Benthic faunas of streams of low pH but contrasting water chemistry in New Zealand

Water chemistry and benthic invertebrate communities were investigated at 37 sites on acid streams originating on the Stockton-Denniston Plateau, North Westland, New Zealand. The region is characterised by high rainfall and runoff, highly acidic soils and the presence of extensive coal measures that have been mined for over 120 years. Four groups of streams were identified: naturally acid plateau streams with clear water and very low conductivity; brown water (humic) streams with pH<4 and low conductivity; coastal plains streams with higher pH and conductivity; and streams contaminated by acid mine drainage. TWINSPAN and cluster analysis also grouped streams into four groups based on presence-absence of invertebrate taxa, the groups being similar to, but not identical to those based on physico-chemical factors. Diptera (mainly Chironomidae) were represented by most species in all site groups followed by Plecoptera in plateau streams, and Trichoptera on the coastal plain and where mine drainage occurred. Species of Orthocladiinae (Chironomidae), Plecoptera and Trichoptera were found at the most contaminated sites (pH <3, conductivity >900 µS cm^–1, total reactive aluminium >25 mg 1^–1). The leptophlebiid mayfly Deleatidium was found at 32 of the 37 sites, and some species of Plecoptera, Trichoptera and Chironomidae were also widely distributed. Our findings indicate that species tolerant of low pH (i.e., <4.5) are not confined to humic waters as has been postulated, but also occur in soft, non-humic waters where concentrations of labile, non-organically bound aluminium may be elevated.