Lake turnover.
Water is densest at 4°C. In winter, the coldest water in the lake (0°C) lies just below the surface ice; water is progressively warmer at deeper levels of the lake, resulting in thermal stratification. In spring, as ice melts, the surface water warms to 4°C and sinks, eliminating temperature differences. Winds mix the water, bringing oxygen to the bottom waters and nutrients to the surface. In summer, the lake regains thermal stratification, with warm surface water separated from cold bottom water by a narrow zone of rapid temperature change, called a thermocline. In autumn, surface water cools and sinks below the underlying layers, remixing the water until the surface begins to freeze and the winter temperature profile is reestablished.