Influence of side-arms on aquatic macroinvertebrate drift in the main channel of a large river

1. Drift from lotic and lentic side-arms to the main channel was studied over 2 years in different sections of the Upper Rhone River (France). The hypothesis of an influence of drift on the main channel was tested by measuring drift in the main river at sites downstream and upstream from the confluences with two side-arms, as well as in the two side-arms themselves. 2. Side-arms were differentiated from the main channel by having a typical composition of macrofauna with a high propensity to drift, particularly in spring-summer and during spates. Drift densities in side-arms averaged more than twice those measured in main channel sites, but these inputs did not significantly affect taxon richness and total drift density in the main channel. 3. Nevertheless, some 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 throughout the sampling period for the more lentic side-arm. The densities of two competitive filter feeders (Hydropsyche and Simuliidae) were significantly higher below the confluences than above, suggesting that side-arms were also contributing substantial amounts of seston to the main channel. 4. Therefore, side-arms influence the community structure of drifting macroinvertebrates in the Upper Rhone River, both directly through immigration-depending on the degree of connectivity to the river-and indirectly via food supply.