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 treated by hypolimnetic oxygenation, while the south basin has served as a reference. In summer 1989, hypolimnetic dissolved oxygen (DO) concentrations 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. There were no apparent treatment effects on biomass, abundance, or distribution of zooplankton in surface waters (0-10 m). However, in summer 1989, hypolimnetic abundances of two of four species (Daphnia longiremis and Daphnia pulex) were greater in the treatment than in the reference basin (P < 0.03) and than in the north basin in summer 1984 (4 years pretreatment). In the hypolimnion, D. longiremis, D. pulex, Bosmina longirostris, and Diacyclops bicuspidatus thomasi were found at greater median depths (P < 0.001) in the treatment than in the reference basin. Median depths of these four species in the hypolimnion tracked species-specific DO concentrations of between 1.5 and 2.3 mg.L-1. Coincident with enhanced DO concentrations in the treatment basin in August, D. longiremis migrated vertically in the treatment basin (median depth day versus night, P < 0.001, median migration 4 m), but not in the reference basin (P > 0.9).