Accounting for the effects of lipids in stable isotope (δ13C and δ15N values) analysis of skin and blubber of balaenopterid whales
C. Ryan, Marine Biodiversity Research Group, Department of Life Science, Galway-Mayo Institute of Technology, Galway, Ireland.
Stable isotope values (δ13C and δ15N) of darted skin and blubber biopsies can shed light on habitat use and diet of cetaceans, which are otherwise difficult to study. Non-dietary factors affect isotopic variability, chiefly the depletion of 13C due to the presence of 12C-rich lipids. The efficacy of post hoc lipid-correction models (normalization) must be tested.
For tissues with high natural lipid content (e.g., whale skin and blubber), chemical lipid extraction or normalization is necessary. C:N ratios, δ13C values and δ15N values were determined for duplicate control and lipid-extracted skin and blubber of fin (Balaenoptera physalus), humpback (Megaptera novaeangliae) and minke whales (B. acutorostrata) by continuous-flow elemental analysis isotope ratio mass spectrometry (CF-EA-IRMS). Six different normalization models were tested to correct δ13C values for the presence of lipids.
Following lipid extraction, significant increases in δ13C values were observed for both tissues in the three species. Significant increases were also found for δ15N values in minke whale skin and fin whale blubber. In fin whale skin, the δ15N values decreased, with no change observed in humpback whale skin. Non-linear models generally out-performed linear models and the suitability of models varied by species and tissue, indicating the need for high model specificity, even among these closely related taxa.
Given the poor predictive power of the models to estimate lipid-free δ13C values, and the unpredictable changes in δ15N values due to lipid-extraction, we recommend against arithmetical normalization in accounting for lipid effects on δ13C values for balaenopterid skin or blubber samples. Rather, we recommend that duplicate analysis of lipid-extracted (δ13C values) and non-treated tissues (δ15N values) be used. Copyright © 2012 John Wiley & Sons, Ltd.