Influence of Fluctuating Water Levels on Mercury Concentrations in Adult Walleye

Abstract

Atmospheric deposition of mercury (Hg) has been shown to be a significant source of Hg on the landscape (Rada et al. 1989; Swain et al. 1992) and is believed to contribute to increased Hg concentrations in aquatic food webs (Sorensen et al. 1994; Edwards et al. 1999). Methylmercury (MeHg) is the organic, bioavailable form of Hg that accumulates to toxic levels in top-level predators in aquatic systems (Suedel et al. 1994). Although limnological conditions in lakes and rivers can affect Hg methylation and concentrations in fishes, these relationships often vary among water bodies and fish species (McMurtry et al. 1989; Bodaly et al. 1993). Thus, regional studies are needed to identify mechanisms of local MeHg production and factors associated with Hg contamination in fishes. The Prairie Pothole Region of North America has a unique variety of natural wetlands and glacial lakes that are important for fish, shorebirds, waterfowl, and humans. Cyclical climate, characterized by extended wet–dry periods, cause high variation in water surface area within the region (Rosenberry 2003). For example, consecutive years of high precipitation during the mid-1990s caused dramatic surface area increases in many glacial lakes and wetlands of eastern South Dakota. After lake levels increased, several fish populations were found to contain elevated Hg concentrations (>1 lg/g), prompting local officials to post fish consumption advisories (South Dakota Game, Fish and Parks, 2006). The discovery of elevated Hg concentrations in fishes was surprising because (1) there were no apparent pointsource inputs of Hg, and (2) the limnological conditions of most of these lakes (eutrophic, high pH) generally do not favor Hg methylation or bioaccumulation (Grieb et al. 1990; Pickhardt et al. 2002). Moreover, lakes that experienced large increases in surface area generally contained fast growing fish populations, a situation that usually lowers Hg concentration due to growth dilution (Rodgers and Qadri 1982; MacCrimmon et al. 1983). In this study, we document changes in lake surface area for glacial lakes in the Prairie Pothole Region and relate this to Hg concentration in adult walleye (Sander vitreus). Although a similar phenomenon is known to occur in reservoirs with fluctuating water levels (Jackson 1988; Snodgrass et al. 2000; Sorensen et al. 2005), widespread effects of surface area changes on Hg concentrations in fishes have not been documented in natural, glacial lakes. We hypothesized that increases in lake surface area enhanced T. M. Selch (&) S. R. Chipps U.S. Geological Survey South Dakota Cooperative Fish and Wildlife Research Unit, Department of Wildlife and Fisheries Sciences, South Dakota State University, Box 2140B, Brookings, SD 57007-1696, USA e-mail: Trevor.Selch@sdstate.edu