Variation in delta N-15 and delta C-13 trophic fractionation: Implicationsfor aquatic food web studies

Use of stable isotope techniques to quantify food web relationships require s a priori estimates of the enrichment or depletion in delta N-15 and delta C-13 values between prey and predator (known as trophic fractionation; her eafter Delta delta N-15 and Delta delta C-13). We conducted a broad-scale a nalysis of Delta delta N-15 and Delta delta C-13 from aquatic systems, incl uding three new field estimates. Carnivores had significantly higher Delta delta (15N) values than herbivores. Furthermore, carnivores, invertebrates, and lab-derived estimates were significantly more variable than their coun terparts (f-test, p < 0.00001). Delta delta C-13 was higher for carnivores than for herbivores (p = 0.001), while variances did not differ significant ly. Excluding herbivores, the average Delta delta N-15 and Delta delta C-13 were 3.4 parts per thousand and 0.8 parts per thousand, respectively. But even with unbiased fractionation estimates, there is variation in isotopic fractionation that contributes to error in quantitative isotope model outpu ts. We simulated the error variance in Delta delta N-15-based estimates of trophic position and two-source delta C-13 diet mixing models, explicitly c onsidering the observed variation in Delta delta N-15 and Delta delta C-15, along with the other potential error sources. The resultant error in troph ic position and mixing model outputs was generally minor, provided that pri mary consumers were used as baseline indicators for estimating trophic posi tion and that end member delta C-13 values in dietary mixing models were su fficiently distinct.