Small-scale movements of lotic macroinvertebrates with variations in flow

1. The small-scale movements and distribution patterns of invertebrates were observed in an attempt to identify the various mechanisms by which organisms may use flow refugia during flow disturbances. The microdistribution of lotic macroinvertebrates was examined in two replicate, non-circulating laboratory flumes with variations in flow among microhabitat patches (approximate to 0.015-0.035 m(2)). The discharge in one experimental flume was manipulated to mimic spates and alter near-bed flow patterns; the other flume acted as a control. After an initial settling period, the position and behaviour of animals within the flumes was recorded before, during and after a simulated spate. Three species with contrasting flow microhabitat preferences and movement behaviour were examined. 2. At low discharge, the microdistribution of all three study species in flumes was broadly consistent with field observations. In the field, the optimum current speed was lowest for adults of the dytiscid beetle, Oreodytes sanmarkii, and highest for mayfly nymphs, Ephemerella ignita, with nymphs of the stonefly, Leuctra inermis, most abundant at intermediate velocities. In the flumes, O. sanmarkii occurred only in very low velocity areas, L. inermis occurred widely throughout the flumes with highest density in low velocity areas and E. ignita also occurred throughout the flumes, but maximum density was in moderately high velocity areas. 3. Increased discharge did not reduce the total number of individuals in experimental versus control flumes for any of the three species studied, although total numbers did decrease over the observation period in both treatments. Simulated spates resulted in a change in the microdistribution of O. sanmarkii and E. ignita, but not L. inermis, such that numbers were reduced in very high velocity microhabitats and animals accumulated in lower flow areas, analogous to flow refugia. These distributional shifts were attributed to movements of individuals among microhabitats. 4. Both active and passive modes of movement contributed to the accumulation of E. ignita and O. sanmarkii in low flow microhabitats (i.e. flow refugia). Some nymphs of E, ignita actively crawled from high to low flow microhabitats. Both species drifted between microhabitats. Drift entry could be active or passive, whereas regaining the substratum was active: O. sanmarkii swam down and E. ignita altered its body posture to promote sinking.