Variability in isotope discrimination factors in coral reef fishes: Implications for diet and food web reconstruction

Abstract

Interpretation of stable isotope ratios of carbon and nitrogen (d13C and d15N) is generally based on the assumption that with each trophic level there is a constant enrichment in the heavier isotope, leading to diet-tissue discrimination factors of 3.4% for 15N (DN) and,0.5% for 13C (DC). Diet-tissue discrimination factors determined from paired tissue and gut samples taken from 152 individuals from 26 fish species at Ningaloo Reef, Western Australia demonstrate a large amount of variability around constant values. While caution is necessary in using gut contents to represent diet due to the potential for high temporal variability, there were significant effects of trophic position and season that may also lead to variability in DN under natural conditions. Nitrogen enrichment increased significantly at higher trophic levels (higher tissue d15N), with significantly higher DN in carnivorous species. Changes in diet led to significant changes in DN, but not tissue d15N, between seasons for several species: Acanthurus triostegus, Chromis viridis, Parupeneus signatus and Pomacentrus moluccensis. These results confirm that the use of meta-analysis averages for δN is likely to be inappropriate for accurately determining diets and trophic relationships using tissue stable isotope ratios. Where feasible, discrimination factors should be directly quantified for each species and trophic link in question, acknowledging the potential for significant variation away from meta-analysis averages and, perhaps, controlled laboratory diets and conditions. © 2010 Wyatt et al.