Tissue-specific isotope turnover and discrimination factors are affected by diet quality and lipid content in an omnivorous consumer

Tissue stable isotopes can be used for dietary reconstruction provided that the factors influencing turnover rates and trophic discrimination factors (TDFs) between consumer tissues and diet are known. This experiment quantified δ13C and δ15N dynamics in muscle and liver tissues in lab-reared juvenile marine fish after a switch from a high quality control diet to medium and low quality diets with decreasing protein and lipid contents. Turnover of δ15N in the liver was strongly influenced by metabolism, equilibrating 3 × faster compared to muscle for both diets. Nitrogen TDFs were dependent on diet quality, with values ranging from 3.0-6.5‰ in the muscle and 1.5-3.0‰ in the liver. The effects of mathematical lipid correction on δ13C turnover and discrimination were examined by developing novel empirical equations involving C:N ratios and lipid δ13C values. Lipid correction affected estimates of isotope turnover in the low quality diet treatment, with lipid-corrected muscle carbon isotopes equilibrating to diet 2 × faster than non-corrected muscle, due to lipid retention increasing turnover estimates for non-corrected values. Conversely, lipid-corrected liver half-lives were 4.4 × slower than non-corrected liver because lipid catabolism increased turnover rates for non-corrected values. A shift in control fish liver carbon TDFs of 2.1 and 1.7‰ for non-corrected and lipid-corrected values, respectively, between the beginning and end of the experiment was attributed to a 32% increase in lipid content. These results demonstrate that metabolic routing of lipid macromolecules strongly influences tissue-specific turnover, and is important to consider when reconstructing trophic dynamics.