Effect of lipid extraction on analyses of stable carbon and stable nitrogen isotopes in coastal organisms of the Aleutian archipelago

Abstract

We tested whether extracting lipids reduced confounding variation in δ13C and δ15N values by analyzing paired lipid-extracted (LE) and non-lipid-extracted (NLE) samples of bald eagle ( Haliaeetus leucocephalus (L., 1766)) whole eggs, muscle tissue from nine seabird and one terrestrial bird species, muscle tissue from four marine fish species, and blue mussels ( Mytilus edulis L., 1758) collected from the Aleutian archipelago, Alaska. Lipid extraction significantly increased δ13C by an average of 2.0‰ in whole eggs, 0.8‰ in avian muscle, 0.2‰ in fish muscle, and 0.6‰ in blue mussels. Lower δ13C values in NLE samples covaried positively with lipid content across all sample types. Lower δ13C values in NLE samples were not correlated with lipid content within bald eagle eggs and blue mussels, but covaried positively with percent lipid in avian and fish muscles. Neither lipid extraction nor percent lipid significantly changed δ15N values for any sample type. Lower δ13C values in most NLE avian and fish muscle tissues should not confound interpretation of pelagic versus nearshore sources of primary production, but lipid extraction may be necessary when highly precise estimates of δ13C are needed. Lipid extraction may not be necessary when only δ15N is of interest.

Stable isotopes and trace elements as indicators of diet and habitat use in cetaceans: predicting errors related to preservation, lipid extraction, and lipid normalization

“…Other proposals, for instance, to not account for lipids when restricting analyses to a specific tissue, species, or ageclass (Sotiropoulos et al 2004) or when C:N ratio is < 3.5 (i.e. below ~5% lipids for aquatic species) or when lipid content is <15% (Ricca et al 2007), are also not supported by our study (Fig. 2).…”

“…Lipid extraction caused an enrichment of δ 13 C values that was within the range of those from a variety of tissues and aquatic organisms. The magnitude of 13 C depletion varied among studies, primarily as a result of fat content, with lipid-rich tissues generally showing the largest depletions (Todd et al 1997, Pinnegar & Polunin 1999, Kurle & Worthy 2002, Schlechtriem et al 2003, Sotiropoulos et al 2004, Kiljunen et al 2006, Murry et al 2006, Sweeting et al 2006, Bodin et al 2007, Ricca et al 2007, Smyntek et al 2007, Barrow et al 2008, Mintenbeck et al 2008. In these studies, the effects of lipid extraction on δ 15 N values were more variable but were generally toward a mean, not always significant, enrichment of ≤1.6 ‰, a range that encompassed values obtained in the present study.…”

“…Sotiropoulos et al 2004, Murry et al 2006, Søreide et al 2006. In general for animal species, the effects of lipid extraction (Δδ 13 C) increase as tissue becomes depleted in 13 C (Bodin et al 2007, the present study) or with lipid content (Tieszen & Boutton 1989, Focken & Becker 1998, Sweeting et al 2006, Ricca et al 2007, Smyntek et al 2007, Mintenbeck et al 2008. In turn, tissue lipid content is also positively correlated with C:N ratio (McConnaughey & McRoy 1979, Schmidt et al 2003, Sweeting et al 2006, Smyntek et al 2007, Mintenbeck et al 2008.…”

“…Errors of the magnitude observed in our study when not accounting for lipids are equivalent to or greater than the 1 ‰ trophic increment in δ 13 C values observed for marine food webs (DeNiro & Epstein 1978, Peterson & Fry 1987, Caut et al 2009) and so may introduce significant biases in interpretation of food-web structure and relative placement in the isotopic space (Murry et al 2006). Circumstances where lipid content or lipid extraction have little effect on isotope signatures and our interpretation of trophic ecology or habitat use are exceptions rather than the norm, although the significance of biases depend on the study questions (Sotiropoulos et al 2004, Ricca et al 2007). We therefore recommend extracting lipids or correcting for them systematically when interested in δ 13 C values unless their insignificance is unequivocally demonstrated for the tissue and species of interest.…”

“…However, recent studies on a variety of freshwater or marine invertebrates and vertebrates documented an enrichment in δ 15 N as a result of lipid extraction (e.g. Pinnegar & Polunin 1999, Sotiropoulos et al 2004, Murry et al 2006, Sweeting et al 2006, Mintenbeck et al 2008, although other studies documented negative or no effects of lipid extraction on δ 15 N values (Bodin et al 2007, Ricca et al 2007, Barrow et al 2008. The variability observed in the degree of N enrichment indicated the need to evaluate solvent effects on various tissues and species (Sotiropoulos et al 2004).…”