Changes of the macroinvertebrate communities in the dammed and by‐passed sections of the French upper rhocne after regulation

Abstract

Abstract The macroinvertebrate communities of the French Upper Rhǒne have been studied since 1975. Three hydroelectric development schemes were erected in this part of the river between 1981 and 1984. The original component of these schemes is the construction of a diversion canal parallel to the old bed, which is closed by a dam and only receives a very reduced compensation flow. The changes of the macrofauna were studied until 1988. The 10 year study of the Chautagne scheme, the oldest scheme, pointed out important alterations in the benthic macro‐invertebrate fauna after the completion of this hydroelectric scheme. In the by‐passed section, hydrological patterns governed the presence or absence of species, whereas abundance was greatly influenced by substratum stability, food quality and quantity, development of aquatic macrophytes and periphyton. The low hydrological fluctuations in the reservoirs of run of the river schemes favoured limnophilic species, which were previously accidental. The long‐term survey of the three schemes (14 years) showed that the period (i.e. before or after the regulation) had a greater influence on the changes in the structure and functioning of the Upper Rhǒne macroinvertebrate communities than the location (upstream or downstream) or the type of environment (i.e. reservoir, by‐passed section) considered. No recovery of unregulated and/or upstream fauna appeared. The serial discontinuity concept was not supported because of the absence of abiotic (temperature) discontinuities and of large tributaries. However, the seasonal rhythm was not affected by the regulation.