Effects of contrasting land use on physico-chemical conditions and benthic assemblages of streams in a Canterbury (South Island, New Zealand) river system

Physico-chemical conditions and benthic invertebrate assemblages in streams draining catchments dominated by different land use activities were investigated near Hanmer Springs, South Island. Four streams in pastoral, scrubland, exotic pine plantation forest (primarily Pinus spp.), and native beech forest (Nothofagus spp.) catchments were sampled in four seasons. Alkalinity, pH, and calcium concentrations were highest among scrubland streams, whereas iron and potassium concentrations were highest in pastoral streams. Both taxonomic richness and invertebrate biomass were greatest in the beech forest streams where most species of Ephemeroptera, Plecoptera, Coleoptera, and Diptera were found. Forest streams were dominated by the mayflies Deleatidium and Coloburiscus humeralis, the stonefly Stenoperla prasina, and the caddisfly Olinga feredayi. The facultative shredder Austroperla cyrene was also abundant in pine forest streams. However, mayfly, stonefly and caddisfly taxa were poorly represented in pastoral streams. In contrast, molluscs were most prolific in pastoral streams, where the hydrobiid snail Potamopyrgus antipodarum dominated and the chironomid Eukiefferiella sp. was also abundant. Streams in the four land use types represented a series of progressively more modified systems, ranging from pristine beech forested streams to highly modified streams draining agriculturally developed catchments. The structure of the stream communities changed along this ''ecological gradient''.