Meromictic lakes are unusual bodies of water that fascinate scientists from many disciplines. Most deep lakes follow a seasonal cycle of stratification and complete mixing, but meromixis is a condition in which a lake does not mix completely. The term was introduced in 1935 by an Austrian limnologist, Ingo Findenegg, to describe the circulation behavior of subalpine lakes in Corinthia. Since then, examples have been reported from many other parts of the world.

Lakes are warmed by the sun and mixed by the wind. In shallow lakes, wind-driven currents keep the water mixed all year round, but in deep lakes there comes a time, during the warming cycle, when the currents are no longer able to overcome the density differences that develop between top and bottom waters, and the lake becomes stratified into layers (an upper epilimnion, a lower hypolimnion, and an intervening boundary layer termed the metalimnion or thermocline). Later, as the air cools, the density difference is diminished, the currents penetrate deeper, and the lake eventually is restored to complete circulation (holomixis). In Temperate Zones the cycle of temperature is aligned with seasonal changes, but even in tropical and Polar Regions the interplay of sun and wind may impose short-lived periods of incomplete circulation. Meromixis, then, is a global phenomenon.

In meromictic lakes the bottom water usually contains dissolved salt, so that the density difference between top and bottom is reinforced by both salinity and temperature. The effect may be aided by the size, shape, and orientation of the lake basin. A deep hole in a shallow basin, or a lake sheltered from the prevailing wind by surrounding mountains or tall vegetation, would have a greater propensity to retain stratification. Depending on the shape and exposure of the basin, and the intensity of the stratification, meromictic conditions can last anywhere from one year to centuries.

A meromictic lake has an upper mixolimnion (a stratum where mixing by wind occurs), a lower monimo-limnion (the more dense stratum that is not mixed by wind), and an intervening boundary layer or chemo-cline (usually the thinnest stratum, but contains the greatest chemical or density gradient). The mixolimnion may behave like a typical lake and become seasonally stratified (thus, an epilimnion and hypolimnion) and later mix completely. However, the monimolimnion remains isolated from the atmosphere and does not participate in these periods of mixing. The resistance of the total water column to mixing depends mainly on the density differences across the chemocline.

In a meromictic lake, the monimolimion is usually anoxic (without oxygen) and uninhabited by organisms other than bacteria, some protozoans, and possibly nematodes. Some of the dipteran 'fly' larvae of the Chironomidae and Chaoboridae families are able to survive for considerable periods of time without oxygen. The hypolimnion of a thermally stratified lake may also be seasonally depleted in oxygen, if the lake is nutrient rich or eutrophic, but oxygen is redistributed throughout the water column when the lake does undergo mixis.

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