Field and laboratory experiments show that when nutrients (N and P) are added to a mixed population of algae, blue-greens predominate. However, if CO2 is added also, or if pH is lowered with HCl, greens predominate. The phenomenon is reproducible, and works with most lakes. Although most successful at pH 5.5, the shift to greens can be made to happen at pH values as high as 8.5. Most blue-greens appear to be susceptible to the shift, and Scenedesmus and Chlorella are the predominant greens resulting. If pH is raised, the shift is reversed. The reason for the shift is not known. It may involve competition by the algae for CO2, but other evidence suggests that the lowered pH stimulates cyanophage production and lysis of the blue-greens, with release of nutrients which then are used by the greens. Analysis of results of lake circulation data from the literature and from experiments suggests that the algal shifts resulting from circulation may involve the same phenomena. Understanding of these phenomena should lead to predictable use of circulation as a lake-management tool.