The effects of water velocity and sediment size on Acroneuria abnormis (Plecoptera: Perlidae) entrainment

Abstract

We investigated the roles that velocity and substrate size play in the drift of stonefly larvae (Acroneuria abnormis). Acroneuria were introduced into an experimental flume containing homogeneous substrates of different size (small gravel, large gravel, or cobble), or a mixture of various substrate sizes. For each substrate Acroneuria were subjected to three different velocities (0.3, 0.6 and 0.9 ms−1) corresponding to base flow conditions in real streams, and the number of individuals entrained into the water column was measured. Entrainment was nearly two times higher from the smallest substrates than larger substrates and the heterogeneous mixture of substrate sizes. Higher entrainment from the small particles appeared to result from lower availability of refugia among particles and movement of particles under the higher water velocities. Entrainment generally increased with increasing discharge on small and large, but not intermediate-sized, particles. Our results suggest there may be dynamic Acroneuria entrainment under low flow conditions in streams.