A meromictic lake has layers of water which do not intermix. In ordinary, "holomictic" lakes, at least once each year there is a physical mixing of the surface and the deep waters. This mixing can be driven by wind, which creates waves and turbulence at the lake's surface, but wind is only effective at times of the year when the lake's deep waters are not much colder than its surface waters. In a "monomictic" lake, this mixing occurs once a year; in "dimictic" lakes, the mixing occurs twice a year (typically Spring and Autumn), and in "polymictic" lakes the mixing occurs several times a year. In meromictic lakes, the layers of the lake water remain unmixed for years, decades, or centuries.
The term "meromictic" was coined by the Austrian Ingo Findenegg in 1935, apparently based on the older word "holomictic;" the concepts and terminology used in describing meromictic lakes were essentially complete following some additions by G. Evelyn Hutchinson in 1937.
Among the consequences of this stable layering (or stratification) of lake waters is that the deeper layer (the "monimolimnion") receives little oxygen from the atmosphere. The monimolimnion becomes depleted of oxygen. While the surface layer (the "mixolimnion") may have 10 mg/l or more dissolved oxygen in summer, the monimolimnion in a meromictic lake has less than 1 mg/l. Very few organisms can live in this oxygen-poor environment. One exception is purple sulfur bacteria. These bacteria, which are commonly found at the top of the monimolimnion in meromictic lakes, use sulfur compounds for photosynthesis; sulfur compounds are one of the products of sediment decomposition in "anoxic" (oxygen poor) environments.
This type of lake may form for a number of reasons:
The layers of sediment at the bottom of a meromictic lake remain relatively undisturbed because there is very little physical mixing and few living organisms to stir them up, and very little oxygen or chemical decomposition. For this reason corings of the sediment at the bottom of meromictic lakes are important research tools in tracing climate history at the lake.
Occasionally carbon dioxide (CO2) or other dissolved gasses can build up relatively undisturbed in the lower layers of a meromictic lake. When the stratification is disturbed, as could happen due to an earthquake, a limnic eruption may result. In 1986, a notable event of this type took place at Lake Nyos in Cameroon, causing nearly 1,800 deaths.