ABSTRACT: We used stable-isotope analysis (SIA) to evaluate trophic relationships in an Antarctic seabird community. We determined natural abundances of stable-nitrogen (d15N) and stable-carbon (d13C) isotopes from blood samples (n = 283) from adults and chicks of 4 Antarctic fulmarine petrel species (Fulmarus glacialoides, Thalassoica antarctica, Daption capense and Pagodroma nivea) during 2 consecutive breeding seasons, 1994/1995 and 1995/1996, and from representative prey items. Our objectives were to use the isotope approach to infer trophic status and diet composition within and between species, addressing interspecific and temporal variability within this seabird community, and to investigate potential age-related differences in assumed trophic position within species. Prey d13C values ranged from -26.8[per thou] in amphipods to -23.9[per thou] in adult Antarctic silverfish. Seabird d13C values ranged from -25.3[per thou] in Antarctic petrel chicks to -23.8[per thou] in cape petrel adults. Prey d15N values ranged from 4.0[per thou] in euphausiids to 10.7[per thou] in adult Antarctic silverfish. Seabird d15N values ranged from 8.4[per thou] in Antarctic petrel adults to 12.0[per thou] in snow petrel chicks. There was considerable interspecific overlap in assumed trophic positions amongst the 4 petrel species, and we conclude all species consumed fish and krill. Despite this apparent overlap, the range in d15N values for petrels corresponded to the equivalent of 1 full trophic level, and estimated trophic level varied with both species and age. A simple trophic level model, constructed based on the d15N data, predicted trophic levels ranging from 2.3 in krill to 4.7 in snow petrel chicks. Snow petrels and Antarctic fulmars tended to have higher d15N values than Antarctic and cape petrels, suggesting a higher proportion of fish in their diets. Petrel chicks consistently had higher d15N values than adults, which suggests trophic segregation between adults and chicks. We discuss advantages of selectively provisioning chicks with higher trophic level prey. Extensive overlap and a relatively narrow range of d15N values are consistent with a food web comprised of few trophic steps.

KEY WORDS: Trophic relationships · Diet · Stable-isotope analysis · Pagodroma nivea · Daption capense · Thalassoica antarctica · Fulmarus glacialoides · Antarctica · Chick-provisioning

Published in MEPS Vol. 198 (2000) on June 5
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 2000