Contributions of stable-isotope data to elucidating food webs of Mediterranean rocky littoral fishes

The food webs of rocky infra-littoral ecosystems in the Mediterranean have been little studied. In this investigation stable isotopes and dietary data were compared in an attempt to describe features of the food webs concerned. delta(13)C and delta(15)N were determined for plants, invertebrates and fishes from the Bay of Calvi, Corsica. Dietary data were derived from the literature. delta(13)C of plants ranged from -8.59 parts per thousand to -33.74 parts per thousand, of benthic invertebrates from -17.0 parts per thousand to -20.52 parts per thousand, of planktonic invertebrates from -20.08 parts per thousand to -22.34 parts per thousand and of fishes from -16.27 parts per thousand to -19.59 parts per thousand. delta(15)N was generally greater at higher trophic levels. delta(15)N Of plants was 0.95-2.92 parts per thousand, of benthic invertebrates 1.69-6.54 parts per thousand of planktonic invertebrates 3.51-6.82 parts per thousand and of fishes 4.63-9.77 parts per thousand. C-13 enrichment tended to be associated with benthic food chains and C-13 depletion with planktonic chains. Stable-isotope data suggested more varied diets for many species than implied by gut-contents data. Omnivory and trophic plasticity were widespread, and many consumers fed lower down the food chain than previous studies had suggested. Both stable-isotope and gut-contents analysis resolved differences between fishes feeding on planktonic and benthic prey and indicated that the herbivorous fish Sarpa salpa fed on a diet substantially different from that of other fishes. Zooplankton were important in the diets of several consumers (both; primary and secondary), as was plankton derived detritus. One species of fish previously identified as planktivorous was shown to feed largely on benthic organisms, whilst several species of benthic invertebrates may feed on plankton-derived detritus. Although herbivores seemed to obtain most of their C from macroalgae, delta(15)N data suggested that many of these animals supplemented their intake of N, although gut-contents analysis did not provide evidence for such uptake. The isotopic data have elucidated several features of the food web which we would not otherwise have detected.