Lipid extraction has little effect on the delta super(15)N of aquatic consumers

Proper application of stable isotopes (e.g., delta super(15)N and delta super(13)C) to food web analysis requires an understanding of all nondietary factors that contribute to isotopic variability. Lipid extraction is often used during stable isotope analysis (SIA), because synthesized lipids have a low delta super(13)C and can mask the delta super(13)C of a consumer's diet. Recent studies indicate that lipid extraction intended to adjust delta super(13)C may also cause shifts in delta super(15)N, but the magnitude of and reasons for the shift are highly uncertain. We examined a large data set (n = 854) for effects of lipid extraction (using Bligh and Dyer's [1959] chloroform-methanol solvent mixtures) on the delta super(15)N of aquatic consumers. We found no effect of chemically extracting lipids on the delta super(15)N of whole zooplankton, unionid mussels, and fish liver samples, and found a small increase in fish muscle delta super(15)N of ~0.4ppt. We also detected a negative relationship between the shift in delta super(15)N following extraction and the C:N ratio in muscle tissue, suggesting that effects of extraction were greater for tissue with lower lipid content. As long as appropriate techniques such as those from Bligh and Dyer (1959) are used, effects of lipid extraction on delta super(15)N of aquatic consumers need not be a major consideration in the SIA of food webs.