Abstract
In Revellata Bay Gulf of Calvi, Corsica, France, the sparid fish Sarpa salpa L. is the main macro‐consumer of Posidonia oceanica L. D elile leaf. Stomach contents were analysed and 13 C/ 12 C isotopic ratios were measured in fish muscle and potential food sources algae, P. oceanica leaf and its epiphytes to determine their relative contribution to the fish diet. S. salpa has an age‐related mixed diet: juveniles are plankton feeders, young, sub‐adults and adults are herbivorous, and, the older the individuals, the higher the relative contribution of P. oceanica to the diet. Our results and former studies of carbon stocks and fluxes in the P. oceanica bed of Revellata Bay have enabled an estimation of the general impact of S. salpa grazing on infralittoral communities: the studied species consumes 24 g C.m ‐2 ‐a ‐1 from P. oceanica leaf, 4.8 from epiphytes and 13 from epilithic algae. The fish net production and biomass turnover in that zone have been estimated to be 1.2gC.m ‐2 .a ‐1 and 1.5 a ‐1 , respectively.