Effects of chemical lipid extraction and arithmetic lipid correction on stable isotope ratios of fish tissues

Abstract

For accurate interpretation of fish trophodynamics from carbon stable isotope data it is necessary to extract tissue lipids. This is because lipid content varies within and among tissues in both space and time, and because lipids are 13C-depleted relative to proteins. However, lipid extraction may affect delta15N, thus requiring costly and time-consuming separation of delta13C and delta15N analyses. These problems have prompted the development of arithmetic correction techniques for delta13C, but the techniques and their underlying assumptions have not been systematically tested. This study compared the effects of lipid extraction and arithmetic correction techniques on delta13C and delta15N of European sea bass (Dicentrarchus labrax) tissues. Following Folch lipid extraction from muscle and liver, there was a mean increase in delta15N of 0.77 per thousand, but enrichment varied with lipid content such that effects on delta15N were hard to predict. Changes in delta13C and C:N between untreated and lipid-extracted samples reflected the quantity of lipid removed. The arithmetic correction techniques of mass balance and lipid correction were sensitive to the C:N of the lipid-extracted tissue and to the assumed depletion of lipid delta13C relative to protein delta13C. However, the mass balance approach was appropriate for the mathematical correction of bulk tissue data in most circumstances, provided that the C:N of lipid-extracted tissue could be determined for a small proportion of samples. Application of mass balance arithmetic correction can lead to significant time and cost savings in trophodynamic studies, because the majority of delta13C and delta15N analyses would not need to be run separately.

Linking pollutant exposure of humpback whales breeding in the Indian Ocean to their feeding habits and feeding areas off Antarctica

“…Lipid content may be variable among samples and thus the potential influence of lipid content on d 13 C values in bulk tissue must be considered (Ryan et al, 2012). Therefore, lipid extraction is often recommended when the C:N ratio is too high (because of lipid content) to apply an equation for lipid normalization (Post et al, 2007;Ryan et al, 2012 (Lesage et al, 2010;Ryan et al, 2012;Sweeting et al, 2006). Lipids were discarded by rinsing skin samples with chloroform:methanol (2:1, v/v).…”

Mercury bioaccumulation in the food web of Three Gorges Reservoir (China): Tempo-spatial patterns and effect of reservoir management

“…Freeze-dried biota samples for δ 13 C analysis were treated with 1 N HCl to remove carbonate, followed by treatment with 1% dichloromethane in methanol to eliminate lipids (Sweeting et al, 2006). Samples for δ 15 N analysis did not undergo any further treatment after lyophilization.…”

Use of carbon and nitrogen stable isotopes to study the feeding ecology of small coastal cetacean populations in southern Brazil

“…There are several equations or mass balance approaches that have been developed to correct for lipid content (Fry et al 2003, Post et al 2007, Sweeting et al 2006, Abrantes et al 2012. However, the equation developed by Post et al (2007) is simple because it depends exclusively on the C:N ratios of tissues, having shown to be very applicable when tested in lipid-rich salmonids (Abrantes et al 2012).…”