Using Stable Isotopes to Estimate Trophic Position: Models, Methods, and Assumptions

The stable isotopes of nitrogen (δ15N) and carbon (δ13C) provide powerful tools for estimating the trophic positions of and carbon flow to consumers in food webs; however, the isotopic signature of a consumer alone is not generally sufficient to infer trophic position or carbon source without an appropriate isotopic baseline. In this paper, I develop and discuss methods for generating an isotopic baseline and evaluate the assumptions required to estimate the trophic position of consumers using stable isotopes in multiple ecosystem studies. I test the ability of two primary consumers, surface-grazing snails and filter-feeding mussels, to capture the spatial and temporal variation at the base of aquatic food webs. I find that snails reflect the isotopic signature of the base of the littoral food web, mussels reflect the isotopic signature of the pelagic food web, and together they provide a good isotopic baseline for estimating trophic position of secondary or higher trophic level consumers in lake ecosystems. Then, using data from 25 north temperate lakes, I evaluate how δ15N and δ13C of the base of aquatic food webs varies both among lakes and between the littoral and pelagic food webs within lakes. Using data from the literature, I show that the mean trophic fractionation of δ15N is 3.4‰ (1 sd = 1‰) and of δ13C is 0.4‰ (1 sd = 1.3‰), and that both, even though variable, are widely applicable. A sensitivity analysis reveals that estimates of trophic position are very sensitive to assumptions about the trophic fractionation of δ15N, moderately sensitive to different methods for generating an isotopic baseline, and not sensitive to assumptions about the trophic fractionation of δ13C when δ13C is used to estimate the proportion of nitrogen in a consumer derived from two sources. Finally, I compare my recommendations for generating an isotopic baseline to an alternative model proposed by M. J. Vander Zanden and J. B. Rasmussen. With an appropriate isotopic baseline and an appreciation of the underlying assumptions and model sensitivity, stable isotopes can help answer some of the most difficult questions in food web ecology.