Functional Ecology 2005 19, 777–784 © 2005 British Ecological Society 777 Blackwell Publishing, Ltd. Variance in isotopic signatures as a descriptor of tissue turnover and degree of omnivory C. J. SWEETING,*†‡ S. JENNINGS§ and N. V. C. POLUNIN‡ ‡School of Marine Science and Technology, University of Newcastle, Newcastle Upon Tyne NE1 7RU, UK, and §The Centre for Environment, Fisheries and Aquaculture Science, Lowestoft Laboratory, Pakefield Road, Lowestoft NR33 OHT, UK Summary 1. Diet analyses using C and N stable isotopes commonly focus on mean isotopic sig- natures; however, isotopic variance among individuals is likely to also contain useful information including details of omnivory. 2. 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. 3. Temporal changes in δ 15 N and δ 13 C variance in juvenile European Sea Bass (Dicen- trarchus 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 indi- viduals were used to model changes in variance over time. 4. 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. 5. 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. 6. 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. Key-words: Fractionation, generalist, niche, specialist Functional Ecology (2005) 19, 777–784 doi: 10.1111/j.1365-2435.2005.01019.x Introduction C and N stable isotopes are used by ecologists to inves- tigate differences in feeding patterns or habitat utiliza- tion by consumers in time and space (Hansson et al. 1997; Hobson 1999; Kurle & Worthy 2001; Carmichael et al . 2004). Stable isotopes have been applied to inves- tigations of migration (Hansson et al . 1997), settlement (Herzka, Holt & Holt 2002), size-related dietary shifts (Badalamenti et al . 2002) and seasonal oscillations of prey abundance or isotopic signature through differ- ences in dietary or basal food-web isotopic signatures (Goering, Alexander & Haubenstock 1990; Altabet et al . 1999; Vizzini & Mazzola 2003). Such analyses often assume a large constant trophic step enrichment of δ 15 N, which can be used to define trophic level, and a smaller constant trophic step enrichment of δ 13 C, which can be used to define basal sources of produc- tivity (Gannes, O’Brien & del Rio 1997). However consumers typically vary their diet over time and 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 (Fry & Arnold 1982; Hobson & Clark 1992). Thus isotopic signatures at any point in time may reflect neither the current nor †Author to whom correspondence should be addressed. E-mail: christopher.sweeting@ncl.ac.uk *Present address: CNR-IAMC, Laboratorio di Biologia, via G. Da Verrazzano 17, 91014 Castellammare del Golfo (TP) Sicily.