The spatio-temporal variability of benthic communities was studied in a modified system whose main physical characteristics are controlled by humans: the Rance basin, with its tidal power station. Soft-bottom communities were sampled in 1976 and 1995 by a quantitative survey of 105 and 113 stations, respectively, to assess spatial and temporal changes, to determine factors involved in benthic assemblage changes and to lay a base for the management of this coastal area. A total of 205 and 240 species were recorded in 1976 and 1995, respectively. Ascendant hierarchical classification and factorial correspondence analysis identified 7 and 6 assemblages in 1976 and 1995, respectively. The main groups are: 1) the Amphioxus lanceolatus-Glycymeris glycymeris; 2) the Abra alba-Corbula gibba and 3) the oligospecific Macoma balthica communities. A large consortium of common species linked the main assemblages described in 1976 as in 1995. From comparison of their distribution and structure, local spatio-temporal changes were identified in response to modifications of the habitats due to 1) silting in the brackish sector; 2) the decrease in the mean low water level and 3) the proliferation of the mollusc Crepidula fornicata. As a consequence, species richness 1) decreased in the brackish part as a result of the ‘homogenisation’ of habitats induced by silting and 2) increased in parts of the basin through the proliferation of Crepidula fornicata and the apparent diversification of the associated habitat. However, despite reduction in species abundances, the structure of the identified assemblages between 1976 and 1995 remained stable at the scale of the Rance basin. This long-term relative stability seems to reflect stable environmental conditions. In the basin, tide is moderate with reduced range variations and benthic communities are preserved from the effects of swells. However, the new balance is entirely dependant upon the mode of the installation: by modifying water levels with appropriate time intervals, managers can chose to favour intertidal or subtidal communities according to their respective ecological interests for avifauna and flatfishes, local socio-economical preoccupations or recreational activities carried out in the basin.

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