Interactive effects of near-bed flow and substratum texture on the microdistribution of lotic macroinvertebrates

Abstract

The interactive effects of substratum texture and near-bed velocity on the small scale spatial distribution of lotic macroinvertebrates was investigated in laboratory flumes with variations in flow among microhabitat patches (∼0.015 to 0.035 m 2 ) There were three different substratum textures, coarse, intermediate and smooth, and three species with contrasting velocity preferences, Gammarus pulex, Oreodytes sanmarkii and Baetis rhodani. The number of individuals retained in the flumes was highest for B. rhodani and O. sanmarkii on the rough substratum, but maximum numbers of G. pulex occurred on the intermediate substratum texture. Microdistribution patterns with respect to near-bed flow differed among species and with substratum texture. Oreodytes sanmarki and G. pulex occurred predominantly in the low velocity areas on all substrata; only on the roughest substratum were nymphs of B. rhodani able to maintain position in fast velocities that typify their preferred microhabitat. These results suggest that local abundances of animal species in rivers and streams may be related to the mechanical problems of clinging to substrata that differ in texture as much as to an increase in resource availability with the physical complexity of the surface. Further, local abundance was related to the suitability of conditions within a microhabitat, but also to the unsuitability of adjacent areas which may act as barriers to dispersal at very small scales.