Rapid turnover of tissue nitrogen of primary consumers in tropical freshwaters.

Stable isotopes are widely used as time-integrating tracers of trophic interactions, but turnover rates of isotopes in animal tissues remain poorly understood. Here, we report nitrogen (N) isotope turnover rates in tissues of four primary consumer species: Ancistrus triradiatus armored catfish (muscle, fins, and whole blood), Tarebia granifera snails (muscle), and Rana palmipes tadpoles (muscle) from a Venezuelan river, and Lavigeria grandis snails (muscle) from Lake Tanganyika, East Africa. Turnover was estimated from the dilution of a 15N label introduced into consumer tissues by feeding on 15N-enriched periphyton. Muscle turnover rates were rapid (0.5-3.8% per day), and were attributable to metabolic replacement of N as well as growth in catfish and snails. N turnover in catfish muscle decreased with size, and fin tissue turned over more rapidly than whole blood or muscle, though the difference was not significant. Our results indicate that stable isotope signatures of these tropical species could change markedly within weeks following a shift in diet. However, generalization across taxa or latitudes is complicated by the strong size-dependence of isotope turnover rates. The enrichment-dilution approach outlined here may facilitate measurement of isotopic turnover in a wide variety of consumers under field conditions.