Artificial circulation and hypolimnetic aeration are management techniques for oxygenating eutrophic lakes subject to water quality problems, algal blooms, and fishkills. Artificial circulation is achieved by injecting diffused air into lower waters, by mechanical pumping of water from one depth stratum to another, or by inducing turbulence at the surface using large axial-flow pumps. In contrast, hypolimnetic aeration by air or oxygen injection affects primarily bottom waters. In some instances low dissolved oxygen concentrations persist in the metalimnion. In general, both methods lower the concentrations of reduced compounds in lake waters, providing benefits for water supply systems. Aeration may cause supersaturation of nitrogen gas, thereby raising the potential danger of gas bubble disease in downstream fishes.