SHORT NOTE 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 Corinne Pomerleau • Gesche Winkler • Akash Sastri • R. John Nelson • William J. Williams Received: 19 December 2013 / Revised: 26 June 2014 / Accepted: 30 June 2014 Ó Springer-Verlag Berlin Heidelberg 2014 Abstract Stable isotope ratios of carbon (d 13 C) in zoo- plankton are widely applied in ecosystem-level studies of energy flux and trophic interactions. Carbonate (CaCO 3 ) and lipid content are highly variable both among and within zooplankton species. Such variability can arise from the d 13 C-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 acid- ification is poorly understood and applied in an inconsistent manner. Here, we investigated the effect of lipid extraction and sample acidification (CaCO 3 removal) on d 13 C 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, d 13 C can be corrected for carbonate content using the linear equation developed in this study. In contrast, the d 13 C for all species was signif- icantly enriched by a combination of lipid extraction and acidification (up to ?4.9 %) prior to stable isotope ana- lysis. Our data were used to develop simple, predictive species-specific correction models for lipid-acid-normal- ized d 13 C using C:N and/or untreated d 13 C values. Our results indicate that the d 13 C 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 d 13 C lipid-normalization models for accurate determination of carbon sources and dynamics for arctic and sub-arctic marine zooplankton. Keywords Acidification  Arctic  Carbon  Carbonates  Lipid extraction  Stable isotopes  Zooplankton Introduction Zooplankton is a key component of marine ecosystems as they transfer energy from primary producers to higher trophic levels. They also play a major role in carbon and nutrient cycles and influence ecosystem structure and function (Keister et al. 2012). Ratios of carbon stable isotopes (d 13 C) are often used to study energy pathways in food webs and ecosystems (Peterson and Fry 1987; Dalerum and Angerbjo ¨rn 2005; McMahon et al. 2013). The d 13 C of organisms is indicative of the carbon source(s) at the base of food webs because it is only moderately enriched with trophic level ( \ 1.0 %; Post 2002; Caut et al. 2009). However, correct interpreta- tion of d 13 C must also account or normalize for in vivo variability introduced by physiological partitioning of Electronic supplementary material The online version of this article (doi:10.1007/s00300-014-1540-8) contains supplementary material, which is available to authorized users. C. Pomerleau (&)  W. J. Williams Institute of Ocean Sciences, Fisheries and Oceans Canada, 9860 West Saanich Road, Sidney, BC V8L 4B2, Canada e-mail: corinne.pomerleau@dfo-mpo.gc.ca G. Winkler Universite ´ du Que ´bec a ` Rimouski, 310 Alle ´e des Ursulines, Rimouski, QC G5L 3A1, Canada A. Sastri Ocean Network Canada, P.O. Box 1700 STN CSC, Victoria, BC V8W 2Y2, Canada R. J. Nelson University of Victoria, 3800 Finnerty Road, Victoria, BC V8W 3N5, Canada 123 Polar Biol DOI 10.1007/s00300-014-1540-8