1.
Stable isotope analyses coupled with mixing models are being used increasingly to
evaluate ecological management issues and questions. Such applications of stable isotope
analyses often require simultaneous carbon and nitrogen analyses from the same
sample. Correction of the carbon isotope values to take account of the varying content
of
13
C-depleted lipids is then frequently achieved by a lipid-normalization procedure
using a model describing the relationship between change in
δ
13
C following lipid
removal and the original C:N ratio of a sample. 2.
We evaluated the applicability of two widely used normalization models using
empirical data for muscle tissue from a wide range of fish and for aquatic invertebrates.
Neither normalization model proved satisfactory, and we present some modifications
that greatly improve the fit of one of the models to the fish muscle data. For invertebrates
we found no clear relationship between change in
δ
13
C following lipid removal and the
original C:N ratio. 3.
We also examined the effect of lipid-normalization on the output of a mixing model
designed to calculate the proportional contribution of prey items to the diet of a consumer.
Mixing model output was greatly influenced by whether prey or consumer values
alone or together were lipid-normalized and we urge caution in the interpretation of
results from these models pending further experimental evidence. 4.
Synthesis and applications
. We describe a revised lipid-normalization model that should
be applicable to a wide range of marine and freshwater fish species in studies applying
stable isotope analyses to ecological management issues. However, we strongly advise
against applying these kinds of lipid-normalization models to aquatic invertebrate data.
The interpretation of outputs from mixing models is greatly influenced by whether the
carbon isotope data have been lipid-normalized or not.