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

Physico-chemical conditions and macroinvertebrate faunas of 20 South Island lake outlets were investigated in summer-early autumn of 1990-94. Four outlet types were recognised: unregulated alpine, unregulated mid/low elevation, regulated hypolimnetic and regulated epilimnetic outlets. Alpine outlet waters had significantly lower (P < 0.05) alkalinities and conductivities (6-12mg l(-1) CaCO3 and 13-45 mu S cm(-1)) than all other outlets (11-36mg l(-1) CaCO3 and 36-93 mu S cm(-1)) whereas pH values were similar among outlet types. Substrate size was significantly greater in modified epilimnetic than natural mid/low elevation outlets (P < 0.01). Total number of taxa and invertebrate densities were significantly lower in alpine and hypolimnetic outlets than natural mid/low elevation outlets (P < 0.05, and P < 0.01 respectively), and distinct differences were observed in the relative abundances of major taxonomic groups. Alpine streams were dominated by the mayflies Deleatidium (Leptophlebiidae), and Nesameletus (Siphlonuridae), the gripopterygid stonefly Zelandoperla decorata and predatory hydrobiosid caddisflies; molluscs were absent, and filter-feeding hydropsychid caddisflies rare. In contrast, the hypolimnetic outlets were dominated by filter-feeding hydropsychids (87-93 % of the fauna), and no stoneflies were found. Epilimnetic and unregulated mid/low elevation outlets had similar communities, however invertebrate densities were significantly lower in regulated surface release outlets. Altitude, outlet width, velocity, conductivity, and percentage of the catchment in forest, were all implicated as factors explaining differences in community composition.