We assessed the temporal changes in and the relationships between the structures of the macroinvertebrate communities and the environmental conditions of the French Rhône River (the river from Lake Geneva to the Mediterranean Sea) over the last 20 years (1985-2004). Multisite environmental and biological datasets were analysed using multiple CO-inertia analysis (MCOA) and Procrustean analysis. Changes in environmental conditions were mainly marked by an improvement in water quality between 1985 and 1991 and by an increase in water temperature from 1985 onwards due to climate change. Improvement in water quality seemed to delay changes in community structures under global warming. We then observed trends in community structures coupled with high temperatures and a decrease in oxygen content. Interestingly, we observed both gradual changes and rapid switches in community states. These shifts seemed coupled to extreme hydroclimatic events (i.e. pulse disturbances). Floods and the 2003 heatwave enhanced the development of eurytolerant and invasive taxa which were probably able to take advantage of gradual warming environmental conditions. Despite various site-specific "press" constraints (e.g. hydropower schemes, nuclear power plants), similar changes in community structures were observed along the French Rhône River. Such consistency in temporal processes at large geographical scales underlined the strength of hydroclimatic constraints on community dynamics compared to specific local disturbances. Finally, community structures did not show any sign of recovery, and their relative sensitivities to extreme hydroclimatic events seemed to increase with time. Thus, our results suggest that global changes may reduce the resilience of current community states.

Embankments of the last century and contemporary channelization and development of urban and industrial areas along the Rh6ne Valley have greatly changed the river. The impacts of these on three biological descriptors of the hydrosystem (benthic invertebrates, fish communities and aquatic birds) have been studied. A reduction in the morphological diversity of the regulated hydrosystem has caused a reduction in biological diversity. This system has become a slow potamic system as a consequence of the more uniform environmental conditions: the benthic macroinvertebrate fauna has become eurytopic and pollution tolerant, with very localized potamic species; fish communities are dominated by limnophilic cyprinids; and water bird communities are limited by the absence of typical species of fluvial areas, such as terns.

Multitable techniques are rarely used for investigating patterns in ecological data surveys despite their ability to deal with the spatial and/or temporal stability of assemblages. Based on a covariance optimisation criterion, Multiple Co-inertia analysis (MCOA) enables the simultaneous ordination of several tables. Such analysis allows the representation of the stable vs. unstable part of the assemblage structure in comparison to a reference derived from each table. We used MCOA on multiple time series of invertebrate sampling to show that synchrony in the temporal variability of communities can establish between geographically distant locations despite the spatial and temporal plasticity of the faunistic responses to long-term changes in environmental conditions.

In running water, the main natural factor of disturbance is hydraulic. Investigation of its biological impact is currently complicated by the increase of anthropogenic disturbances which tend to mask the natural functioning of rivers. Monitoring of the impacts of the artificial alterations undergone by the Middle Rhône River for some decades took place in the mid-1980s. Discharge and temperature could constitute the major events controlling the biological dynamics in terms of variations in species richness, diversity, and abundance. The between-years changes of the structure and diversity of macroinvertebrate communities were connected with the amplitude of discharge fluctuations. This was in accordance with the disturbance-diversity concept and it seems to confirm the intermediate disturbance hypothesis. The fish community mainly depended on hydroclimatic factors that influenced breeding success. Variations in nutrient input played a secondary role in controlling changes in some communities (particularly the periphyton). Therefore it appears that the study of long-term changes in river systems, including the dynamics of their biological communities, requires continuous observations and data collection that only medium-to longterm studies can provide, implying the setting up of ecological monitoring centres for the natural environment such as the Long Term Ecological Research (LTER) sites in United States. En eau courante, le principal facteur naturel de perturbation est d'ordre hydraulique. A l'heure actuelle, ce problème se trouve particulièrement compliqué par l'accroissement des perturbations anthropiques qui tendent à masquer le fonctionnement naturel des cours d'eau. Un suivi des impacts des perturbations anthropiques subies depuis plusieurs décennies par le Rhône Moyen a été mis en place au milieu des années 80. L'hydrologie et la température semblent les principaux événements régissant la dynamique biologique en terme de variation de la richesse spécifique, de la diversité et de l'abondance des différentes espèces. Les variations inter-annuelles de la structure et de la diversité des peuplements de macoinvertébrés sont ainsi liées à l'amplitude des fluctuations de débit. Ceci est tout à fait en accord avec le concept de perturbation-diversité et la théorie des perturbations moyennes. Pour sa part, le peuplement piscicole dépend des facteurs hydroclimatiques pour le succès de la reproduction. En second lieu, la dynamique des. apports nutritifs gouverne également l'évolution de certains peuplements (en particulier le periphyton). Dans ce cadre, l'étude de l'évolution des hydrosystèmes fluviaux et de la dynamique de leurs peuplements impliquent une continuité d'observation et de récolte de données que seule permet une surveillance à moyen ou long terme, nécessitant la mise en place de véritables observatoires écologiques du milieu naturel, tels que les sites du programme LTER (Long Term Ecological Research) aux Etats-Unis.