IMPACT OF HYPOLIMNETIC OXYGENATION ON PROFUNDAL MACROINVERTEBRATES INA EUTROPHIC LAKE IN CENTRAL ALBERTA .1. CHANGES IN MACROINVERTEBRATE ABUNDANCE AND DIVERSITY

Hypolimnetic oxygenation from 1988 to 1991 in the north basin of Amisk Lake increased mean summer (June-August) dissolved oxygen (DO) concen trations in the deep hypolimnion (25 m) from <0.4 mg.L-1 during pretre atment years (1980-1987) to 2.7 mg.L-1 during treatment. Mean summer h ypolimnetic temperatures at 25 m also increased from 6.3 (pretreatment ) to 8.1 degrees C (treatment). Profundal (15-25 m) Chironomus spp. ab undance in the treated basin increased from <100 to >2000.m(-2) from 1 988 to 1991 whereas Chaoborus spp., Tubificidae, and other taxa remain ed uncommon at similar depths. Shannon-Weaver indices of diversity dec reased as oxygenation progressed, in contrast with previous studies. S imilar patterns in densities and diversity were observed in the south basin of Amisk Lake, although the increase in DO concentrations was sm aller relative to the north basin. The impact of increased water tempe rature on macroinvertebrates appeared minor relative to the effects of increased DO concentrations. No changes in macroinvertebrate communit ies were noted in a nearby reference lake during the study period. Res ponses to hypolimnetic oxygenation in Amisk Lake differed markedly fro m those reported for smaller, single-basined water bodies.