Brominated and chlorinated flame retardants in Pacific bald eagles: Sources and dietary insights from stable isotopes...

Many flame retardants are persistent, bioaccumulative, and toxic contaminants that raise ongoing concerns about their sources, environmental fate, and impacts on apex predators. This study investigated dietary exposure pathways and spatial variation of polybrominated diphenyl ethers (PBDEs) and alternative halogenated flame retardants (AHFRs) in bald eagles (Haliaeetus leucocephalus) across a marine-freshwater-terrestrial habitat gradient. Blood samples were collected from 91 nestlings (n = 64 nests) from 2021 to 2023 across British Columbia, Canada, including the Fraser Valley, Delta, Burrard/Howe Sound, North/South Salish Sea, Southeast Vancouver Island, Thompson River, and a reference site on the West Coast of Vancouver Island. Nestling plasma was analyzed for 15 PBDEs, 7 AHFRs, and stable isotopes (δ¹³C, δ³⁴S, δ¹⁵N) alongside the novel application of fatty acid methyl ester (FAME) analysis to distinguish between aquatic and terrestrial diets. Isotope signatures and FAMEs revealed a clear spatial gradient, with marine-derived diets prevalent on the outer coast (high δ¹³C, δ¹⁵N, δ³⁴S and ∑Omega3), transitioning to terrestrial and freshwater diets inland. PBDEs were dominated by BDE-47, -99, and -100, with the highest ∑PBDE concentrations detected in the North Salish Sea (mean ∑PBDEs = 2.19 ± 1.04 ng/g, range = 0.12-14.4 ng/g ww), particularly in a nestling near an electrical substation. Interestingly, nestlings near the Delta landfill did not exhibit the highest PBDE burdens. δ¹⁵N and ∑Omega3:6 were significant predictors (p < 0.05) for penta-BDEs, while BDE-209 was associated with terrestrial feeding (↑ ∑Omega6). AHFRs (Dechlorane Plus) were detected at low levels (<1 ng/g) in 10% of samples, suggesting limited exposure and bioavailability. Despite complex interactions between local sources and diet, ∑PBDE concentrations remained below thresholds known to affect circulating thyroid hormone and retinol levels. To our knowledge, this is the first study integrating multiple stable isotopes and fatty acids to assess flame retardant exposure in a top predator, such as the bald eagle.