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

Diet-tissue fractionation factors and metabolic turnover rates of delta N-15 and delta C-13 were assessed in laboratory-reared black fly (Simulium vittatum IS-7) larvae fed isotopically distinct diets. Five treatments consisted of using food with different delta N-15 signatures throughout the experiments (19-26 days), a sixth shifted from a low to high delta N-15 signature diet (uptake) on day 14, and the last shifted from a high to low delta N-15 signature diet (elimination) on day 14. In the larvae, diet-tissue fractionation factors for delta C-13, which were in steady state with food, ranged from -0.61 to 2.0, with a median of 1.87. The delta N-15 diet-tissue fractionation factors were mostly negative, ranging from +2.85 to -24.96 parts per thousand, with a single positive value from the elimination treatment in which larval delta N-15 did not achieve steady state with the food. Diet-tissue fractionation factors also had a significant negative relationship (r(2) = 0.98) with delta N-15 values in the food suggesting that nitrogen diet-tissue fractionation factors are N-15 concentration-dependent. The delta N-15 of shed head capsules and feces were enriched in N-15 and could be mechanisms for elimination of N-15 by the larvae. For delta N-15, metabolic turnover values based on the Hesslein model were highly consistent (0.40 to 0.43 delta N-15*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 (C) 2008 John Wiley & Sons, Ltd.