Explaining isotope trophic-step fractionation: why herbivorous fish are different

1. An assumed constant trophic fractionation of N-15/N-14 between consumer and diet (usually 3.4 parts per thousand for diet-muscle tissue differences) allows inferences to be made about feeding interactions and trophic level in food web studies. However, considerable variability surrounds this constant, which may conceal subtle differences about the trophodynamics of consumers. 2. The feeding ecologies of herbivores and carnivores differ in terms of diet quality (in C : N terms) and food processing mechanisms, which may affect fractionation. 3. We present a new model that explores how consumer feeding rates, excretion rates and diet quality determine the N-15/N-14 ratios in the consumer's tissues and hence influence the magnitude of trophic fractionation. 4. Three herbivorous reef fish Acanthurus sohal, Zebrasoma xanthurum and Pomacentrus arabicus were chosen as study organisms. Empirical estimates of diet-tissue stable isotope fractionation were made in the field, and model parameters were derived from feeding observations and literature data. 5. The trophic fractionation values of A. sohal, Z. xanthurum and P. arabicus were 4.69 parts per thousand, 4.47 parts per thousand and 5.25 parts per thousand, respectively, by empirical measurement, and 4.41 parts per thousand, 4.30 parts per thousand and 5.68 parts per thousand, respectively, by model, indicating that herbivores have a higher trophic fractionation than the currently accepted value of 3.4 parts per thousand. 6. The model was most sensitive to the excretion rate, which may differ between herbivores and carnivorous animals. This model is the first to determine stable isotope signatures of a consumer's diet mixture without applying a constant fractionation value.