Biotic Responses along the Recovery Gradient of a Regulated Stream

Abstract

Samples were collected year-round over a 1-yr period at six sites located downstream from a reservoir with a hypolimnetic release on the upper Colorado River to examine the longitudinal patterns of macroinvertebrates along an environmental gradient induced by river regulation. The impoundment had only minimal effects on river chemistry, and chemical variables did not exhibit a distinct downstream gradient. Macroinvertebrate density was highest at sites near the Granby Reservoir dam, but diversity increased with distance from the impoundment. A percentage similarity matrix identified two areas of rapid transition in macroinvertebrate assemblages along the regulated river gradient. Detrended correspondence analyses (DCA) showed a sequential faunal gradient with the most rapid change occurring within the first 3.0 km below the impoundment. Detrended canonical correspondence analysis (DCCA) suggested that downstream decreases in periphyton standing crop and minimum temperatures and downstream increases in most food resources and maximum temperatures were major variables structuring faunal assemblages. The results of this study, when compared with research conducted in free-flowing and other impounded rivers, indicated the predictability of some longitudinal patterns and the recovery potential of regulated lotic systems.