Variability in tissue-specific trophic discrimination factors (∆13C and ∆15N) between Antarctic krill Euphausia superba and free-ranging Pygoscelis penguins

For top consumers in marine environments, trophic discrimination factors (∆13C and ∆15N) between food and consumers’ tissues are expected to be similar among related species. However, few studies conducted in the laboratory indicate a large variability among species, which should be potentially higher in free-ranging animals. Here, we test for differences in tissue-specific ∆13C and ∆15N values of two wild penguin species (Chinstrap Pygoscelis antarctica and Gentoo P. papua) breeding in sympatry at Livingston Island, Antarctica. A total of 41 adults and 28 chicks, and food items comprised exclusively by Antarctic krill (Euphausia superba, n = 22) in Chinstraps and almost exclusively in Gentoos, were sampled for stable isotope analyses. Overall, Δ13C values varied between -1.8 and 4.0 ‰ and Δ15N values ranged from 1.2 to 6.1 ‰, and these differed between species, tissues and age-classes. Δ13C in adult penguins differed between species for feather and blood. Species-specific differences in Δ13C and Δ15N were seen in chick nail and muscle, while only Δ13C values differed between species in feathers. Our results show that trophic discrimination factors can differ substantially between closely related species consuming similar prey, especially in Δ13C value. Variation in Δ13C was driven by species, tissue and age-class, while variation in Δ15N was mostly driven by tissue type. Trophic discrimination factors may be associated to physiological and/or stress factors which may fluctuate in the wild, and this was particularly evident on chicks. This study highlights the use of diet-specialised species for the determination of trophic discrimination factors in the wild.