A steady-state energy flow model is proposed for the benthic fine sand community of the Bay of Saint-Brieuc. The main objective is to assess the relative importance of benthic processes associated with the carbon cycle in an eutrophicated bay. Available data include biomass estimates of the major trophic groups and measurements of the most critical fluxes : particulate organic carbon sedimentation, community respiration, microbial production. A balanced annual carbon budget is estimated using an inverse method, particularly appropriate in food web research where the number of unknowns is far higher than the number of independent observations. The method offers the possibility of taking dynamical constraints into account, by introducing global flow measurements, for example, and of reducing the initial uncertainty in the fluxes. The annual carbon budget provided by the model outlines the major features of the flows of the carbon cycle. The potential supply of energy (328 g C m-2 y-1) is significantly higher than the total demand of the benthic consumers. So a large fraction (46%) of the total carbon input is not used by the benthic food web. Then the role of this eutrophicated bay as potential source of carbon for the adjacent sea is discussed.