VARIATIONS IN BENTHIC FAUNA BETWEEN DIFFERING LAKE OUTLET TYPES IN NEW-ZEALAND

Physico-chemical conditions and macroinvertebrate faunas of 20 South I sland lake outlets were investigated in summer-early autumn of 1990-94 . Four outlet types were recognised: unregulated alpine, unregulated m id/low elevation, regulated hypolimnetic and regulated epilimnetic out lets. Alpine outlet waters had significantly lower (P < 0.05) alkalini ties and conductivities (6-12mg l(-1) CaCO3 and 13-45 mu S cm(-1)) tha n all other outlets (11-36mg l(-1) CaCO3 and 36-93 mu S cm(-1)) wherea s pH values were similar among outlet types. Substrate size was signif icantly greater in modified epilimnetic than natural mid/low elevation outlets (P < 0.01). Total number of taxa and invertebrate densities w ere significantly lower in alpine and hypolimnetic outlets than natura l mid/low elevation outlets (P < 0.05, and P < 0.01 respectively), and distinct differences were observed in the relative abundances of majo r taxonomic groups. Alpine streams were dominated by the mayflies Dele atidium (Leptophlebiidae), and Nesameletus (Siphlonuridae), the gripop terygid stonefly Zelandoperla decorata and predatory hydrobiosid caddi sflies; molluscs were absent, and filter-feeding hydropsychid caddisfl ies rare. In contrast, the hypolimnetic outlets were dominated by filt er-feeding hydropsychids (87-93 % of the fauna), and no stoneflies wer e found. Epilimnetic and unregulated mid/low elevation outlets had sim ilar communities, however invertebrate densities were significantly lo wer in regulated surface release outlets. Altitude, outlet width, velo city, conductivity, and percentage of the catchment in forest, were al l implicated as factors explaining differences in community compositio n.