CHIRONOMIDAE (DIPTERA) OF THE COLORADO RIVER, GRAND-CANYON, ARIZONA, USA, II - FACTORS INFLUENCING DISTRIBUTION

Biogeographic, flow regulation (water clarity and temperature), and te mporal influences affect the composition of the chironomid midge assem blage in the Colorado River between Glen Canyon Dam and Lake Mead. Thi s assemblage is dominated by euryecious Nearctic and Holarctic orthocl adine taxa (23 of 38 total species, total weighted relative abundance [WRA] = 0.972) and includes a minor Neotropical component. Chironomid species richness increases over distance downstream from the dam, and dominance shifts across 3 turbidity segments. Eleven species occur in the cold-stenothermic clearwater (CW) segment between the dam and the Ist perennial tributary (the Paria River, 26 km from the dam). Chirono mid diversity increases from 18 to 24 species in the variably turbid ( VT) and usually turbid (UT) segments downstream, respectively. Total C ricotopus spp. WRA is negatively correlated with distance (turbidity), while total Chironominae WRA shows the opposite pattern. In contrast to chironomid diversity, species density decreases from 0.42 species/k m in the CW segment to 0.19 and 0.08 species/km in the VT and UT segme nts, respectively. Seasonal dominance shifts slightly from orthocladin e Eukiefferiella spp, in winter (WRA = 0.101) to Cricotopus spp. (WRA = 0.165) in summer. Total WRA is lowest in spring (0.191). The assembl age is depauperate compared with other western rivers and has changed over post-dam time.