A Statewide Assessment of Mercury Dynamics in North Carolina Water Bodies and Fish

—Mercury contamination of aquatic systems has received much attention recently because of potential health effects on humans and wildlife. Although the factors affecting mercury deposition, its conversion to biologically active methylmercury, and its bioaccumulation in aquatic systems have been identified, equivocal results for particular species and systems have hampered policy making. Our study addresses this problem through a comprehensive, statewide synthesis of the available data on fish mercury contamination in North Carolina and the environmental factors associated with methylmercury formation and transport through aquatic food webs. Using data collected by the North Carolina Department of Environment and Natural Resources, the U.S. Environmental Protection Agency, and others, we examined the relationships between a suite of biotic and abiotic factors and tissue mercury concentrations in fish from North Carolina water bodies. Multivariate tests were conducted to create predictive models relating environmental variables to mercury levels in fish, and Akaike’s information criterion was used to examine the relative strengths of the candidate models. The best model in our analyses (R1⁄4 0.81) included species, fish trophic status, ecoregion, and pH. Other important drivers of mercury accumulation were land use patterns (the percentage of the subbasin that is agricultural) and site type (swamps versus lakes, rivers, and bays). Although previous investigations have indicated similar individual relationships, our study is unique in that we examined the relative importance of a large number of biotic and abiotic variables across a range of environments, ecosystems, and species. The results of these analyses should help policymakers in making risk assessment decisions and serve as a template for future investigations. Since the industrial revolution, mercury concentration in the atmosphere has increased substantially, largely from anthropogenic inputs (Schuster et al. 2002). Owing to the ease with which mercury is transported in the atmosphere and its long residence time, contamination in freshwater and marine ecosystems has become a global problem (Schuster et al. 2002; Trudel and Rasmussen 2006). Human exposure to mercury is almost entirely from the consumption of fish contaminated with methylmercury (Clarkson 2002). Methylmercury (MeHg) is the organic, highly toxic form of mercury that enters the food web following chemical transformation by bacteria and constitutes a threat to wildlife (Brant et al. 2002; Kenow et al. 2003) and ultimately humans (Holloway and Weech 2003). Nearly all (95–99%) of the mercury in fish tissue exists in the form of MeHg (Bloom 1992; Wiener and Spry 1996; NCDENR 2000). Though global in nature, this contaminant’s effects can be concentrated in particular regions; for example, fish in the southeastern portion of North Carolina contain MeHg levels that are among the highest documented in the United States (NCDENR 2000). The mechanisms associated with MeHg concentration in fish tissue, however, remain poorly understood, and for that reason human health officials are often forced to recommend fairly nonspecific advisories. North Carolina, for example, recently issued a statewide consumption advisory for largemouth bass Micropterus salmoides, one of the state’s most popular freshwater sport fish (Rundle et al. 2004), though mercury concentrations in this species vary widely among water bodies across the state. A more comprehensive understanding of the factors that drive mercury bioaccumulation in aquatic systems should permit better predictive measures for