Nitrogen and carbon isotope values of individual amino acids : a tool to study foraging ecology of penguins in the Southern Ocean

We determined the delta N-15 and delta C-13 values of individual amino acids (AAs) isolated from chick blood of 4 penguin species that forage in different oceanic regions (from the subtropics of the Indian Ocean to Antarctica) to test if: (1) the delta N-15 values of phenylalanine (delta N-15(phe)) revealed different foraging areas among the species; (2) the difference between glutamic acid and phenylalanine delta N-15 values (Delta delta N-15(glu-phe)) accurately predicted trophic levels; and (3) the delta C-13 value of AAs could resolve species foraging locations, similar to bulk delta C-13 values. The delta C-13 values of all AAs decreased with latitude, were positively correlated with bulk delta C-13 data, and, therefore, tracked the isotopic baseline. However, we were not able to discern additional ecological information from these delta C-13 values. In contrast, the delta N-15 values of AAs distinguished the isotopic value of the nitrogen at the base of the food web from the trophic level of the consumer, providing new insight for the study of the trophic ecology of seabirds. The difference in the bulk delta N-15 values of northern and southern rockhopper penguins Eudyptes chrysocome ssp. was due to both a difference in their foraging location (different delta N-15(phe)) and their trophic levels (different Delta delta N-15(glu-phe)). The delta N-15(phe) values of king Aptenodytes patagonicus and Adelie penguins Pygoscelis adeliae were higher than those of rockhoppers, which could reflect a foraging on mesopelagic prey for king penguins and, in the highly productive Antarctic shelf waters, for Adelie penguins. The Delta delta N-15(glu-phc) accurately reflected the relative trophic level of penguins, but further work is required to determine the trophic enrichment factors for compound-specific isotope analysis