Temporal and spatial dynamics in large rivers: example of a long-term monitoring of the middle Rhone River

In running water, the main natural factor of disturbance is hydraulic. Inve stigation of its biological impact is currently complicated by the increase of anthropogenic disturbances which tend to mask the natural functioning o f rivers. Monitoring of the impacts of the artificial alterations undergone by the Middle Rhone River for some decades took place in the mid-1980s. Di scharge and temperature could constitute the major events controlling the b iological dynamics in terms of variations in species richness, diversity, a nd abundance. The between-years changes of the structure and diversity of m acroinvertebrate communities were connected with the amplitude of discharge fluctuations. This was in accordance with the disturbance-diversity concep t and it seems to confirm the intermediate disturbance hypothesis. The fish community mainly depended on hydroclimatic factors that influenced breedin g success. Variations in nutrient input played a secondary role in controll ing changes in some communities (particularly the periphyton). Therefore it appears that the study of long-term changes in river systems, including th e dynamics of their biological communities, requires continuous observation s and data collection that only medium- to long-term studies can provide, i mplying the setting up of ecological monitoring centres for the natural env ironment such as the Long Term Ecological Research (LTER) sites in United S tates.