Fluxes of organic matter are the foundation of the functioning of ecosystems and the understanding of their origin, production and their uses by biological and ecological processes is therefore essential. In anthropized systems, such as coastal ecosystems, disruptions caused by human activities at different scales can mobilize a significant part of the organic matter, which is no longer available locally for natural processes. As many coastal and marine ecosystems, the megatidal bay of Saint-Brieuc (BSB) faces cumulated impacts of strong anthropogenic pressures, mainly eutrophication-related proliferation of green algae, invasive species (slipper limpet) and shellfish farming. To assess these cumulative impacts, this study performs a quantitative assessment of the food web using the mass balanced Ecopath model, at two spatial scales: the whole Bay vs its intertidal fraction. Models outputs demonstrate the importance of the spatial scale considered on conclusions drawn. The global model showed that invasive species constitute a non-negligible trophic impasse. The intertidal submodel evidenced the effects of both green algae (trophic impasse) and farmed mussels (export), i.e. main producers and consumers, respectively, as additional bottlenecks limiting the trophic transfer. The BSB is thus characterized by tidal flats approaching their productive carrying capacity and low trophic transfer, hindered by three main trophic impasses.