LAUSR

Invertebrate drift is an important ecological process but factors affecting the downstream transport of invertebrates are difficult to assess. The influence of water velocity on drift entry rates is still unclear and has rarely been quantified. In this study, conducted in spring and autumn, we investigated the drift propensity for different sizes of Baetis, Simulium and Chironomus larvae which, over a 24-h period, were subjected to low, moderate or high water velocities (14, 30 and 40 cm s−1, respectively) in either fine or coarse gravel beds. Interspecific differences were detected: Baetis drifted the most, Chironomus showed intermediate values and Simulium drifted the least. Chironomus was the only taxon that responded to water velocity. Baetis and Simulium were only slightly constrained to drift by the experimental conditions, while, in contrast, Chironomus larvae were very sensitive to flow increase. Substrate size (fine/coarse gravel bed) had no effect on any taxa. Small Simulium and Chironomus drifted more than larger ones. A seasonal effect was also detected, with Baetis drifting more in fall and Simulium drifting more in spring. Results suggest that community structure may be as or more important than hydraulics as a determinant of drift rates.