Variation in d 15 N and d 13 C trophic fractionation : Implications for aquatic food web studies

Use of stable isotope techniques to quantify food web relationships requires a priori estimates of the enrichment or depletion in d15N and d13C values between prey and predator (known as trophic fractionation; hereafter Dd15N and Dd13C). We conducted a broad-scale analysis of Dd15N and Dd13C from aquatic systems, including three new field estimates. Carnivores had significantly higher Dd15N values than herbivores. Furthermore, carnivores, invertebrates, and lab-derived estimates were significantly more variable than their counterparts ( f-test, p , 0.00001). Dd13C was higher for carnivores than for herbivores (p 5 0.001), while variances did not differ significantly. Excluding herbivores, the average Dd15N and Dd13C 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 d15N-based estimates of trophic position and two-source d13C diet mixing models, explicitly considering the observed variation in Dd15N and Dd13C, 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. Measurement of carbon (13C/12C; d13C) and nitrogen (15N/ 14N; d15N) stable isotope ratios has enhanced our understanding of food webs and energy flow in aquatic ecosystems. Nitrogen isotope ratios become enriched at successive trophic levels, thereby allowing estimates of consumer trophic position (Cabana and Rasmussen 1996; Vander Zanden and Rasmussen 1999). Stable carbon isotopes can indicate feeding and carbon flow pathways because there is little fractionation from prey to predator, and different energy sources can have distinct d13C values (Hecky and Hesslein 1995). Traditionally, food web studies using stable isotopes have provided qualitative depictions of trophic structure of single systems. Yet major advances in the use of stable isotopes have involved quantitative estimates of food web parameters, allowing examination of food web patterns and treatment of applied problems. Such quantitative applications require that the investigator assume the behavior of the isotopic tracer, in particular, the change in d15N or d13C values from prey to predator (hereafter called trophic fractionation; Dd15N and Dd13C). Provided Dd15N and Dd13C are known, variables such as omnivory, trophic position, food chain length, and energy flows and sources can be calculated. It is often assumed that the d15N value of a consumer is enriched by 3.4‰ over that of its diet based on the results of Minagawa and Wada (1984). This average Dd15N value was, in fact, the mean of 16 individual estimates with a standard deviation (hereafter 1 SD) of 1.1‰, although a close examination of this paper reveals that consumer diets were poorly characterized. Similarly, the common assumption that d13C exhibits little or no trophic enrichment is based on early studies (Fry and Sherr 1984; Peterson and Fry 1987). For both 15N and 13C, fractionation estimates have sometimes deviated from these values (Pinnegar and Polunin 1999). No studies have explicitly examined variation in trophic fractionation and the resultant error in quantitative stable isotope model outputs. In this study we address a few fundamental questions relevant to the application of stable isotopes to quantitative food web studies: Do the mean and variance in trophic fractionation vary systematically with taxon, habitat, or food type? How much error does the observed variation in trophic fractionation introduce into isotope-based trophic position estimates and d13C mixing model outputs? To address these questions, we synthesize field and laboratory Dd15N and Dd13C estimates from aquatic systems and examine patterns of variation in isotopic fraction. Using our estimates of variation in trophic fractionation and the known error in the other variables used in these models, we quantify the resultant error in d15N-based trophic position estimates and d13C mixing model outputs. Field estimates of trophic fractionation—During detailed dietary and isotopic studies of more than 20 lakes in Ontario and Quebec lakes, we identified three lakes (Lake Memphremagog [central basin], Lake Opeongo, and Victoria Lake) that could be used to provide field estimates of Dd15N and Dd13C because (1) the top predator (lake trout, Salvelinus namaycush) was known from gut contents to feed exclusively (.95% by volume) on a single prey species (either cisco [Coregonus artedii] or rainbow smelt [Osmerus mordax]), and (2) both predator and prey were relatively large and long lived, thereby minimizing temporal isotopic variation. Adult lake trout, cisco, and rainbow smelt were collected during the summer of 1995 and prepared for stable isotope analysis as described elsewhere (Vander Zanden et al. 1999a,b, 2000). Stable isotope ratios are expressed in delta (d) notation, defined as the parts per thousand (‰) deviation from a standard material (PDB limestone for d13C and atmospheric nitrogen for d15N); d13C or d15N 5 ([Rsample/ Rstandard] 2 1) 3 1000, where R 5 13C/12C or 15N/14N. The standard error of the mean for replicates of the same tissue was 0.13‰ for d13C and 0.15‰ for d15N. The mean d15N of lake trout varied by more than 6‰ among lakes (Table 1). Enrichment in d15N from prey to predator (Dd15N) was relatively consistent across lakes, with an average of 3.49‰ 6 0.23 (Dd15N 6 1 SD). Trophic fractionation for d13C averaged 0.05‰ 6 0.63. ANOVA comparing d15N and d13C values of lake trout and their prey