The impact of river regulation on the lotic macroinvertebrate communities of the lower rhǒne,France

Channelization and the construction of 12 dams on the Lower Rhǒne during the last 30 years increased the morphological and biological effects of 19th century embanking along the river. These trends were demonstrated on the main channel, i.e. the central axis of the alluvial floodplain, by the synchronic study of the benthic macrofauna of six lowhead hydroelectric impoundments. The variability of the most significant taxa was investigated by an ANOVA, and the data-samples matrix was processed with a Principal Components Analysis in order to analyse the communities organization and their significance. The benthic macroinvertebrate communities of the channel include some lentic potamic species topographically related with this kind of river (metapotamon), as well as eurytopic and pollution-tolerant species. Thus, the Lower Rhǒne has become a slow potamic river, in the same way as other large mid-European rivers. Flow velocity appears to be the most important structural factor in a large regulated river. The homogeneity of the macroinvertebrate communities and the absence of longitudinal typology show that, in a very intensely regulated river, the River Continuum Concept is too theoretical, and the disturbing effects of dams on longitudinal changes and species response to regulation must be taken into account.     

Environmental and biological responses of former channels to river incision: A diachronic study on the upper rhǒne river

The impacts of river incision should be a lowering of the river level, which would increase the rate of water flow from the hillslope aquifer to the river and its former channels. If this aquifer is nutrient-poor, it should favour the oligotrophication of the former channels. However, if the lowering of the water-table exceeds the depth of the former channels, then it should lead to the drainage of these channels. These hypotheses were tested on two former channels located in a degrading reach of the Rhǒne River (France); the former channels were influenced by floods, seepage water from the river and hillslope groundwater in 1989. The rate of river incision in this reach has increased since 1980 because of gravel extraction from the river bed (0.5 m between 1989 and 1993). Between 1989 and 1993, the reduction of river infiltration and the increase in the amount of water from the hillslope aquifer in the former channels were demonstrated by the decrease in the phosphate and ammonia content of the water, and its increased alkalinity and conductivity. The responses of aquatic vegetation to river incision depended on the vegetation zone investigated. Two floristic zones that were frequently flooded and species-poor in 1989 remained species-poor in 1993. Two floristic zones, which were species-rich and patchily organized, became dry or very shallow in 1993; as a consequence, aquatic vegetation disappeared, and was replaced by helophytes and terrestrial species. In the last vegetation zone, the increase in the amount of water coming from the hillslope aquifer caused the appearance and development of Chara hispida, an oligotraphent species. Unforeseen impacts of river incision were a decrease in the spatial and temporal heterogeneity of the former channels and lower aquatic macrophyte richness.     

Spring ecotone and gradient study of interstitial fauna along two floodplain tributaries of the river rhǒne,France

The hyporheic zone of two small tributaries of the upper Rhǒne River where they cross the floodplain (approximately 2 km wide) was studied. The streams originate in karstic areas which directly border the floodplain. The surveys were conducted at 11 sites along the course of the two tributaries at a depth of 50 cm in bed sediments. Ten litre samples were collected six times a year with a Bou-Rouch sampler. The paper examines the distribution of interstitial communities along two transverse (floodplain) profiles, defines the spring ecotone, analyses the heterogeneity, and underlines the importance of the tributaries in the functioning of the floodplain. A comparison between the spatial distributions of fauna and organic matter content along the two profiles underlines the important relationships between the groundwater and surface environment, and makes it possible to differentiate parts of the floodplain where the exchanges are most intense (interstitial spring, karst/floodplain interface). Along the longitudinal course of these streams, we observe the influence of groundwater circulation on community structure/i.e. the interactions between stream channel, alluvial aquifer and adjacent karstic aquifer.     

Juvenile fishes as functional describers of fluvial ecosystem dynamics: Applications on the river rhǒne,France

Biological indicators may signal changes in freshwater quality but offer little information on long-term trends in ecosystem succession. As an improvement over biological indicators in river management we propose the use of ‘functional describers’, abiotic and biotic elements that reflect alterations in ecosystem function and succession through changes in their concentration or abundance, including the appearance and/or extinction of organisms or groups of organisms. Juvenile (0 +) fishes have been found to be useful functional describers in many fluvial situations encountered in Europe, not least because the ontogeny of European fishes is strongly reflected in the type of habitat chosen for reproduction. Using the regulated River Rhǒne, France, as an example, the absence or presence of 0 + juveniles is shown to provide a direct reflection of a biotope's potential as a spawning area, particularly if recent hydrological conditions have not facilitated 0 + fish displacement and the lone presence of a predator species has not suggested the elimination of other fish species. Identification of the ecological function and successional state of a fluvial ecosystem is aided by consideration of species reproductive styles and tolerances. When time, labour, and/or funding is limited, the function and succession of numerous biotopes within a catchment or large fluvial system can be monitored cost effectively by analysing the 0 + fishes assemblages via the functional describer approach.     

Oligochaete worms as biological describers of the interactions between surface and groundwaters: A first synthesis

The paper focuses on the definition of the vulnerability of phreatic waters to surface water pollution. The amplification of water exchanges between surface and underground waters, because of human activities, is considered as a risk of contamination of aquifers. The aim of this paper is to present a first synthesis of oligochaetes as biological describers of these exchanges. Field investigations have shown that several rare and/or stygophilous taxa are found in surface (coarse) sediments of rivers upstream and/or downstream of dams or gravel operations. These observations relate to species such as Propappus volki, Pristina spp., Chaetogaster parvus, Rhyacodrilus carsticus, R. falciformis, Phallodrilus spp. The data suggest that these species invade surface waters from phreatic or interstitial waters when water flows are amplified between the two. Flow amplification is considered as a consequence of dams' and gravel operations' hydraulic impact. But the lack of hydraulic validation of the presumed active exchanges describer's (AED) ecological status given to those species remains a problem, giving the opportunity to examine the link between hydraulics and biology. Preliminary results on two gravel pits of La Wantzenau (Rhine alluvial valley, France) have shown relations between the distribution of oligochaetes, especially Lumbriculidae, and soil filtration properties. Two other studies were planned along an industrially polluted tract of the River Rhǒne (Grand Gravier Island). Horizontal and vertical zonation of the river banks were performed by studying simultaneously hydraulics and oligochaetes. Preliminary results have shown that the absolute and relative abundances of taxa such as Phallodrilus sp. and S. velutinus increase in relation to the permeability of superficial coarse substrate in the horizontal plane, and to the current velocity in underground porous media in the vertical plane. The increasing percentages of P. barbatus and the decreasing percentages of Tubificidae reveal active exchange zones where superficial waters are renewed by groundwaters. Some other species such as P. vejdovskyi seem to be potential describers of water exchanges.