effect of experimental flow reductions on macroinvertebrate drift in natural and streamside channels

Understanding how much water must remain in a stream to maintain a healthy functioning ecosystem has become an important focus in stream ecology research. The drift of stream invertebrates is important as a mechanism of dispersal, recolonization and as a food source for fish in flowing water. Drift behaviour of stream invertebrates in response to flow reduction was examined in natural and streamside channels in two countries (Canada and New Zealand). We hypothesised that the drift of some taxa would increase following flow reduction as they attempted to avoid unfavourable conditions. Taxa such as Baetis sp. (Ephemeroptera) in Canadian streamside channels and Coloburiscus humeralis (Ephemeroptera) and Austrosimulium sp. (Simulidae) in streams in New Zealand exhibited a short-term increase in drift following flow reduction. This appears to be in response to decreased velocities and available habitat in flow reduced areas. The majority of taxa displaying this response were filter feeders, suggesting a decline in food delivery with reduced flow contributed to increased drift. Some taxa (e.g. the amphipod Paracalliope fluviatilis) had a sustained increase in drift throughout the reduced flow period, probably because a preference for reduced flows increased their abundance or levels of activity. Water allocation decisions should consider potential impacts on the drift behaviour of the more commonly drifting taxa in a stream. Copyright © 2007 John Wiley & Sons, Ltd.