Nitrogen and carbon isotope fractionations of zooplankton consumers in ponds: potential effects of seston C:N stoichiometry

Abstract

C : N stoichiometry in food sources is known to influence nitrogen isotope fractionation of consumers, but the relationship between C : N stoichiometry and isotope fractionation has never been tested in the field. In this study, we investigated the effects of the C : N ratio of food sources on nitrogen and carbon isotope fractionation (Δδ15N and Δδ13C) of consumers, using zooplankton communities in 15 ponds in Japan. The fractionation variations we found contradicted the assumption of a 3.4‰ enrichment in δ15N of consumers per trophic level. However, the negative relationships between Δδ15N of filter-feeding zooplankton and the C : N ratio of seston supported the isotope homeostasis hypothesis, which predicts isotopic homeostasis of consumers to vary with respect to their body nutrient composition. The Δδ15N of Eodiaptomus sp. and cyclopoid copepods did not correlate with the C : N ratio of the seston. Further studies should pay attention to the isotope fractionation of consumers in the field, especially given the increased use of isotope fractionation to elucidate the structure of natural food webs.