Lipid removal and acidification affect nitrogen and carbon stable isotope ratios of beluga whales (Delphinapterus leucas) and their potential prey species in the Beaufort Sea ecosystem

Carbon and nitrogen stable isotope ratios are ecological tracers that can provide insights into the diets of marine mammals. As a generalist predator, beluga whales (Delphinapterus leucas) consume a variety of prey; however, differences in lipid content and the presence of inorganic carbon in prey may cause variability in the delta C-13 signal that is not related to food sources. We examined the effects of carbonate and/or lipid removal in beluga and potential prey and tested whether the C:N ratio was a valid indicator of lipid content. The C: N ratio was a good predictor of the change in delta C-13 after lipid removal in capelin (Mallotus villosus), octopus (Cirroteuthis muelleri), green shrimp (Argis dentata), and circumpolar eualid (Eualus gaimardii). Despite relatively low C:N ratios, lipid removal significantly increased delta C-13 values, but also affected delta N-15. Removal of carbonates from invertebrate samples significantly decreased delta C-13 values and had variable effects on delta N-15. Overall, the variability in delta C-13 within a species decreased after removing lipids and carbonates. Variability in delta N-15 did not change for species requiring only lipid removal, but increased after acidification. We also evaluated the effect of these sample preparation methods on niche dispersion metrics. After lipid and carbonate treatments, centroid locations differed significantly in all species except beluga muscle, and niche breadth and mean distance to the centroid decreased. Failure to remove lipids and carbonates for delta C-13 values may lead to incorrect interpretations for isotopic niche, which may have major ecological implications, such as predicting the impacts of invasive species or determining the dietary linkages of beluga whales.