Effect of temperature and ration size on carbon and nitrogen stable isotope trophic fractionation

Abstract

Summary Stable isotope data are widely used to track the origins and transformations of materials in food webs. Reliable interpretation of these data requires knowledge of the factors influencing isotopic fractionation between diet and consumer. For practical reasons, isotopic fractionation is often assumed to be constant but, in reality, a range of factors may affect fractionation. To investigate effects of temperature and feeding rate on fractionation of carbon and nitrogen stable isotopes in a marine predator, we reared European sea bass Dicentrarchus labrax on identical diets at 11 and 16 °C on three ration levels for 600 days. Nitrogen trophic fractionation (Δδ 15 N) was affected by temperature. Bass Δδ 15 N was 4·41‰ at 11 °C and 3·78‰ at 16 °C. Carbon fractionation (Δδ 13 C) was also affected by temperature. Bass Δδ 13 C was 1·18‰ at 11 °C and 1·64‰ at 16 °C. The higher lipid content in the tissues of bass reared at cooler temperatures accounted for the temperature effect on Δδ 13 C. When Δδ 13 C was determined using mathematically defatted values, there was a direct effect of ration size and Δδ 13 C was 2·51, 2·39 and 2·31‰ for high, medium and low rations, respectively. Reported Δδ 15 N for all treatments exceeded the mean of 3·4‰ widely used in ecological studies of fish populations and communities. This would confound the interpretation of δ 15 N as an indicator of trophic level when comparing populations that are exposed to different temperatures. The Δδ 13 C of 0–1‰ commonly applied in food web studies did not hold under any of the temperature or feeding regimes considered and a value of 2‰ would be more appropriate.