Mercury and Other Mining-Related Contaminants in Ospreys Along the Upper Clark Fork River,Montana, USA

We investigated links between mining-related contaminants in river sediment and their occurrence in nestling ospreys (Pandion haliaetus) in the Clark Fork River Basin, Montana, USA. Blood and feather samples from 111 osprey chicks were collected during 4 years from nests along river sections with greatly different sediment concentrations of arsenic (As), cadmium (Cd), copper (Cu), lead (Pb), zinc (Zn), and mercury (Hg). No significant differences between river sections were found among Zn (3,150 ± 160 μg L⁻¹) and Cd (<5 μg L⁻¹) concentrations in blood. Cu, Pb, and As concentrations in blood were significantly increased in chicks from the most contaminated river sections (mean values of 298, 8.9, and 100 μg L⁻¹, respectively). Cu, Zn, and Pb concentrations increased significantly during a year of above-average river runoff combined with high suspended sediment loads in rivers. Total Hg concentrations in blood and feathers were highly correlated and depended on the geographic locations of the nests. The lowest blood concentrations of Hg were observed in the most upstream river section (mean 151 μg L⁻¹) where total sediment concentrations were increased (0.80 mg kg⁻¹). River sections with intermediate blood concentrations (mean 206 and 303 μg L⁻¹) were associated with low to intermediate sediment concentrations (0.058 and 0.46 mg kg⁻¹). The highest concentrations of Hg in ospreys (mean 548 μg L⁻¹) were observed downstream from a contaminated tributary (1–4 mg kg⁻¹ in sediment). In river sections with lower Hg concentrations in sediment, there was a negative correlation between blood Hg concentration and chick mass, presumably due to high deposition rates into growing feathers. This relationship was absent in sections of high Hg exposure. Osprey blood and feathers are suitable for monitoring Hg in aquatic ecosystems; however, responses of As, Cd, Cu, Pb, and Zn are more subtle.