Variance in isotopic signatures as a descriptor of tissue turnover and degree of omnivory

Abstract

Summary Diet analyses using C and N stable isotopes commonly focus on mean isotopic signatures; however, isotopic variance among individuals is likely to also contain useful information including details of omnivory. Changes in isotopic signature as a result of dietary shifts are not instantly manifest in the isotopic signature of consumer tissues, but lagged over a period of time required for equilibration. Tissue turnover times have not previously been described in terms of variance in isotopic signature among individuals, and variance among individuals following equilibration with a constant diet is limited. Temporal changes in δ 15 N and δ 13 C variance in juvenile European Sea Bass ( Dicentrarchus labrax ) muscle, heart and liver were monitored following a shift from a wild diet to two single‐source diets administered under seminatural conditions in captivity. Exponential decay functions of the standard deviation of δ 15 N and δ 13 C among individuals were used to model changes in variance over time. All tissues exhibited a similar rate of tissue turnover using variance. However, variance among individuals within tissue types differed once fishes were equilibrated with the laboratory diet. The coefficients of variation of δ 13 C and δ 15 N were smallest in muscle and greatest in liver and greater among sampling dates than within. Analysis of δ 15 N and δ 13 C in different tissues will not therefore provide equivalent power to detect differences in diet or to track changes in patterns of omnivory. Analysis of omnivory should be restricted to variance from a single tissue type. Of the tissues considered here, white muscle is most appropriate for this purpose. Variance estimates derived here provide minimum values expected for a highly specialist feeding population. Departure from these values can be used to describe the degree of omnivory within a population.