Mercury (Hg) is a leading contaminant across U.S. water bodies, warranting concern for wildlife species that depend upon food from aquatic systems. The risk of Hg toxicity to large herbivores is little understood, even though some large herbivores consume aquatic vascular plants (macrophytes) that may hyper-accumulate Hg. We investigated whether total Hg and methylmercury (MeHg) in aquatic forage may be of concern to moose (Alces alces) and beaver (Castor canadensis) by measuring total Hg and MeHg concentrations, calculating sediment-water bioconcentration factors for macrophyte species these herbivores consume, and estimating herbivore daily Hg consumption. Abiotic factors impacting macrophyte Hg were assessed, as was the difference in Hg concentrations of macrophytes from glacial lakes and those created or expanded by beaver damming. The amount of aquatic-derived Hg that moose move from aquatic to terrestrial systems was calculated, in order to investigate the potential for movement of Hg across ecosystem compartments by large herbivores. Results indicate that the Hg exposure of generalist herbivores may be affected by macrophyte community composition more so than by many abiotic factors in the aquatic environment. Mercury concentrations varied greatly between macrophyte species, with relatively high concentrations in Utricularia vulgaris (>80ngg-1 in some sites), and negligible concentrations in Nuphar variegata (~6ngg-1). Macrophyte total Hg concentration was correlated with water pH in predictable ways, but not with other variables generally associated with aquatic Hg concentrations, such as dissolved organic carbon. Moose estimated daily consumption of MeHg is equivalent to or below human reference levels, and far below wildlife reference levels. However, estimated beaver Hg consumption exceeds reference doses for humans, indicating the potential for sub-lethal nervous impairment. In regions of high moose density, moose may be ecologically important vectors that transfer Hg from aquatic to surrounding terrestrial systems.
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We thank Brandon Seitz, Mark Romanski, Paul Brown, and other colleagues at Grand Portage National Monument and Isle Royale National Park for their collegiality and assistance with field logistics; and Dave Krabbenhoft, John DeWild, Jacob Ogorek and colleagues at the USGS Wisconsin Water Science Center for their collegiality and support. We are grateful to Charles Kerfoot, Mae Gustin, and three anonymous reviewers for providing helpful comments on an earlier draft of this manuscript; to Luke Obermeyer and Ruth Bennett for their assistance with field data collection; to Luis Verissimo for field logistics support; and to Robert Bump for sharing watercraft. This research was funded in part by the NASA Michigan Space Grant Consortium , the National Science Foundation ( DGE 0841073 ), and the Michigan Tech University's Centers for Water and Society and Ecosystem Science .