INFLUENCE OF EXTENDED WATER COLUMN MIXING DURING THE FIRST 2 YEARS OFHYPOLIMNETIC OXYGENATION ON THE PHYTOPLANKTON COMMUNITY OF AMISK LAKE, ALBERTA

The phytoplankton community, physical variables, and nutrient and chlo rophyll a (Chl a) concentrations were monitored during the first two o f six open-water seasons of hypolimnetic oxygenation in double-basined Amisk Lake, Alberta. Deep mixing of the water column in the treated b asin (Z(max) = 34 m) in spring was enhanced by hypolimnetic oxygenatio n. Oxygenation began in June 1988, when stratification was likely alre ady established, but subsequent year-round treatment favoured an exten ded spring diatom bloom (Asterionella formosa and Cyclotella spp.), fo llowed by a delay in the development of, and reduction in the severity of, cyanobacterial blooms (Aphanizomenon flos-aquae and Anabaena flos -aquae) in 1989. Historically, mean summer Chl a and total phosphorus (TP) concentrations in the euphotic zone (0-6 m) of the treated basin were 15.9 +/- 1.6 and 33.5 +/- 1.5 mu g.L-1, respectively, indicating a eutrophic lake. In 1988 and 1989, mean summer Chl a (10.0 +/- 0.6 an d 8.1 +/- 0.7 mu g.L-1, respectively) and TP concentrations (29.0 +/- 0.5 and 22.5 +/- 0.9 mu g.L-1, respectively) in this stratum were lowe r than historic values (P < 0.05), indicating that the trophic status of Amisk Lake had shifted towards mesotrophy.