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

We determined the δ15N and δ13C values of individual amino acids (AAs) isolated from chick blood of four penguin species that forage in different oceanic regions (from the subtropics of the Indian Ocean to Antarctica) to test if (1) the δ15N values of phenylalanine (δ15Nphe) revealed different foraging areas among the species, (2) the difference between glutamic acid and phenylalanine δ15N values (Δδ15Nglu-phe) accurately predicted trophic levels (TL), and (3) the δ13C value of AAs could resolve species foraging locations, as bulk δ13C values did. The δ13C values of all AAs decreased with latitude, were positively correlated with bulk δ13C data, and therefore, tracked the isotopic baseline. However, we were not able to discern additional ecological information from these δ13C values. In contrast, the δ15N analysis of individual AAs in blood 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 δ15N values of northern and southern rockhopper penguins was due to both a difference in their foraging location (≠δ15Nphe) and their trophic levels (≠Δδ15Nglu-phe). The δ15Nphe values of king and Adelie penguins were higher than those from rockhoppers and we hypothesize that this difference reflects foraging on mesopelagic prey and in the highly productive Antarctic shelf waters respectively. The Δδ15Nglu-phe accurately reflected relative penguin's TL but further work is required to determine the trophic enrichment factors for compound-specific isotope analysis.