Applied to a lake in which two seasonal periods of free circulation occur, as is typical of lakes in mid-latitude climates. In summer, thermal stratifica-tion occurs as surface waters are warmed and cease to mix with the denser, colder, deep waters. In winter, when they cool to below 4°C, surface waters expand, so becoming less dense than warmer waters beneath them, giving a reverse stratification. Free circulation through the depth of the lake is possible only in spring, when the surface temperature rises to above 4°C, and the water becomes heavier than that beneath and so sinks and mixes, and in autumn, when the surface (epilimnion) waters cool to the temperature of the deep (hypolimnion) waters.
Subjects: Plant Sciences and Forestry — Ecology and Conservation.