MICRODISTRIBUTIONS OF FILTER-FEEDING CADDISFLIES (INSECTA, TRICHOPTERA) IN A REGULATED ROCKY-MOUNTAIN RIVER

A year-round study was conducted in the Blue River, Colorado, U.S.A., to examine microdistributions of the filter-feeding caddisflies Arctop syche grandis and Brachycentrus occidentalis along a regulated stream environmental gradient. There was a well-defined longitudinal species replacement, with A. grandis reaching maximum densities 2.3 km below t he impoundment and declining downstream as B. occidentalis increased. Arctopsyche grandis was often the only caddisfly found near the dam. T he two species co-occurred at two downstream sites, with A. grandis nu merically dominant 5.2 km below the impoundment and B. occidentalis at the farthest downstream site (11 km). Near the dam, where densities o f filter-feeders were highest, there were significant relationships be tween individual rock surface area and the number of larvae during mos t sampling periods. Contrary to our predictions of selection of curren t-exposed microhabitats, both species occurred primarily on the bottom and lower lateral sides of rocks at all sites. Clumped distribution p atterns in the absence of spatial segregation by size class suggests c ommon microhabitat preferenda for species and instars. This use of sim ilar spaces indicates that competitive displacement may occur, partial ly explaining the longitudinal distribution patterns. At a macroscale the reservoir-induced environmental alterations clearly limit the dist ribution of organisms, especially at sites directly below the impoundm ent. As the stream regains its preimpoundment character farther downst ream, factors at a microscale may play a more important role in produc ing the distinct longitudinal distribution patterns.