Dealing with biases introduced by lipids in stable carbon and nitrogen isotope analyses: a solution based on 28 marine invertebrate, fish, and mammal species

Stable carbon (delta 13C) and nitrogen (delta 15N) isotope ratios are widely used in marine food web and habitat use studies. However, lipids are naturally depleted in 13C relative to proteins and are variable in content, biasing delta 13C of bulk samples, with consequences for the accuracy of conclusions. This issue can be resolved either by extracting lipids from samples prior to analysis, a resource-intensive process that can also alter delta 15N, or by estimating lipid-free delta 13C using one of several equations that differ in degree of sophistication and generalization across taxa. Here, delta 13C and delta 15N were measured in bulk and lipid-extracted muscle samples from over 2000 specimens of 28 species of marine invertebrates, fishes, and mammals. Our objectives were to compare the effect of lipid extraction on delta 13C and delta 15N across taxa and evaluate the performance of 5 normalization models, overall and using subsets of species, to propose a model to revert lipid-extracted delta 15N back to their bulk values and to identify the best approach for dealing with lipid-related biases. Lipid extraction caused an uneven enrichment in delta 13C and delta 15N across species. Model taxonomic specificity increased estimation accuracy for both isotopes. While models from Logan et al. (2008; J Anim Ecol 77:838-846) and McConnaughey & McRoy (1979; Mar Biol 53:257-262) were the best at predicting lipid-free delta 13C, a linear model reliably estimated delta 15N values of lipid-free samples using delta 15N values of bulk samples. This study presents a method for reliably estimating delta 13C and delta 15N values of muscle tissue without resorting to duplicate analyses. This represents a major step toward the harmonization of data sets generated using bulk and lipid-extracted samples.