A HIERARCHICAL STUDY OF MACROINVERTEBRATE RECOLONIZATION OF DISTURBEDPATCHES ALONG A LONGITUDINAL GRADIENT IN A PRAIRIE RIVER

1. Macroinvertebrate densities and community composition were examined at three spatial scales after substratum disturbance; among reaches a long a longitudinal gradient, within reaches and within plots. Reaches consisted of sandstone outcrops that were separated by approximately 2 km of highly mobile sandy silt substratum. 2. Substrata were disturb ed by scraping sandstone plots (0.3 x 0.3 m). Body-sized depressions c reated by Trichoptera in the sandstone were removed along with the upp er 5 mm of sandstone, resulting in areas of newly exposed, smooth sand stone. 3. The spatial scale of examination determined whether patterns of macroinvertebrate distribution and densities were discernible. Ini tially there were no significant differences in community composition or total densities among reaches or among upstream/ downstream locatio ns within reaches. Following substratum disturbance and 30 days recolo nization, total macroinvertebrate densities did not differ significant ly between undisturbed plots and disturbed plots. However, densities o f Petrophilia (Pyralidae: Lepidoptera) differed along the longitudinal gradient and the Simuliidae had its highest density in the upstream r each. Significant differences were found in total macroinvertebrate de nsities between the upstream and the downstream halves of disturbed pl ots, with higher densities occurring in the downstream portions. 4. Re colonized plots had similar macroinvertebrate densities and community composition to undisturbed plots, suggesting that the stream community was highly resilient.