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 (mero, partial; mixis, circulation) lakes, there are no periods of overturn or complete mixing because seasonal changes in the thermal gradient either are small or are overridden by the stability of a chemical gradient, or the deeper waters are physically inaccessible to the mixing energy of the wind. Commonly in meromictic lakes, the vertical stratification in density is stabilized by a chemical gradient.