6I5N and 6°C were determlned for plants invertebrdtes and f~s h e s collected from 3 s~t e s on the southern coast of the Mediterranean Island of Mallorca Spdln The sites were separated by distances of 1250 to 3750 m The mean Si5N of plants was 1 1 to 4 1 " W , , benthic Invertebrates 5 9 to 6 g':%) planktonic Invertebrates 5 5 to 5 8% and fishes 8 4 to 13 8%" 6"N became ennched with increasing trophic level The mean 6I3C of plants was -11 4 to -16 3'Xo benthlc invertebrates -14 8 to -16 8'X,

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... anktonlc Invertebrates -19 3 to -19 8 O W b and fishes -16 1 to -19 2"y) There were s~g n~f i c a n t d~f f e iences in the lsotopic c o m p o s~t~o n of ~ndividual specles w~t h i n the plant invertebrate 01 flsh groupings at each slte and there were s~g n~f i c a n t d~fferences in the ~s o t o p~c composit~on of the same specles at dlfferent sites Depleted ' < C was assoc~ated w~t h b e n t h~c food chalns and enrlched "C w~t h planktonlc chalns The data suggest that b e n t h~c food chalns are Important to the rocky reef assoclated f~s h e s studled as might be expected In a nutrlent poor system where planktonlc product~on IS relatively low Hoivever, the vanance In 6I1C composltlon between sltes was such that the r e l a t~v e s~g n~f l c a n c e of the 2 pdthways could not be determlned I5N measurements indicated that some of the flsh species studled had adopted d~fferent feedlng strategies at d~fferent sites and, as a result ~n d l v~d u a l s of the same species could sometimes be a s s~g n e d to different t r o p h~c groups at different sltes The data suggest that these f~s h e s e x h i b~t plast~clty in t h e~r feedlng strateg~es and this may p r o v~d e them wlth greater adaptlve flexibll~ty to respond to s~t e -s p e c~f~c changes In food avallabllity Moreover the data p r o v~d e emplrlcal support for current theones of food web dynamlcs w h~c h suggest that trophlc levels a r e dynamlc rather than flxed and that multlchannel omnivory IS an Important feature of food webs KEY WORDS Stable lsotopes T r o p h~c interact~ons Variablllty Mediterranean Reefs Feeding strategy O Inter-Research 1997 Resale of full artlcle not permitted