Diet-tissue fractionation of δ15N by consumers from streams and rivers

Variation in the diet-tissue fractionation of stable nitrogen isotopes (Delta 15N) is a major source of uncertainty in mixing model outputs and the calculation of trophic level in food web studies in aquatic systems. Using delta N-15 and delta C-13 of algae and consumers collected from a broad range of streams and rivers in Australia and New Guinea, we calculated Delta 15N using a gradient approach, and compared these estimates with those from the literature. Riverine invertebrates and fishes from this region have Delta 15N diet-tissue fractionation at the low end of the range of values reported for literature summaries, but different trophic groups had different Delta 15N estimates. Source-consumer regressions based on delta C-13 had lower slope estimates and lower R-2 values compared with those based on delta N-15. This implies that although consumers on average obtain a portion of their organic carbon from higher plants, they derive most of their organic nitrogen from N-rich algae. Use of appropriate Delta 15N for riverine consumers will lead to more satisfactory results with isotope mixing models, and the uncertainty associated with Delta 15N estimates can be incorporated in the latest iterations of these models that are probabilistic rather than deterministic.