Reconstructing Diet and Trophic Position: Baseline Issues in Stable Isotope Studies

The stable isotopes of N, C, and O are commonly used to assess diet, trophic position, and habitat in modern and ancient organisms, both terrestrial and aquatic. Numerous factors affect the isotopic signature (e.g., d¹³C_org) of an organism including the isotopic signature of the primary producers at the base of the food chain (d¹³C_base), the number of food chains within the food web (base1 and base2), the relative contributions of each food chain to the diet of the organism (α), the trophic position of the organism (t_org), and the trophic fractionation (Δ_c high for N, low for C): d¹³C_org  = d¹³C_base2 – [ α (d¹³C_base2 - d¹³C_base1)] - Δ_ct_org. Our ability to resolve changes in diet (α) depends on our understanding of (1) the isotopic disparity of the two dietary sources of carbon or end members (e.g., C₃ vs C₄ plants, littoral vs pelagic producers), (2) the amount of variation in isotopic signature around those end members, (3) the magnitude of the observed isotopic difference, and (4) the experimental error around those measurements. The d¹³C values of modern aquatic primary producers exhibit great heterogeneity and shift with lateral changes along shore, onshore-offshore gradients, water depth, lake size, primary productivity, latitude, seasonality, etc. In the fossil record we must consider these factors in addition to time averaging and long-term trends such as changes in the pCO₂ of the atmosphere. Where isotopic disparity of the end members is large, and the variation around those end members is small or quantified using an independent proxy, modest differences in the observed isotopic signature between samples (~2-3‰) may be considered robust. However, where end member disparity is low, or variation around those end members is not addressed, and the observed differences are small (~1‰) specific interpretations related to diet, habitat, or organism behavior may be suspect.