A standard protocol for stable isotope analysis of zooplankton in aquatic food web research using mass balance correction models

Stable isotope analysis has become a crucial tool for aquatic food web ecologists, but a lack of methodological standardization hinders comparisons between studies. One methodological inconsistency in stable isotope food web research is the decision whether to extract lipids before stable isotope analysis. The depletion in zooplankton stable carbon isotope values ( delta C-13) due to fatty acid content and the accuracy of mathematical correction models designed to predict this depletion were examined for a range of zooplankton species from nine lakes of diverse size and productivity. Large differences of up to 5 parts per thousand observed between delta C-13 values of nonextracted and lipid-extracted zooplankton samples correlated with zooplankton fatty acid content. A mass balance delta C-13 correction model for fatty acid content using atomic C: N ratios and directly measured delta C-13 values of fatty acids accurately predicted ( R-2 = 0.95) lipid-extracted delta C-13 values for both copepod and cladoceran zooplankton. Researchers should use mass balance lipid corrections as an efficient method to eliminate bias in comparisons of zooplankton and fish delta C-13 values and allow their results to be more easily compared with other studies.