The Effects of Density, Sediment Particle Size and Velocity on Drift of Acroneuria abnormis (Plecoptera)

Abstract

A laboratory experiment was conducted to test the effects of population density, sediment type and flow rate on the drift of a stonefly. Three types of sediment patches were placed in artificial stream channels. A. abnormis (Neuman) individuals were introduced on these sediments in a range of population densities. Drift from the patches was greatest from gravel, intermediate from cobbles and least from stones. From all substrates drift increased with increasing benthic density. Current velocity differentially affected drift only from gravel. Analysis of variance showed that all main effects and interactions were highly significant. Drift was judged density-independent from gravel and density-dependent from cobbles and stones. Interference competition probably led to the density-dependent drift. The implications for proposed active versus passive mechanisms for drift are discussed. Passive mechanical removal seems to be an inadequate explanation for these findings.