Three main intertidal macrofauna communities are identified in Saint-Brieuc Bay: a Macoma balthica community, a Tellina tenuis community; and a third community with Cerastroderma and Tellina tenuis. Physiological parameters such as filtration, respiration and excretion are examined, for the filter-feeding molluscs with the highest biomass (the cockle Cerastoderma edule, the tellinid Tellina tenuis, and the blue mussel Mytilus edulis), either through laboratory experiments, or through previous literature. These results are used together to build up an annual energy budget for the complete soft bottom community, and to observe trophic relations. Results show that the mussel population has a negative influence on the cockle beds that lie just above.