Spatial, temporal, and habitat-based patterns of mercury exposure in bald eagles in interior Maine

DeSorbo, Christopher R.
Jon Atwood, PhD
Department of Environmental Studies
Heightened concern about the combined effects of increased mercury (hg) deposition and habitat sensitivity to methylmercury production in the northeastern U.S. has instigated numerous studies investigating Hg exposure and impacts on wildlife. In this study, I investigated spatial- and habitat-based patterns of both short-term and chronic Hg exposure in bald eagles (Haliaeetus leucocephalus) in interior Maine.Eagle nesting territories throughout interior Maine were visited to collect eaglet blood, non-viable eggs, and shed adult feathers over the 2004-2006 period. Mercury levels in eaglet blood ranged from 0.08 to 1.51 ug/g (ww)(n=200 eaglets from 148 territories); shed adult feathers ranged 7.5 to 93.0 ug/g (fw)(103 territories), and egg Hg levels ranged 0.09-0.90 ug/g (ww)(n=16 territories). Tissue analyses indicated significantly higher Hg levels in eagles at lakes versus rivers (p<0.05). Mercury differences between habitats was most pronounced in short-term indices from blood at statewide and watershed scales and was suggested in chronic indices from feathers (p<0.10). This study provides baselines for Hg exposure in interior portions of Main's major watersheds. Eaglet blood Hg levels varied significantly among major watersheds (p<0.05). Habitat-specific watershed comparisons indicated highest Hg exposure in lake-based eaglets in the Saint Croix watershed and lowest exposure in river-based eaglets in the Kennebec watershed. The effects of habitat and watershed could not be isolated in analyses, and findings demonstrate both variables should be considered in evaluations of spatial Hg patterns and Hg risk assessments. Measure of hg exposure within habitat subclasses did not suggest higher Hg levels in eagles at impounded water bodies (reservoirs, enhanced lake impoundments, run-of-river impoundments) versus unimpounded sites (natural lakes, rivers), or within/among habitat subclasses. Confounding factors (e.g., abundance of wetland habitat, water chemistry parameters, food chain length) need to be integrated into analyses to accurately detect potential differences among habitat subclasses. Eaglet blood Hg levels were elevated in natural lakes, many in remote locations, suggesting strong interactive influence of both atmospheric deposition and Hg sensitivity in some regions. Measures of Hg in eagle tissues sampled in this study demonstrate Hg is prevalent in the freshwater food web used by eagles, and both eaglets and adults are bioaccumulating Hg to levels that outpace natural depuration and demethylation mechanisms. Mercury levels in eaglet blood, eggs, and adult feathers, particularly in lakes, resemble those found at sites in North American with significant Hg point source pollution problems. Eagles in Maine contain Hg levels exceeding levels associated with reproductive, physiological, and behavioral impairment in other piscivorous birds. Further investigations on the potential negative impacts of Hg on Maine's interior eagle populations are warranted.

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