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

We determined the δ 15 N and δ 13 C 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 δ 15 N values of phenylalanine (δ 15 N phe ) revealed different foraging areas among the species; (2) the difference between glutamic acid and phenylalanine δ 15 N values (Δδ 15 N glu-phe ) accurately predicted trophic levels; and (3) the δ 13 C value of AAs could resolve species foraging locations, similar to bulk δ 13 C values.The δ 13 C values of all AAs decreased with latitude, were positively correlated with bulk δ 13 C data, and, therefore, tracked the isotopic baseline.However, we were not able to discern additional ecological information from these δ 13 C values.In contrast, the δ 15 N 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 δ 15 N values of northern and southern rockhopper penguins Eudyptes chrysocome ssp.was due to both a difference in their foraging location (different δ 15 N phe ) and their trophic levels (different Δδ 15 N glu-phe ).The δ 15 N phe values of king Aptenodytes patagonicus and Adélie 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 Δδ 15 N glu-phe accurately reflected the relative trophic level of penguins, but further work is required to determine the trophic enrichment factors for compound-specific isotope analysis.