Trophic Position of Wintering Common Loons (Gavia immer ) in Barataria Bay, Louisiana Following the Deepwater Horizon Oil Spill

Abstract. Common Loons (Gavia immer) wintering in watercourses of Barataria Bay, in coastal Louisiana were sampled in 2011–2014 following the Deepwater Horizon oil spill of 2010. Blood samples were analyzed for stable isotope ratios of carbon, nitrogen and sulfur as proxies for habitat use and diet in order to expand our understanding of the trophic position of wintering loons. The δ 13C and δ 34S values indicated that these Common Loons feed in coastal estuarine habitats. Trophic position was estimated indirectly by comparing loon stable isotope ratios with those of Brown Pelicans (Pelecanus occidentalis), a known piscivore, sampled concurrently in 2014. The isotopic signatures of the two species were not significantly different; this is consistent with the hypothesis that both species foraged primarily in coastal estuarine habitats and mainly as piscivores. No significant differences were found between subadult and adult Common Loons with respect to isotopic signatures, suggesting similar habitat usage and diet. Adults weighed more and were in better body condition than subadults. Stable isotope composition and body condition were not significantly related. Using a parallel data set of polycyclic aromatic hydrocarbons (PAH, an indicator or oil contamination) in the blood of the same loons, there was no significant relationship between PAH contamination and stable isotopic composition. Therefore, PAH-contamination could not be linked to a distinctive foraging habitat or diet.