An oligotrophic and a eutrophic lake in Alberta were comparatively investigated for 14 months to elucidate relationships between the physicochemical environment and the species composition, seasonal succession, vertical distribution, and seasonal cycles of their phytoplankton. One hundred and seven algal species were found in Muir Lake and 63 species in Hastings Lake. Increases in the numbers of Anabaena flos-aquae accompanied decreases in the numbers of several green algae, viz Pediastrum and Scenedesmus spp., suggesting antagonistic effects. Three groups of vertical distribution patterns of phytoplankton occurred in both lakes: buoyant and some motile forms accumulated at or near the surface, a second group was more or less uniformly dispersed throughout the water column, and a third group occurred only in bottom waters. Intermittent species were classed as either stenochronic or eurychronic, depending on the length of their pulses. In Muir Lake, Asterionella formosa was inversely correlated with silica concentrations. Dinobryon increases were associated with increases in phosphates and bicarbonates. Up to 5 times more orthophosphate occurred in the ice than in the water beneath. Ice may be of heretofore unrealized ecological significance as a phosphate reservoir for the spring aquatic community.