Diel Migration and Microhabitat Distribution of a Benthic Stream Assemblage

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Abstract
Macroinvertebrate microspatial distributions were determined on nonglazed brick substrates at three sites on the upper Colorado River. Site 1 was located above a reservoir, whereas Sites 2 and 3 were located 0.4 and 4.0 km, respectively, downstream from a deep-release dam. Replicate samples from each substrate surface (top, sides, front, back, and bottom) and from the interstices below brick substrates were collected during the day (ca 14:00–16:00) and at night (ca 21:00–23.00) in the summer and autumn of 1982. Although most macroinvertebrates occurred on "bottom" surfaces, "top," and "side" surfaces and the interstices were often utilized. Macroinvertebrates were least abundant on "back" and especially "front" surfaces. Total macroinvertebrate density on upper substrate surfaces was not significantly different at night compared with the day. However, there was a 47% increase in the density of Baetis spp. on upper surfaces during the night at Site 1, a diel shift in benthic substrate utilization that corresponded to drift periodicity. The density of Baetis spp. on upper surfaces during the night compared with the day was not significantly different at Sites 2 and 3. Large-sized Baetis spp. at Site 1, but not at Sites 2 and 3, used a greater proportion of upper substrate surfaces at night compared with the day.