INCREASED ABUNDANCE AND DEPTH DISTRIBUTION OF PELAGIC CRUSTACEAN ZOOPLANKTON DURING HYPOLIMNETIC OXYGENATION IN A DEEP, EUTROPHIC ALBERTA LAKE

Since June 1988, the north basin of eutrophic Amisk Lake has been trea ted by hypolimnetic oxygenation, while the south basin has served as a reference. In summer 1989, hypolimnetic dissolved oxygen (DO) concent rations were greater than or equal to 1 mg.L-1 in the treatment basin, while they fell to <1 mg.L-1 in the reference basin by early July. Th ere were no apparent treatment effects on biomass, abundance, or distr ibution of zooplankton in surface waters (0-10 m). However, in summer 1989, hypolimnetic abundances of two of four species (Daphnia longirem is and Daphnia pulex) were greater in the treatment than in the refere nce basin (P < 0.03) and than in the north basin in summer 1984 (4 yea rs pretreatment). In the hypolimnion, D. longiremis, D. pulex, Bosmina longirostris, and Diacyclops bicuspidatus thomasi were found at great er median depths (P < 0.001) in the treatment than in the reference ba sin. Median depths of these four species in the hypolimnion tracked sp ecies-specific DO concentrations of between 1.5 and 2.3 mg.L-1. Coinci dent with enhanced DO concentrations in the treatment basin in August, D. longiremis migrated vertically in the treatment basin (median dept h day versus night, P < 0.001, median migration 4 m), but not in the r eference basin (P > 0.9).