CHANGES OF THE MACROINVERTEBRATE COMMUNITIES IN THE DAMMED AND BY-PASSED SECTIONS OF THE FRENCH UPPER RHONE AFTER REGULATION

The macroinvertebrate communities of the French Upper Rhone have been studied since 1975. Three hydroelectric development schemes were erect ed in this part of the river between 1981 and 1984. The original compo nent of these schemes is the construction of a diversion canal paralle l to the old bed, which is closed by a dam and only receives a very re duced compensation flow. The changes of the macrofauna were studied un til 1988. The 10 year study of the Chautagne scheme, the oldest scheme , pointed out important alterations in the benthic macroinvertebrate f auna after the completion of this hydroelectric scheme. In the by-pass ed section, hydrological patterns governed the presence or absence of species, whereas abundance was greatly influenced by substratum stabil ity, food quality and quantity, development of aquatic macrophytes and periphyton. The low hydrological fluctuations in the reservoirs of ru n of the river schemes favoured limnophilic species, which were previo usly accidental. The long-term survey of the three schemes (14 years) showed that the period (i.e. before or after the regulation) had a gre ater influence on the changes in the structure and functioning of the Upper Rhone macroinvertebrate communities than the location (upstream or downstream) or the type of environment (i.e. reservoir, by-passed s ection) considered. No recovery of unregulated and/or upstream fauna a ppeared. The serial discontinuity concept was not supported because of the absence of abiotic (temperature) discontinuities and of large tri butaries. However, the seasonal rhythm was not affected by the regulat ion.