Diet discrimination factors are inversely related to delta N-15 and delta C-13 values of food for fish under controlled conditions

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Rapid Communications in Mass Spectrometry





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A recent literature review reported negative relationships between diet discrimination factors (DDFs X-fish - X-food; X = delta N-15 or delta C-13) and the values of delta N-15 and delta C-13 in the food of wild organisms but there has been no laboratory-based confirmation of these relationships to date. Laboratory reared guppies (Poecilia reticulata) fed a series of diets with a range of delta C-13 (-22.9 to -6.6%) and delta N-15 (6.5 to 1586%) values were used to magnify diet-tissue dynamics in order to calculate DDFs once the fish had achieved equilibrium with each of the diets. Values of DDFs range widely for delta N-15 (7.1 to -849%) and delta C-13 (1.1 to -7.0%) and showed a strong negative correlation with the stable isotope value in the food for delta N-15 (slope = -0.59 +/- 0.02, r(2) = 0.95) and delta C-13 (slope = -0.56 +/- 0.02, r(2) = 0.94). Based on these relationships, the magnitude of DDF change over environmentally relevant values of delta N-15 or delta C-13 would be significant and could confound the interpretation of stable isotopes in the environment. Using highly enriched experimental diets, our study adds to a growing number of studies that undermine the consistent trophic enrichment paradigm with results that demonstrate the currently poor mechanistic understanding of how DDFs arise. The results of our study highlight that the magnitude of the stable isotope values in prey must be considered when choosing DDF values. Future laboratory studies should therefore be directed at uncovering the mechanistic basis of DDFs and, like others before, we recommend the determination of diet-dependent DDFs under laboratory conditions before modeling dietary proportions or calculating trophic positions. Copyright (C) 2010 John Wiley & Sons, Ltd.