Response of stream macroinvertebrates in flow refugia and high scour areas to a series of floods: A reciprocal replacement study

Recreational releases from dams result in frequent floods (days vs. months) creating a static mosaic of scoured patches in high shear stress areas and patches of algae in low shear stress areas (refugia).  We examined macroinvertebrate community responses to these frequent floods in high scour and flow refugia areas using a reciprocal replacement experiment.  Sets of 34 trays filled with natural substrata were embedded in both high shear stress (HSS) and low shear stress (LSS) areas, and then 17 trays were exchanged between the two areas. 20 trays were sampled from each area after 2, 6 and 12 releases: 5 resident HSS trays, 5 resident LSS trays, 5 LSS trays moved to the HSS area, and 5 HSS trays to the LSS area.  Lower macroinvertebrate densities in LSS trays reflect losses of taxa (Isonychia and several net-spinning caddisflies) that require higher current velocities; these taxa were probably sensitive to decreased flow in LSS areas during periods following releases when low water levels/velocities occurred due to the slow rebound of the reservoir level.  Baetidae and Chironomidae densities varied between trays and dates; these taxa often recover quickly following floods in other studies, but in our study their numbers varied between HSS and LSS areas under the frequent flood regime.  Our results suggest that some macroinvertebrate taxa respond quickly (12-21 days) to changes in flow regimes either by drifting from HSS trays when placed in LSS areas or through colonization of LSS trays when placed in HSS areas.