STREAM HYDRAULICS AND THE DISTRIBUTION OF MICROCRUSTACEA - A ROLE FORREFUGIA

1. The relationship between microcrustacean abundance and flow habitat in three contrasting reaches of a single stream was examined. 2. Thre e methods of characterizing stream hydraulics (shear stress frequency distributions, Froude numbers and mean reach velocity) showed the same pattern among the three reaches, which were subsequently termed 'fast ', 'slow' and 'medium'. 3. Distributions of epibenthic microcrustacea matched flow characteristics of the three reaches, being most abundant in the slow reach and least abundant in the fast reach. In contrast, densities of three out of four species of interstitial Harpacticoida d id not vary significantly with reach, and none of the parameters measu red explained much of their between-patch variation in density. Hydrau lic conditions at the substratum surface may not affect the distributi on of species with interstitial habits. 4. Patterns in the abundance o f organic matter (a possible food source) were examined. Measures of o rganic matter were highly correlated with hydraulic variables, with de tritus being most abundant in the slow reach and least in the fast rea ch. Despite their strong response to reach, local densities of most mi crocrustacea (including many epibenthic species) did not vary with org anic matter. Food for detritivorous microcrustacea may not be limiting in this highly retentive stream. 5. There was some evidence to sugges t that different numbers of animals were retained in the three reaches during flow disturbances. One reach may function as a 'flow refugium' . Models illustrating the processes by which lotic organisms may use f low refugia are presented and provide a framework for future empirical and theoretical studies.