Effects of nutritional restriction on nitrogen and carbon stable isotopes in growing seabirds

TLDR: The results show that 15N enrichment is not always associated with food deprivation and argue effects of growth on diet–tissue fractionation of nitrogen stable isotopes (Δ15N) need to be considered in stable isotope studies.

Abstract: When using stable isotopes as dietary tracers it is essential to consider eVects of nutritional state on isoto- pic fractionation. While starvation is known to induce enrichment of 15 N in body tissues, eVects of moderate food restriction on isotope signatures have rarely been tested. We conducted two experiments to investigate eVects of a 50-55% reduction in food intake on 15 N and 13 C values in blood cells and whole blood of tufted puYn chicks, a species that exhibits a variety of adaptive responses to nutritional deWcits. We found that blood from puYn chicks fed ad libitum became enriched in 15 N and 13 C compared to food-restricted chicks. Our results show that 15 N enrich- ment is not always associated with food deprivation and argue eVects of growth on diet-tissue fractionation of nitro- gen stable isotopes ( 15 N) need to be considered in stable isotope studies. The decrease in 13 C of whole blood and blood cells in restricted birds is likely due to incorporation of carbon from 13 C-depleted lipids into proteins. EVects of nutritional restriction on 15 N and 13 C values were rela- tively small in both experiments ( 15 N: 0.77 and 0.41‰, 13 C: 0.20 and 0.25‰) compared to eVects of ecological processes, indicating physiological eVects do not preclude the use of carbon and nitrogen stable isotopes in studies of seabird ecology. Nevertheless, our results demonstrate that physiological processes aVect nitrogen and carbon stable isotopes in growing birds and we caution isotope ecologists to consider these eVects to avoid drawing spurious conclu- sions.