MEPS prepress abstract   -  DOI: https://doi.org/10.3354/meps14595

ABSTRACT: Stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope ratios are widely used in marine food web and habitat use studies. However, lipids are naturally depleted in ¹³C relative to proteins and variable in content, biasing δ¹³C of bulk samples, with consequences on accuracy of conclusions. This issue can be resolved by extracting lipids from samples prior to analysis, a resource-intensive process that can also alter δ¹⁵N, or by estimating lipid-free δ¹³C using one of several equations that differ in degree of sophistication and generalization across taxa. Here, δ¹³C and δ¹⁵N 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 δ¹³C and δ¹⁵N across taxa, and the performance of 5 normalization models, overall and using subsets of species, to propose a model to revert lipid-extracted δ¹⁵N back to their bulk values, and to identify the best approach for dealing with lipid-related biases. Lipid extraction caused an uneven enrichment in δ¹³C and δ¹⁵N across species. Model taxonomic specificity increased estimation accuracy for both isotopes. While some models were the best at predicting δ¹³C_lipid-free, a linear model reliably estimated δ¹⁵N_bulk from δ¹⁵N_lipid-free values. This study presents a method for estimating reliably δ¹³C and δ¹⁵N values of muscle tissue without resorting to duplicate analyses. This represents a major step toward the harmonization of datasets generated using bulk and lipid-extracted samples.