Invertebrate drift and benthic exhaustion during disturbance: Response of mayflies (Ephemeroptera) to increasing shear stress and river-bed instability

We manipulated hydraulic conditions over patches of sands and fine gravels in an upland, gravel-bed river using a portable flume and assessed changes in the drift rates of three mayflies (Baetis, Ecdyonurus and Caenis). On average, manipulations doubled water velocity in experimental locations and resulted in shear stresses that exceeded the critical threshold for entrainment of patch sediments. Bedload transport rates were used as an indicator of bed disturbance during the manipulations. Partial Least Squares Regression (PLSR) indicated that between 59 and 81% of the variation in the drift rates of the three taxa could be accounted for by the combined effects of shear stress and bed disturbance. Shear stress was more important than bed disturbance as a cause of drift in Ecdyonurus and Baetis, while for Caenis the two variables were equally important. Drift rates fell markedly (10-fold) after only 5¿mins of increased shear stress and bed disturbance. The experiments indicate that (a) dramatic loss of mayflies can occur rapidly from patches of fine and relatively unstable sediment during periods of increased discharge and (b) for some taxa, losses may cause a depletion of the benthic source-pool and hence result in the exhaustion of supply.