervasive in northeastern North America. The map below shows model estimates of total mercury deposited on the landscape, and predicts higher mercury loading to some areas of the Northeast than previously projected. While the map is limited by the lack of mercury monitors in urban areas and near large emissions sources, it shows elevated mercury across the region and particularly high levels in montane forests.
Extensive water and fish data further illustrate the widespread nature of the mercury problem. Water samples from more than 1,000 locations identified particularly high mercury in the Adirondack Mountains of New York as well as the Canadian provinces of Nova Scotia and Newfoundland. The waters with high mercury levels are generally distant from direct point sources and urbanized land use, suggesting airborne mercury is a likely source. However, the data also demonstrate that large sources can have a considerable impact in local areas.
An analysis of fish showed that 15% and 42% of the water bodies sampled for brook trout and yellow perch, respectively, had average fish mercury concentrations exceeding the U.S. Environmental Protection Agency (EPA) criterion of 0.3 ppm. Moreover, most species sampled had average nationwide mercury concentrations above this criterion.
2. Until now, most research has focused on mercury in fish and fish-eating birds in aquatic environments. New research shows that many animals, even forest songbirds, have elevated mercury burdens. Based on these findings, it is increasingly clear that mercury can no longer be viewed as strictly an aquatic pollutant. Conventional thinking holds that mercury is limited to aquatic environments, since mercury is more readily converted to its toxic form (methylmercury) in water. However, elevated mercury levels in Bicknell’s thrush and other forest songbirds demonstrate that methylmercury can be produced in terrestrial ecosystems as well. This finding has implications for the way scientists and policy makers view the nature and extent of mercury in northeastern North America.
3. Mercury is commonly evaluated as an environmental issue at national and global scales. Yet this approach can overlook small locales with regionally significant mercury pollution. As a part of this project, biological hotspots that pose an ecological risk were identified and mapped. This was the first such map created for North America.
Hotspots can form in watersheds with high mercury deposition or within highly sensitive ecosystems. In northeastern North America, areas of high mercury loading prevail in upper elevation ecosystems that receive more mercury deposition from surrounding lowlands, as well as areas near large mercury sources. Often, however, biological hotspots also develop in watersheds where conditions are conducive to methylmercury production or the build-up of mercury in the food chain. This finding illustrates that watershed characteristics can be as important as mercury loading in determining mercury sensitivity. Moreover, the high mercury levels documented in these biological hotspots suggest the need for stronger mercury standards to protect fish and wildlife.
4. It is clear from the analyses conducted for this project thatenvironmental monitoring programs must be expanded in order to fully document the extent and impact of mercury pollution in North America. The current federal monitoring program is limited to the Mercury Deposition Network (MDN). While the 70 existing MDN sites are operating well, they are located primarily in rural areas and are sparsely distributed. They are also limited to collecting mercury in rain and snow. Moreover, connecting air deposition with changes in fish and wildlife is a scientific challenge that must be addressed through an expanded monitoring network. Current programs for measuring water chemistry and fish and wildlife effects are inadequate to detect changing mercury levels and determine ecological effects in a standardized way. A comprehensive system designed to meet mercury monitoring needs nationally is described in a recent paper by Robert Mason and colleagues entitled, “Monitoring the Response to Changing Mercury Deposition,” which appeared in the January 2005 issue of the journal Environmental Science and Technology.
Given the changing levels of mercury in the environment, the increasing global pool of mercury and the risk posed to human and ecological health, a collection system for basic information on mercury in the environment should be a high national priority.
This project included the Canadian Maritime provinces, New York, and New England.
The mercury project in the northeast represented a binational effort that included 71 representatives from 29 state, provincial, federal, nonprofit, independent, and academic institutions. see list of project collaborators
David Evers 2001, 2002, 2004 work on mercury in aquatic ecosystems is now in summary format on the NSRC website: