Fractionation and metabolic turnover of carbon and nitrogen stable isotopes in black fly larvae

Diet‐tissue fractionation factors and metabolic turnover rates of δ15N and δ13C were assessed in laboratory‐reared black fly (Simulium vittatum IS‐7) larvae fed isotopically distinct diets. Five treatments consisted of using food with different δ15N signatures throughout the experiments (19–26 days), a sixth shifted from a low to high δ15N signature diet (uptake) on day 14, and the last shifted from a high to low δ15N signature diet (elimination) on day 14. In the larvae, diet‐tissue fractionation factors for δ13C, which were in steady state with food, ranged from −0.61 to 2.0, with a median of 1.87. The δ15N diet‐tissue fractionation factors were mostly negative, ranging from +2.85 to −24.96‰, with a single positive value from the elimination treatment in which larval δ15N did not achieve steady state with the food. Diet‐tissue fractionation factors also had a significant negative relationship (r2 = 0.98) with δ15N values in the food suggesting that nitrogen diet‐tissue fractionation factors are 15N concentration‐dependent. The δ15N of shed head capsules and feces were enriched in 15N and could be mechanisms for elimination of 15N by the larvae. For δ15N, metabolic turnover values based on the Hesslein model were highly consistent (0.40 to 0.43 δ15N*day−1) between uptake and elimination phases and across experiments and were an order of magnitude greater than growth rates. The rapid turnover of nitrogen in black fly larvae, which was orders of magnitude greater than measured in vertebrates, makes them an excellent indicator of short‐term changes in nitrogen inputs to aquatic systems. Copyright © 2008 John Wiley & Sons, Ltd.