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RationaleNormalizing C-13 values of animal tissue for lipid content is necessary to accurately interpret food-web relationships from stable isotope analysis. To reduce the effort and expense associated with chemical extraction of lipids, various studies have tested arithmetic mass balance to mathematically normalize C-13 values for lipid content; however, the approach assumes that lipid content is related to the tissue C:N ratio.

MethodsWe evaluated two commonly used models for estimating tissue lipid content based on C:N ratio (a mass balance model and a stoichiometric model) by comparing model predictions to measure the lipid content of white muscle tissue. We then determined the effect of lipid model choice on C-13 values normalized using arithmetic mass balance. To do so, we used a collection of fish from Lake Superior spanning a wide range in lipid content (5% to 73% lipid).

ResultsWe found that the lipid content was positively related to the bulk muscle tissue C:N ratio. The two different lipid models produced similar estimates of lipid content based on tissue C:N, within 6% for tissue C:N values <7. Normalizing C-13 values using an arithmetic mass-balance equation based on either model yielded similar results, with a small bias (<1) compared with results based on chemical extraction.

ConclusionsAmong-species consistency in the relationship between fish muscle tissue C:N ratio and lipid content supports the application of arithmetic mass balance to normalize C-13 values for lipid content. The uncertainty associated with both lipid extraction quality and choice of model parameters constrains the achievable precision of normalized C-13 values to about +/- 1.0 parts per thousand. Published in 2015. This article is a U.S. Government work and is in the public domain in the U.S.A.