The Community Trajectory Analysis Framework was used in conjunction with conventional methods to analyse, characterize, and represent long-term functional changes in soft sediment communities in the Rance basin (Normano-Breton Gulf) sampled in 1976, 1995, 2010, and 2020, in connection with the operation of the Rance Tidal Power Station (RTPP). The results of this study have shown a functional stability of assemblages in the downstream/median areas of the basin, driven by the absence of major changes in the dominance of species and the persistence of a common pool of structuring species. On the contrary, in the upstream area, the observed instability over time is largely explained by (i) marinisation caused by changes in the tidal prism and (ii) an increase of surficial sediments grain size in the main channel. Despite the changes observed in 2020, the functional diversity index indicates a relative functional stability at the basin scale, suggesting a high level of ecological resilience.