Predation threat affects isotope ratios of planktonic consumers.

The analysis of stable isotopes of carbon and nitrogen is widely used in ecological investigations. However, sources of variation in isotope ratios of animals are far from being identified, which may bias the interpretation of the results. It has been suggested that stress is a factor that affects isotope ratios and trophic enrichment in animal bodies. Here, we tested the hypothesis that stress due to exposure to a predation threat affects metabolic processes that ultimately result in altered isotope composition of threatened prey. We found that both the nitrogen isotope composition (δ15N) and trophic enrichment in 15N isotope (Δ15N) were affected by threat induced by planktivorous fish in two species of the keystone freshwater pelagic herbivore Daphnia: D. magna and D. pulex. The two species differed from each other with regard to isotope ratios. Despite this, with increasing perceived risk of predation, the δ15N decreased in both species and, as a consequence, individuals of the two species were depleted in 15N with respect to their conspecifics not exposed to predation. We have not found evidence that the carbon isotope ratio (δ13C) was affected by the predation threat. The eco-physiological responses underlying the induction of antipredatory defences may be responsible for the observed pattern. The effect of predation on the nitrogen isotope ratio (δ15N and Δ15N) of consumers should be accounted for when using the analysis of stable isotopes in ecological investigations. Moreover, it could be applied to the fossil record to identify the introduction or removal of planktivorous predators in the paleoenvironment.