Macroinvertebrate responses along a complex regulated stream environmental gradient

Samples were collected year‐round over a one‐year period at sites located downstream from a hypolimnetic‐release reservoir on the Blue River, Colorado, to examine macroinvertebrate responses along a complex environmental gradient induced by river regulation. Six sampling sites were established in riffles downstream of the dam using approximately a geometric progression starting at 0.25 km. Ordination techniques were used to elucidate macroinvertebrate distributional patterns along the complex environmental gradient. The detrended correspondence analysis (DCA) showed a sequential faunal gradient with the most rapid change occurring within the first 2.0 km below the impoundment. Detrended canonical correspondence analysis (DCCA) was used to relate faunal distributions to downstream changes in environmental variables. Downstream decreases in periphyton standing crop and minimum temperatures, and downstream increases in food resources and maximum temperatures, were identified as the major variables structuring faunal assemblages. The combined use of different gradient analysis techniques proved useful for identifying distinct macroinvertebrate distributional patterns and the environmental variables that may be responsible for these patterns.