Long-term changes of benthic communities of the Rance maritime basin

The monitoring and quantification of ecological changes following the construction of the Rance Tidal Power Plant (RTPP) have been extensively discussed in the past and provide us with a valuable reservoir of knowledge on restoration ecology. The Community Trajectory Analysis Framework was used in conjunction with conventional methods to analyse, characterize, and represent long-term changes in soft sediments in the Rance basin (Normano-Breton Gulf) sampled in 1976, 1995, 2010 and 2020. A total of 483 species have been identified since 1976 on soft bottoms. At the basin scale, a group of six species were among the most abundant in each study, revealing a strong impact of dominant species on community structuring in the basin. Benthic assemblages continue to structure themselves nearly 50 years after the construction of the RTPP. The taxonomic trajectories of benthic assemblages were studied at both the basin and local scales. Mainly in response to sedimentological changes, ecological trajectories showed temporal variability upstream and downstream. Local variations in species abundances also contributed to differences in composition observed at the assemblage scale. After a slowdown in the colonization of bottoms in the ‘90s, the number of species experienced a significant increase in 2020, linked to the progression of marine waters, sediment redistribution, the habitat heterogeneity, and the dynamics of seagrass, which have created complex habitat architecture. • Structure of benthic communities is always evolving regarding environmental condition changes. • Whereas it seemed stabilized, the number of species significantly increased during the last decade. • Benthic community structure changed in relation with marine influence, sediment dynamics, and habitat heterogeneity (with a particular importance of biological activity). • Recovering patterns mainly concerned subtidal stations located on the main channel downstream and intertidal stations upstream. • Departing patterns mainly relates to intertidal stations downstream and subtidal stations upstream.