Invertebrate community responses to experimentally reduced discharge in small streams of different water quality

Water abstraction can alter invertebrate community composition by changing the availability or suitability of habitat for invertebrates. The consequences of water abstraction for the physical habitat and invertebrate communities of small permanent streams are not clear. Therefore, we used whole-channel flow manipulations to imitate real water abstractions. We used weirs and diversions to reduce discharge by >85% in 3 small New Zealand streams (mean discharges: 11–84 L/s), ranging in water quality from pristine to moderately polluted. We sampled benthic invertebrates and periphyton at control and impact sites on each stream before and during 2 mo of artificially reduced discharge during summer 2005, and then we left the diversions in operation throughout the following year and sampled invertebrates again after 9 and 12 mo of flow reduction. During the experiment, wetted width decreased by 24 to 34% at impact sites. Water velocity and depth also decreased at impact sites, but only small changes to conductivity, pH, dissolved O2, and temperature were observed. At the pristine site, density of invertebrates and percentage of Ephemeroptera, Plecoptera, Trichoptera (EPT) individuals decreased in response to reduced flows. Only taxonomic richness decreased at the mildly polluted stream, and reduced discharge had no effect on the invertebrate community at the stream with the poorest water quality. Overall community structure also changed in response to flow reduction at all but the poorest-water-quality stream. Alterations to community composition involved changes in the density of common taxa and collection of fewer rare taxa per sample. Our results indicate that the impacts of water abstraction on invertebrate communities differ between streams that vary in water quality, probably because of the relative sensitivities of invertebrate communities to changes in the physical habitat of these streams.