Some lakes never mix completely at any time during the year. Such lakes are called meromictic. The depth below which no mixing occurs is the monimolimnion, and the upper water layer that undergoes the circulation according to the climate of the region is the mixolimnion. A strong vertical chemical gradient or chemocline separates these two zones.
Meromixis develops when the deep water of a lake becomes so heavy with dissolved substances that its density cannot be equaled by the cooling of the surface water. An increase in salt content of 10mgl-1 produces the same increase in density as a decrease in temperature from 5 to 4 °C. In a clear lake with a high salt content in the deep water, the monimolimnion may be warmed with solar radiation well above 4 °C without mixing, even when the surface waters cool to 4 °C.
Different types of meromixis are distinguished on the basis of the source of salt in the monimolimnion: crenogenic meromixis from deep water springs, ecto-genic meromixis from occasional delivery of ocean water in coastal lakes, and biogenic meromixis from minerals released by decomposition in the deep water and released from the sediments. Normally, the accumulation of decomposition products during summer stagnation are not sufficient to prevent circulation in the fall or winter, but in very deep lakes with relatively small surface area inadequate cooling or wind in the fall or winter may result in the lack of a complete circulation. If this happens repeatedly, dissolved substances may accumulate to large enough concentrations to make the lake permanently meromictic. Almost all extremely deep equatorial lakes, such as Lake Tanganyika and Lake Malawi, have this type of meromixis.
A meromictic lake has the same type of vertical chemical gradient as a eutrophic, holomictic lake with an anaerobic hypolimnion. Even unproductive meromictic lakes tend to have an anaerobic monimolimnion, due to the long-term accumulation of decomposing organic matter. The stability of the chemocline also results in much steeper vertical chemical gradients of oxygen and other related substances than in comparable holomictic lakes.
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