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1. In each of twenty-six week-long experiments, the colonization by macroinvertebrates of boxes of natural sediment in a stony stream was measured. The experiments took place between February and November 1992 and environmental conditions prevailing during the weeks (particularly discharge and temperature) differed widely. 2. Colonization rate also varied widely between the weekly experiments and was sensitive to discharge, temperature and background benthic density, depending on the taxa considered. 3. A 'mobility index' measured colonization rate independently of background benthic density. This index was most strongly and positively correlated with discharge for the abundant stonefly Leuctra nigra, the net-spinning caddis Plectrocnemia conspersa and the Chironomidae, perhaps indicating that their mobility is physically driven by the influence of flow on drift. 4. Mobility in a second stonefly (Nemurella pictetii) was greatest in summer, when flows were low but temperature was high. Peak mobility in this species occurred during an ochreous bloom on the stream bed. It is likely the mobility of N. pictetii is more active than the other taxa. 5. There were thus differences among taxa in their mobility at baseflow and in their susceptibility to flow fluctuations. The population consequences of differences in mobility among taxa are discussed. [ABSTRACT FROM AUTHOR]

1. Field experiments were carried out to determine whether animals move into areas of low shear stress during periods of peak flow and therefore avoid hydraulic disturbance. 2. Flow at the scale of 0.05 m2 patches was reduced experimentally by creating artificial 'refugia'. Invertebrate colonization of cages with 1.1 mm mesh sides,which provided such potential refugia by reducing hydraulic forces within the cage, was compared with colonization of cages with coarser (15 mm) mesh which did not restrict the flow. 3. Colonization of these cages was tested over a series of weekly periods in two different stream. Nine trials were completed in a stream with abundant natural flow refugia (Broadstone Stream, SE England), and during three of these trials strong spates occurred. Six trials were completed in a stream with comparatively few natural flow refugia. (Dargall Lane, SW Scotland), and peak flows were relatively less. 4. In Broadstone Stream, the relative colonization of refugium cages was greatest during periods of peak flow, suggesting animals had used these low-flow areas as refugia during spates. Use of the artificial refugia did not occur inDargall Lane, at least at the flows achieved during the trials. 5. Our results are consistent with the hypothesis that stream invertebrates accumulate in refugia during high flow disturbances. Whether theirdistribution among patches is altered by active or passive means remains unclear. [ABSTRACT FROM AUTHOR]