Halfmoon Lake is a small, moderately deep, hypereutrophic, hardwater lake with a relatively small (2.4 km2), partially cleared drainage basin situated in the Boreal Mixedwood Ecoregion of central Alberta. The water is often thermally stratified, and average residence time is 60 years. During mid- to late summer 1988 and 1989, 188 metric tons of Ca(OH)2and 58 metric tons of CaCO3were applied to the surface of Halfmoon Lake to decrease total phosphorus (TP) concentration and algal biomass (estimated as chlorophylla[Chla]). Mean summer (July-September) Chlaand TP in the euphotic zone in 1989 and 1990 decreased to 53 and 63%, respectively, of the pretreatment year (1982). Similarly, sediment P release from June through August was 50% lower in 1989 and 1990 than in the pretreatment year. In contrast water quality deteriorated in three nearby reference lake basins, also hypereutrophic, so that average summer euphotic zone Chlaand TP were 217 and 116% higher, respectively, in 1989 and 1990 than in 1982. When these trends were considered, posttreatment corrected Chlaand TP in Halfmoon Lake were 24 and 54%, respectively, of pretreatment values. Decreased summer Chlain Halfmoon Lake was associated with higher winter dissolved oxygen (DO) concentrations, presumably due to a decrease in the oxygen demand in the water column at fall overturn following treatment and hence increased DO at freeze-up. Thus, Halfmoon Lake, a traditional winterkill lake where the water column went completely anoxic by January, had sustainable DO for fish in the posttreatment winters. We further hypothesize that enhanced sediment calcium content was responsible for reduced P loading from bottom sediments during winter and, consequently, lower summer Chla.