Isotope trophic-step fractionation: a dynamic equilibrium model

1. Trophic position is a fundamental feature of food-web structure, knowledge of which is being improved by stable isotope approaches which assume a constant enrichment in heavier isotopes in consumers relative to their diet. 2. We argue that the typical enrichment reflects a dynamic equilibrium between fractionation vectors associated with assimilation and excretion. 3. We develop a linear model to characterize the relationship between the equilibrium isotopic signature and the feeding rate influenced by isotopic discrimination during assimilation and excretion. 4. We present new data for both diet switching and starvation experiments using a marine worm Nereis virens and use this, and previously published data for a fish, bird and mammal to calculate controlling parameters from observations of the isotopic signature following diet-switching and the onset of starvation. 5. We show that the observed variance in isotopic signatures at each trophic step carries substantially more information than has been used hitherto and is influenced by feeding rate in addition to the isotopic signature of the food source. 6. Using the linear model as a tool we predict that parasitic organisms may be depleted relative to the tissues of their host.