Effects of riparian grazing and channelisation on streams in Southland, New Zealand. 2. Benthic invertebrates

Abstract

Abstract A survey of benthic invertebrate faunas in riparian‐protected, riparian‐grazed, and channelised reaches of five Southland streams with catchment sizes of 3–37 km2 was carried out. It was part of a wider investigation to assess the effects of riparian grazing and channelisation on stream habitat and biota. In small streams (catchment areas 3–10 km2 ; widths 1–4 m), channelisation or intensive grazing by cattle greatly reduced shading by riparian vegetation, resulting in substantial increases in daily maximum temperatures during summer. Channelisation also caused gross changes in channel morphology and intensive grazing of a reach with moist streamside soils was associated with increased bed sedimentation and bank damage. Marked changes in invertebrate communities were associated wilh these habitat modifications. In general, taxa favoured by cool water and low periphyton abundance (e.g., Plecoptera, Paraleptamphopus caeruleus, Deleatidium sp., and Helicopsyche albescens) decreased in density, whereas densities of taxa favoured by an abundance of periphyton (e.g., Chironomidae and Oxyethira albiceps) increased. In contrast, differences in physical habitat and invertebrate communities were minor between paired grazed and riparian‐protected reaches of the larger streams (catchment areas 10–33 km2 ; median widths 6–16 m) where grazing had little or no effect on stream shading. These results indicate that in small streams, with median natural channel widths below c. 6 m, the effects on benthic invertebrates decrease in the following order, channelisation > intensive grazing by cattle > extensive grazing by cattle and/or sheep. Shade provided by riparian vegetation appears to play a vital role in maintaining cool, headwater, stream habitats for benthic invertebrate communities in these streams.