title: Fish tissue lipid-C:N relationships for correcting δ(13) C values and estimating lipid content in aquatic food-web studies.
abstract: BACKGROUND
  Normalizing δ(13) C 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 δ(13) C values for lipid content; however, the approach assumes that lipid content is related to the tissue C:N ratio.
  
  
  METHODS
  We 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 δ(13) C 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).
  
  
  RESULTS
  We 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 δ(13) C 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.
  
  
  CONCLUSIONS
  Among-species consistency in the relationship between fish muscle tissue C:N ratio and lipid content supports the application of arithmetic mass balance to normalize δ(13) C values for lipid content. The uncertainty associated with both lipid extraction quality and choice of model parameters constrains the achievable precision of normalized δ(13) C values to about ±1.0‰.
authors.affiliation: US EPA Office of Research and Development, National Health and Environmental Effects Research Laboratory, Mid-Continent Ecology Division, 6201 Congdon Blvd, Duluth, MN, 55804, USA.
authors.forename: Joel C
authors.identifier: http://orcid.org/0000-0002-5413-8799
authors.initials: JC
authors.lastname: Hoffman
authors.affiliation: US EPA Office of Research and Development, National Health and Environmental Effects Research Laboratory, Mid-Continent Ecology Division, 6201 Congdon Blvd, Duluth, MN, 55804, USA.
authors.forename: Michael E
authors.initials: ME
authors.lastname: Sierszen
authors.affiliation: US EPA Office of Research and Development, National Health and Environmental Effects Research Laboratory, Mid-Continent Ecology Division, 6201 Congdon Blvd, Duluth, MN, 55804, USA.
authors.forename: Anne M
authors.initials: AM
authors.lastname: Cotter
mesh_terms: D000818:Animals; D002247:Carbon Isotopes / Q000032:analysis*; D005399:Fishes / Q000378:metabolism*; D020387:Food Chain; D050356:Lipid Metabolism; D008055:Lipids / Q000737:chemistry*; D009132:Muscles / Q000737:chemistry / Q000378:metabolism; D009587:Nitrogen Isotopes / Q000032:analysis*
chemical_list: D002247:Carbon Isotopes; D008055:Lipids; D009587:Nitrogen Isotopes