Microdistributions of filter-feeding caddisflies (Insecta: Trichoptera) in a regulated Rocky Mountain river

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Abstract
A year-round study was conducted in the Blue River, Colorado, U.S.A., to examine microdistributions of the filter-feeding caddisflies Arctopsyche 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 the impoundment and declining downstream as B. occidentalis increased. Arctopsyche grandis was often the only caddisfly found near the dam. The two species co-occurred at two downstream sites, with A. grandis numerically dominant 5.2 km below the impoundment and B. occidentalis at the farthest downstream site (11 km). Near the dam, where densities of filter-feeders were highest, there were significant relationships between individual rock surface area and the number of larvae during most sampling periods. Contrary to our predictions of selection of current-exposed microhabitats, both species occurred primarily on the bottom and lower lateral sides of rocks at all sites. Clumped distribution patterns in the absence of spatial segregation by size class suggests common microhabitat preferenda for species and instars. This use of similar spaces indicates that competitive displacement may occur, partially explaining the longitudinal distribution patterns. At a macroscale the reservoir-induced environmental alterations clearly limit the distribution of organisms, especially at sites directly below the impoundment. As the stream regains its preimpoundment character farther downstream, factors at a microscale may play a more important role in producing the distinct longitudinal distribution patterns.