FIELD EXPERIMENTS ON FLOW REFUGIA IN STREAMS

1. Field experiments were carried out to determine whether animals mov e into areas of low shear stress during periods of peak Bow and theref ore avoid hydraulic disturbance. 2. Flow at the scale of 0.05 m(2) pat ches was reduced experimentally by creating artificial 'refugia'. Inve rtebrate colonization of cages with 1.1 mm mesh sides, which provided such potential refugia by reducing hydraulic forces within the cage, w as 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 streams. Nine trials were completed in a stream with abundant natural flow refugia (Broadst one Stream, SE England), and during three of these trials strong spate s occurred. Six trials were completed in a stream with comparatively f ew natural Bow refugia (Dargall Lane, SW Scotland), and peak flows wer e 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 o f the artificial refugia did not occur in Dargall Lane, at least at th e flows achieved during the trials. 5. Our results are consistent with the hypothesis that stream invertebrates accumulate in refugia during high flow disturbances. Whether their distribution among patches is a ltered by active or passive means remains unclear.