Crowded waters: short-term response of invertebrate drift to water abstraction

Water abstraction modifies the environmental conditions of stream ecosystems, which can affect invertebrate assemblages by altering drift. We examined this issue with a before–after–control–impact design experiment in which we diverted 90% of the natural flow from a headwater stream. We measured flow-reduction effects on drift densities (animals/m³), total drift rates (animals leaving the reach per hour) and net balance of invertebrates entering or leaving the Impact reach. We also identified the specific taxa and traits that drove these responses. The sudden decrease in flow promoted a 12-fold increase in overall drift density at the Impact reach, which was primarily driven by filterers, shredders and taxa associated with fast velocities, such as simulids. By contrast, drift densities of other abundant taxa, such as Baetis, Esolus and chironomids, increased less than what could be expected from the magnitude of flow reduction. While drift rates remained unchanged after water abstraction, the Impact reach became a net invertebrate exporter indicating that many taxa drifted actively as a response to stressful conditions rather than passively, which would be reduced by water abstraction. Therefore, our results suggest that water abstraction influences drift, with potentially important consequences for the invertebrate assemblages and ecosystem processes further downstream.