The effect of acidification and the combined effects of acidification/lipid extraction on carbon stable isotope ratios for sub-arctic and arctic marine zooplankton species

Stable isotope ratios of carbon (δ13C) in zooplankton are widely applied in ecosystem-level studies of energy flux and trophic interactions. Carbonate (CaCO3) and lipid content are highly variable both among and within zooplankton species. Such variability can arise from the δ13C-depleted nature of lipids as well as differences in carbon incorporation among tissues (e.g., relative amount of carbonate). Critically, the impact of the common lipid- and carbonate-normalization steps of extraction and acidification is poorly understood and applied in an inconsistent manner. Here, we investigated the effect of lipid extraction and sample acidification (CaCO3 removal) on δ13C in sub-arctic and arctic marine zooplankton species. Our results indicate that, with the exception of the shelled mollusc Limacina helicina, acidification of samples can be omitted for all other marine zooplankton considered in this study. In the case of L. helicina, δ13C can be corrected for carbonate content using the linear equation developed in this study. In contrast, the δ13C for all species was significantly enriched by a combination of lipid extraction and acidification (up to +4.9 ‰) prior to stable isotope analysis. Our data were used to develop simple, predictive species-specific correction models for lipid-acid-normalized δ13C using C:N and/or untreated δ13C values. Our results indicate that the δ13C value for all species, including those with lower C:N ratios (~3–4), should be corrected for lipid content. We recommend lipid extraction whenever possible, or else the use of species/taxon-specific δ13C lipid-normalization models for accurate determination of carbon sources and dynamics for arctic and sub-arctic marine zooplankton.