Variation in ¹⁵N and ¹³C trophic fractionation: Implications for aquatic food web studies
M. Jake Vander Zanden, Joseph B. Rasmussen
Limnol. Oceanogr., 46(8), 2001, 2061–2066

Use of stable isotope techniques to quantify food web relationships requires a priori estimates of the enrichment or depletion in ¹⁵N and ¹³C values between prey and predator (known as trophic fractionation; hereafter ¹⁵N and ¹³C). We conducted a broad-scale analysis of ¹⁵N and ¹³C from aquatic systems, including three new field estimates. Carnivores had significantly higher ¹⁵N values than herbivores. Furthermore, carnivores, invertebrates, and lab-derived estimates were significantly more variable than their counterparts ( f-test, p < 0.00001). ¹³C was higher for carnivores than for herbivores (p = 0.001), while variances did not differ significantly. Excluding herbivores, the average ¹⁵N and ¹³C were 3.4‰ and 0.8‰, respectively. But even with unbiased fractionation estimates, there is variation in isotopic fractionation that contributes to error in quantitative isotope model outputs. We simulated the error variance in ¹⁵N-based estimates of trophic position and two-source ¹³C diet mixing models, explicitly considering the observed variation in ¹⁵N and ¹³C, along with the other potential error sources. The resultant error in trophic position and mixing model outputs was generally minor, provided that primary consumers were used as baseline indicators for estimating trophic position and that end member d13C values in dietary mixing models were sufficiently distinct.