Estimating turnover rates of carbon and nitrogen in recently metamorphosed winter flounder Pseudopleuronectes americanus with stable isotopes

Stable-isotope ratios of carbon (C-13/C-12) and nitrogen (N-15/N-14) are widely used in the analysis of animal diets. However, using these stable-isotope ratios to infer dietary changes depends on precise knowledge of turnover rates of carbon and nitrogen. In the present study, carbon and nitrogen turnover rates were determined for recently settled juvenile winter flounder Pseudopleuronectes americanus in the laboratory using naturally occurring stable isotopes as dietary tracers. Flounder were reared at 13degreesC on a diet of rotifers Brachionus plicatilis of known isotopic composition from the time that the larvae began feeding until they reached metamorphosis and began to settle to the benthic habitat. At settlement, the fish were assigned to 1 of 2 temperature treatments (13 and 18degreesC). A subset of fish at each temperature was maintained on rotifers to serve as controls. The remaining fish were switched to a diet of brine shrimp Artemia sp. (known to be isotopically distinct from rotifers) and then sampled systematically over a 16 d period. Temperature had a significant effect on both carbon and nitrogen turnover rates. At 13degreesC, the half-life of carbon was 4.1 d (+/-0.6), and of nitrogen, 3.9 d (+/-0.7). At 18degreesC, the half-life of carbon was 2.2 d (+/-0.3), and of nitrogen, 3.1 d (+/-0.3). The change in isotopic composition closely followed predictions based entirely on the production of new tissue.