FLOW REFUGIA AND THE MICRODISTRIBUTION OF LOTIC MACROINVERTEBRATES

Changes in the microdistribution of benthic macroinvertebrates were ob served, between patches identified as refugia from high flow and the r emaining benthic habitat, over a range of discharges in a small stream with prominent flow refugia. Total invertebrate abundances were simil ar across all patch types at baseflow, but local densities were higher in flow refugia after periods of high and fluctuating flow. Different species- and size-specific responses were observed: microdistribution s of larval Chironomidae and small nymphs of the stonefly Leuctra nigr a did not change with discharge. Conversely, the stonefly Nemurella pi ctetii and large nymphs of L. nigra were most abundant in refugia, rel ative to the remaining benthic habitat, during periods of high and flu ctuating discharge. The interactive effects of leaf litter and local f low conditions on species microdistributions were investigated. In som e seasons, some species may respond indirectly to hydraulic conditions through their direct response to litter mass. Observed microdistribut ion changes probably do not reflect very short-term reactions to indiv idual flow events, but perhaps a longer-term response to seasonal flow conditions. These are some of the first field data implicating the ro le of in-stream flow refugia in macroinvertebrate community structure.