| The influence of stream regulation on: (1) macroinvertebrate community structure, (2) mayfly population dynamics, diet composition and microspatial distribution, and (3) the structure of a scraper/collector-gatherer guild, was determined from field studies during the period September 1981-June 1983 at three sites on the upper Colorado River. Site 1, the reference site, was located above Granby Reservoir, whereas sites 2 (the regulated site) and 3 (the recovery site) were located 0.4 and 4.0 km below Granby Reservoir, respectively.; Although macroinvertebrate diversity was reduced at the regulated site compared to both the reference and recovery sites, the number of taxa (43) was considerably higher than most values reported from studies of other regulated streams below deep-release reservoirs. Macroinvertebrate mean annual density in the regulated site was 20x higher than at the reference site and slightly higher than the recovery site. The regulated site was characterized by the absence of heptageniid mayflies, reductions in stoneflies, caddisflies, shredders and predators, and high densities of baetids, Ephemerella infrequens, chironomids and non-insect taxa. Thermal alterations and the input of coarse detritus in the spring (decaying algae) rather than the autumn (deciduous leaves) may account for the reduction in heptageniid mayflies, stoneflies, and shredders in the regulated site.; The annual production of Ephemerella infrequens and Drunella grandis in the regulated and recovery sites, respectively, are considerably higher than any previously published production estimates for mayflies. Mayflies with smaller body sizes (Baetis tricaudatus and Paraleptophlebia heteronea appeared bivoltine at sites blow the dam compared to a univoltine life cycle at the reference location. However, the larger mayflies (Ephemerella infrequens and Drunella grandis) were univoltine above and below the reservoir.; There was no evidence to support the hypothesis that niche overlap on temporal, trophic and microspatial dimensions between members of the scraper/collector-gatherer guild was reduced in the more predictable, benign recovery site compared to the less predictable, frequently disturbed reference site. Rejection of this hypothesis may be explained by an abundance of resources below the dam or, resource limitation may be better related to the relative strengths of inter- versus intraspecific competition rather than patterns of niche overlap. |
