EFFECTS OF CONTRASTING LAND-USE ON PHYSICOCHEMICAL CONDITIONS AND BENTHIC ASSEMBLAGES OF STREAMS IN A CANTERBURY (SOUTH ISLAND, NEW-ZEALAND) RIVER SYSTEM

Physico-chemical conditions and benthic invertebrate assemblages in st reams draining catchments dominated by different land use activities w ere investigated near Hanmer Springs, South Island. Four streams in pa storal, scrubland, exotic pine plantation forest (primarily Pinus spp. ), and native beech forest (Nothofagus spp.) catchments were sampled i n four seasons. Alkalinity, pH, and calcium concentrations were highes t among scrubland streams, whereas iron and potassium concentrations w ere highest in pastoral streams. Both taxonomic richness and invertebr ate biomass were greatest in the beech forest streams where most speci es of Ephemeroptera, Plecoptera, Coleoptera, and Diptera were found. F orest streams were dominated by the mayflies Deleatidium and Coloburis cus humeralis, the stonefly Stenoperla prasina, and the caddisfly Olin ga feredayi. The facultative shredder Austroperla cyrene was also abun dant in pine forest streams. However, mayfly, stonefly and caddisfly t axa were poorly represented in pastoral streams. In contrast, molluscs were most prolific in pastoral streams, where the hydrobiid snail Pot amopyrgus antipodarum dominated and the chironomid Eukiefferiella sp. was also abundant. Streams in the four land use types represented a se ries of progressively more modified systems, ranging from pristine bee ch forested streams to highly modified streams draining agriculturally developed catchments. The structure of the stream communities changed along this ''ecological gradient''.