INFLUENCE OF SIDE-ARMS ON AQUATIC MACROINVERTEBRATE DRIFT IN THE MAINCHANNEL OF A LARGE RIVER

1. Drift from lotic and lentic side-arms to the main channel was studi ed over 2 years in different sections of the Upper Rhone River (France ). The hypothesis of an influence of drift on the main channel was tes ted by measuring drift in the main river at sites downstream and upstr eam from the confluences with two side-arms, as well as in the two sid e-arms themselves. 2. Side-arms were differentiated from the main chan nel by having a typical composition of macrofauna with a high propensi ty to drift, particularly in spring-summer and during spates. Drift de nsities in side-arms averaged more than twice those measured in main c hannel sites, but these inputs did not significantly affect taxon rich ness and total drift density in the main channel. 3. Nevertheless, som e taxa found in the more lotic of the two side-arms were more abundant downstream of the confluence than upstream, indicating that they had drifted into the main channel; no such direct effect was found through out the sampling period for the more lentic side-arm. The densities of two competitive filter feeders (Hydropsyche and Simuliidae) were sign ificantly higher below the confluences than above, suggesting that sid e-arms were also contributing substantial amounts of seston to the mai n channel. 4. Therefore, side-arms influence the community structure o f drifting macroinvertebrates in the Upper Rhone River, both directly through immigration-depending on the degree of connectivity to the riv er-and indirectly via food supply.