Meromictic lake

A lake whose water is permanently stratified and therefore does not circulate completely throughout the basin at any time during the year. Normally lakes in the temperate zone mix completely during the spring and autumn when water temperatures are approximately the same from top to bottom. In meromictic lakes there are no periods of overturn or complete mixing because seasonal changes in the thermal gradient are either small or overridden by the stability of a chemical gradient, or the deeper waters are physically inaccessible to the mixing energy of the wind. Most commonly, the vertical stratification is stabilized by a chemical gradient in meromictic lakes.

The upper stratum of water in a meromictic lake is mixed by the wind and is called the mixolimnion. The bottom, denser stratum, which does not mix with the water above, is referred to as the monimolimnion. The transition layer between these strata is called the chemocline.

Of the hundreds of thousands of lakes on the Earth, only about 120 are known to be meromictic. In general, meromictic lakes in North America are restricted to: sheltered basins that are proportionally very small in relation to depth and that often contain colored water, basins in arid regions, and isolated basins in fiords. See also Fresh-water ecosystem; Lake; Limnology.