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.