Effects of Successive Flow Perturbations on Stream Invertebrates

Abstract

Initial stream flow changes after a period of constant flow have a much larger effect on invertebrate drift than subsequent changes. Constant discharge was maintained in two man-made streams for several months, after which discharge was kept constant on one stream (control) while in the other (treatment) it was increased fivefold and then returned to its original state three times between 17:00 and 21:00 in 1 d. This treatment was repeated on four successive days each week for 3 wk. On the first day of treatment the density of invertebrate drift increased in the treatment stream during the changes in discharge, the increase being less with each successive change. By the final day of treatment, however, the changes in discharge did not cause increases in drift, and this result is attributed to depletion of the benthos by repeated fluctuations in discharge. Surber sample estimates of benthic invertebrate density declined following flow changes. Many of the invertebrates displaced by flow changes were probably resident in sloughed off filamentous algae. I conclude that it is necessary to know the flow history of a river before being able to predict effects of flow perturbations.