Winter feeding ecology and biomagnification of organochlorine contaminants in Alaska polar bears

Abstract

Dietary pathways expose polar bears to a variety of contaminant profiles and concentrations, ranging from bowhead whale (Balaena mysticetus) as one of the least contaminated marine mammals to the more highly contaminated upper trophic level ringed seal (Phoca hispida) which represent the majority of their annual diet. We used stable isotopes [delta]¹⁵N and [delta]¹³C to estimate trophic status of 139 free-ranging polar bears sampled along Alaska's Beaufort Sea coast in spring 2003 and 2004. The [delta]¹⁵N values of polar bear packed blood cells ranged from 18.2% to 21.4% with a mean of 19.5% (SD=0.7) in 2003 and 19.9% (SD=0.7) in 2004. Two-element three-source mixing models indicated that lower trophic level prey, such as scavenged bowhead whale carcasses, may have composed 11-26% (95% CI) of the diet in 2003, and -2-14% (95% CI) of the diet in 2004. Organochlorine (OC) concentrations in subcutaneous adipose tissue were determined for 47 of the polar bears sampled in 2003 and compared to trophic position ([delta]¹⁵N). Although many OCs appear not to biomagnify in polar bears, we found positive relationships with [delta]¹⁵N in both sexes between concentrations of several highly recalcitrant OCs in models incorporating age, lipid content, and [delta]¹³C. [Delta]¹⁵N was important in explaining variation in OC concentrations, indicating structural differences in food webs and biomagnification of OCs among polar bears related to their sex, age, and the apparent use of lower trophic level prey.