Influence of trophic ecology and spatial variation on the isotopic fingerprints of seabirds

Notwithstanding the potential applications of stable isotopes in feeding and migration studies, the simultaneous influence of diet, foraging behavior and spatial variation on the stable isotope signatures of seabirds is poorly understood. Many studies have interpreted their isotopic signatures without considering local baseline and prey isotopic signatures; consequently, the main factors causing isotopic differences between populations have frequently not been discerned. To examine the influence of these factors on the stable isotopic signatures of seabirds, we analyzed the delta N-15, delta C-13, delta S-34 and Hg concentrations of chick feathers of the European shag Phalacrocorax aristotelis, its main fish prey and baseline indicator organisms (mussels), all sampled in 2 sectors of northwest coastal Spain with marked differences in primary productivity. Our results show that the delta N-15 signature and Hg concentration of shags are influenced by both feeding ecology and spatial variation. The delta C-13 and delta S-34 signatures, however, mainly related to spatial differences and can thus be used as reliable geographic markers. Our findings also highlight the importance of assessing spatio-temporal variation in baseline isotopic signatures and their progressive integration through the food web. Omission of potential prey and baseline values, or application of only a single baseline to the food webs of the 2 sectors, assuming isotopic homogeneity because of geographical proximity, would have led to significantly distorted interpretations of feeding ecology of shag chicks.