SPATIAL VARIATION IN THE N-15 AND C-13 STABLE-ISOTOPE COMPOSITION OF PLANTS, INVERTEBRATES AND FISHES ON MEDITERRANEAN REEFS - IMPLICATIONSFOR THE STUDY OF TROPHIC PATHWAYS

delta(15)N and delta(13)C were determined for plants, invertebrates an d fishes collected from 3 sites on the southern coast of the Mediterra nean island of Mallorca, Spain. The sites were separated by distances of 1250 to 3750 m. The mean delta(15)N of plants was 1.1 to 4.1 parts per thousand, benthic invertebrates 5.9 to 6.9 parts per thousand, pla nktonic invertebrates 5.5 to 5.8 parts per thousand and fishes 8.4 to 13.8 parts per thousand. delta(15)N became enriched with increasing tr ophic level. The mean delta(13)C of plants was -11.4 to -16.3 parts pe r thousand, benthic invertebrates -14.8 to -16.8 parts per thousand, p lanktonic invertebrates -19.3 to -19.8 parts per thousand and fishes - 16.1 to -19.2 parts per thousand. There were significant differences i n the isotopic composition of individual species within the plant, inv ertebrate or fish groupings at each site and there were significant di fferences in the isotopic composition of the same species at different sites. Depleted C-13 was associated with benthic food chains and enri ched C-13 with planktonic chains. The data suggest that benthic food c hains are important to the rocky reef associated fishes studied, as mi ght be expected in a nutrient poor system where planktonic production is relatively low. However, the variance in delta(13)C composition bet ween sites was such that the relative significance of the 2 pathways c ould not be determined. N-15 measurements indicated that some of the f ish species studied had adopted different feeding strategies at differ ent sites and,as a result, individuals of the same species could somet imes be assigned to different trophic groups at different sites. The d ata suggest that these fishes exhibit plasticity in their feeding stra tegies and this may provide them with greater adaptive flexibility to respond to site-specific changes in food availability, Moreover, the d ata provide empirical support for current theories of food web dynamic s which suggest that trophic 'levels' are dynamic rather than fixed an d that 'multichannel omnivory' is an important feature of food webs.