Lakes can be classified on the basis of their thermal behavior in the course of a year. The hypothetical lake considered in the last section is just one possibility for the annual changes in thermal regime.
First we need to distinguish between holomictic lakes and meromictic lakes.
Holomictic lakes are of the same salinity (essentially zero) throughout, so that if the temperature becomes the same everywhere in the lake, the density also has to be the same everywhere in the lake, and the water of the lake can circulate freely. In such holomictic lakes, only the temperature, not the salinity, controls the density.
Meromictic lakes, on the other hand, can’t circulate freely even if the temperature becomes the same everywhere, because of greater salinity in the bottom waters. Any higher-salinity water produced within the lake or introduced from outside the lake will find its density level and then spread out horizontally to form a distinctive layer, and the resulting density stratification prevents or at least inhibits vertical circulation.
Holomictic lakes can be further subdivided into dimictic lakes, warm monomictic lakes, and cold monomictic lakes, depending on the history of surface-water temperature in the course of the year, which in turn is a function of the climate.
Dimictic lakes are those in temperate regions where the annual fluctuations in air temperature are such that the surface water temperature of the lake is above 4°C for a part of the year and below 4°C for part of the year. Dimictic lakes circulate freely by overturning twice a year. The lake described in Section 7 is a dimictic lake.
Warm monomictic lakes are those in tropical regions where the air temperature never gets very low during the year. The surface water temperature of thelakestaysabove4°C,sothere’sfreeverticalcirculationonlyatthetimeof coldest surface water temperature.
Cold monomictic lakes are those in polar regions where the air temperature never gets very high during the year. The surface water temperature of the lake staysbelow4°C,sothere’sfreecirculationonlyatthetimeofwarmestsurface water temperature.
The basic reason why lakes show such diverse circulation behavior is that maximum in water density at 4°C.