Seasonal changes in diets of seabirds in the North Water Polynya: a multiple-indicator approach

Each year, millions of seabirds migrate to the North Water Polynya, northern Baffin Bay, to feed in its productive waters during the 6 months that the polynya is free of ice. We evaluated seasonal shifts in diets of 3 species using the North Water: dovekie Alle alle, thick-billed murre Uria lomvia, and black-legged kittiwake Rissa tridactyla. Diets were assessed through a combination of stable isotope analysis of muscle tissue, fatty acid analysis of subcutaneous fat, and stomach content analysis. Dovekies had the lowest delta N-15 values and hence lowest trophic level in spring and summer, corresponding to their consumption of herbivorous copepods. In fall, their delta N-15 values increased as they switched to feeding at a higher trophic level on primary carnivores such as amphipods and fish. Throughout the study period, kittiwakes and murres had stomach contents and stable isotope values similar to dovekies in fall. Fatty acid signatures of black-legged kittiwakes and dovekies feeding in fall were similar, likely due to the reliance of both species on the pteropod Limacina limacina. Our study highlights the seasonal nature of prey availability and seabird diets in the polynya, as well as the utility of simultaneously using conventional stomach content analysis and stable isotope and fatty acid analyses to infer dietary patterns in marine food webs.