Effects of perturbations on the aquatic vegetation of regulated river.

Macrophytes of the River Rance were studied according to the sampling methods of the French Water Agencies (<<protocole MEV>>) : survey of 50 m length stations along the river gradient; the sectorisation of the river was done considering abiotic factors such as drainage order, slope, geology and pollution points. Spatio-temporal variations of the floristic communities were described through species richness, diversity and evenness indexes, macrophyte index and cover of the types and the biological groups. For its first application in regulated parts, MEV protocol gave quite a good result. Floristic structure changed according to the management of the hydroelectric dam of Rophemel. Upstream, chemical industrial and agricultural pollution modified the vegetation too. The more obvious modification occurred for the hydraulic perturbation. This response was obtained with the French macrophyte index too (Indice Biologique Macrophytique : IBM) : The low waters of September decreased the notes of the index. Several propositions are discussed to improve the index according to the English methodology developed trough the Mean Trophic Rank. Great macrophytes cover was observed at the low water level of the end of the summer. Perturbed parts were more affected. The balance between community structures gave a functional indication of the River Rance. The comparison of seasonal variations occurs to be a way of study of the biological response to perturbation. According to the great response of the vegetation downstream from the dam, perspectives are proposed in using regulated parts of the river as an in situ experimental tool to increase the understanding of ecological mechanisms.