Dynamique spatio-temporelle des déplacements de macroinvertébrés dans une grande rivière

The movements of macroinvertebrates in the Rhône River were studied by using suspended artificial substrates that were immersed for 7 days at three depths near a bank and at the centre of the channel at Jons (27 km upstream of Lyon, France). Sampling was carried out monthly on an annual cycle, from December 1978 to March 1980. The 93 taxa found were distributed as follows: chironomid larvae (Diptera), 34.4%; Hydropsyche spp. (Trichoptera), 24.9%; Oligochaeta, 15.7%; Gammarus fossarum and (or) G. pulex (Crustacea), 15.0%; others, 10.0%. Correspondence analysis revealed and summarized the hierarchy of the spatio-temporal variations in fauna. The first level of variation is seasonal and shows contrasts between some summer species (e.g., Ephemerella ignita, Leuctra fusca) and the pool of organisms that subsist during winter. The second level of variation is due to seasonal hydrological conditions. The large flow rate during winter and at the beginning of the summer instigates the drift of rheolithophilous (e.g., Psychomyia pusilla) or limnophilous (Asellus aquaticus) taxa, which results in a more intense spatial homogenization of the fauna. The first increase in flow rate after a slower regime, although minor, has a "washing out" effect on the backwaters connected with the main stream (parapotamic area). On the other hand, the slower flow rate at the end of summer is favourable to some extent for some lentic taxa (Planaria), particularly near the banks. A third level of variation, less important, concerns transverse movements (between bank and channel) and the effects of emptying and cleaning the upstream reservoir.