An experimental test of sub-hourly changes in macroinvertebrate drift density associated with hydropeaking in a regulated river

Abstract

ABSTRACTWe investigated the response of the invertebrate drift to the regulated peaking Magpie River under experimental sub-hourly changes in flow conditions. Benthic invertebrates were also quantified upstream of invertebrate drift sampling sites in order to examine the propensity of nearby invertebrates to drift under hydropeaking cycles. To provide an understanding of the natural patterns of drift in relation to changes in discharge, the proximate natural Batchawana River was similarly sampled. Increased discharge was associated with higher drift densities and greater particulate organic matter in the Magpie River. At the family level, multiple drift responses were observed suggesting that ecological traits and behaviors drive drift response to changes in discharge. The drift densities of some invertebrates varied in proportion to discharge; whereas, other invertebrates in the drift decreased to pre-peaking levels shortly after peak discharge was reached, with some invertebrates exhibiting a secondary peak in density as discharge returned to pre-peaking levels. Although average drift densities were comparable between the two rivers, drift density on the natural Batchawana River was much more stable than that of the Magpie River. Changes in drift density associated with flow manipulation likely impact feeding patterns and behavior of drift feeding fish.