How to account for the lipid effect on carbon stable-isotope ratio (δ13C):: sample treatment effects and model bias

This study investigated the impact of lipid extraction, CaCO3 removal and of both treatments combined on fish tissue delta C-13, delta N-15 and C:N ratio. Furthermore, the suitability of empirical delta C-13 lipid normalization and correction models was examined. delta N-15 was affected by lipid extraction (increase of up to 1.65 parts per thousand) and by the combination of both treatments, while acidification alone showed no effect. The observed shift in delta N-15 represents a significant bias in trophic level estimates, i.e. lipid-extracted samples are not suitable for delta N-15 analysis. C:N and delta C-13 were significantly affected by lipid extraction, proportional to initial tissue lipid content. For both variables, rates of change with lipid content (Delta C:N and Delta delta C-13) were species specific. All tested lipid normalization and correction models produced biased estimates of fish tissue delta C-13, probably due to a non-representative database and incorrect assumptions and generalizations the models were based on. Improved models need a priori more extensive and detailed studies of the relationships between lipid content, C:N and delta C-13, as well as of the underlying biochemical processes. (c) 2008 The Authors Journal compilation (c) 2008 The Fisheries Society of the British Isles.