Factors regulating the bioavailability of methylmercury to breeding rusty blackbirds in northeastern wetlands

Rusty blackbirds are undergoing rapid population decline and have elevated Hg concentrations while breeding in the Acadian ecoregion of North America. Factors regulating the bioavailability of methyl-Hg (MeHg) within this population's habitat were determined using water, invertebrates, and blood from adult rusty blackbirds collected for Hg-speciation, along with additional water column parameters: MeHg and THg, dissolved organic carbon, pH, dissolved oxygen, conductivity, redox potential, and temperature. Both DO2 and pH were negatively related to biota MeHg, while water MeHg concentrations were positively related. Both invertebrate MeHg concentration and %MeHg increased with trophic level. Invertebrate MeHg concentrations were among the greatest reported when compared with those reported elsewhere for wetlands and waterbodies—often several times greater for similar taxa—while percent MeHg of THg were similar. An environment with high bioavailability of MeHg in combination with a high trophic position best explains elevated Hg concentrations for this species regional population. ► DO2 and pH negatively correlate with wetland biota methylmercury. ► Water MeHg concentrations positively correlate with wetland biota methylmercury. ► Rusty blackbird blood-Hg correlates with MeHg in Araneae, Ephemeroptera, and Trichoptera, but not Odonata. ► Habitat with high MeHg bioavailability and a high trophic position explains regionally elevated Hg in the rusty blackbird. Elevated mercury concentrations in the disappearing Acadian population of breeding rusty blackbirds are due to a diet of aquatic macroinvertebrates in an environment with high methylmercury bioavailability.