Résumé: The food web of two intertidal seagrass (Zostera marina and Zostera noltii) beds that may be influenced by the seasonal variation in food source abundance was studied in winter and in summer with delta(13)C and delta(13)N analysis. In spite of high relative variation of abundance of main primary producers at the two sites, the food web did not vary between winter and summer. The delta(13)C range of primary producers was wide. Zostera leaves, the most (13)C-enriched source, were not consumed directly by grazers. Deposit and filter feeders have a similar delta(13)C and could use a mix of suspended and sedimented organic particulate matter, largely composed of detritus from macroalgae to seagrass. This trophic pathway allows the local incorporation of the high biomass produced by seagrasses. The wide delta(15)N range of predators was linked either to a large variety from omnivore to carnivore predators or to the also wide ranges of delta(15)N of primary consumers.