Spatial patterns of mercury contamination and associated risk to piscivorous wading birds of the south central United States.

Piscivorous birds are top predators in aquatic ecosystems and are vulnerable to mercury (Hg) exposure and associated adverse health effects. In some areas of North America, the health risk posed to piscivorous birds by Hg contamination has not been characterized because concentrations of Hg in bird tissues have not been extensively monitored. When data on Hg in tissues of piscivorous birds are not available, the concentration of Hg in the blood of piscivorous birds can be estimated from the concentration of Hg in prey fish. We used concentrations of Hg in different lengths of a proxy prey fish, bluegill (Lepomis macrochirus), to estimate the concentration of Hg in the blood of 4 species of adult piscivorous wading birds (little blue herons [Egretta caerulea], green herons [Butorides virescens], great egrets [Ardea albus], and great blue herons [Ardea herodias]) in 14 ecoregions of the south central United States. The 4 species of birds consume different sizes of fish with different concentrations of Hg and were predicted to have different concentrations of Hg in their blood, with little blue herons < green herons < great egrets < great blue herons. For each species of bird, there were significant differences in average estimated concentrations of Hg in blood between ecoregions, with estimated concentrations of Hg in blood increasing with Hg deposition. The level of predicted risk varied with ecoregion and bird species and was highest for great blue herons. We recommend that future studies of Hg contamination of piscivorous wading birds of the southern United States focus on great blue herons in water bodies within ecoregions that have high Hg deposition. Environ Toxicol Chem 2019;38:160-166.