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

1. 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. 2. 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 degrees C on three ration levels for 600 days. 3. Nitrogen trophic fractionation (Delta delta N-15) was affected by temperature. Bass Delta delta N-15 was 4.41 parts per thousand at 11 degrees C and 3.78 parts per thousand at 16 degrees C. 4. Carbon fractionation (Delta delta C-13) was also affected by temperature. Bass Delta delta C-13 was 1.18 parts per thousand at 11 degrees C and 1.64 parts per thousand at 16 degrees C. The higher lipid content in the tissues of bass reared at cooler temperatures accounted for the temperature effect on Delta delta C-13. When Delta delta C-13 was determined using mathematically defatted values, there was a direct effect of ration size and Delta delta C-13 was 2.51, 2.39 and 2.31 parts per thousand for high, medium and low rations, respectively. 5. Reported Delta delta N-15 for all treatments exceeded the mean of 3.4 parts per thousand widely used in ecological studies of fish populations and communities. This would confound the interpretation of delta N-15 as an indicator of trophic level when comparing populations that are exposed to different temperatures. 6. The Delta delta C-13 of 0-1 parts per thousand commonly applied in food web studies did not hold under any of the temperature or feeding regimes considered and a value of 2 parts per thousand would be more appropriate.