A meromictic lake has layers of water that 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 or warmer than its surface waters.
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.
Most lakes are holomictic; that is, at least once per year, physical mixing occurs between the surface and the deep waters. In monomictic lakes the mixing occurs once per 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.
Meromictic lakes can also be divided into three sections: monimolimnion, chemocline, and the mixolimnion. The monolimnion is the lower portion of the lake that does not circualte much and is generally anoxic and saltier than the rest of the lake. The mixolimnion is the uppermost part of the lake that behaves as holomitic lake. The area in between is referred to as the chemocline. 
This lack of mixing creates radically different environments for organisms to live in. The monimolimnion is stagnant and rich in phosphorus, nitrogen and sulfide. These factors combine to create a perfect environment for bacteria to grow in. The mixolimnion has similar qualities, however the types of bacteria that can grow on the surface is dictated by the amount of light that the surface receives.  Because of the ease of access to these unique bacterial hotspots, they have become the ideal location to study photosynthetic sulfur bacteria and chemoautotrophs.  Because of these unique bacter, sulfur is cycled through the chemocline. During the day dihydrogen sulfide is absent, while at night concentrations build up.
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 basin is unusually deep and steep-sided compared to the lake's surface area
- the lower layer of the lake is highly saline and denser than the higher levels of water
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 cores of the sediment at the bottom of meromictic lakes are important research tools in tracing climate history at the lake.
When the layers do mix for whatever reason the consequences can be devastating for organisms that normally live in the mixolimnion. This layer is usually of a much smaller volume than the monimolimnion and therefore when they mix the oxygen concentration in mixolimnion will decrease dramatically. This may result in the death of many organisms such as fish that require oxygen.
Occasionally carbon dioxide (CO2) or other dissolved gases can build up relatively undisturbed in the lower layers of a meromictic lake. When the stratification is disturbed, as could happen from 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.
While it is mainly lakes that are meromictic, the world’s largest meromictic basin is the Black Sea. The deep waters below 50 metres (150 feet) do not mix with the upper layers that receive oxygen from the atmosphere. As a result, over 90% of the deeper Black Sea volume is anoxic water. The Caspian Sea is anoxic below 100 metres (300 feet). The Baltic Sea is persistently stratified with large hypoxic sediment areas below its halocline.
List of meromictic lakes
There are meromictic lakes all over the world. The distribution appears to be clustered, but this may be due to incomplete investigations. Depending on the exact definition of "meromictic", the ratio between meromictic and holomictic lakes worldwide is around 1:1000.
- Lake Nyos and Lake Monoun in Cameroon
- Lake Kivu in Rwanda, The DRC
- Lake Tanganyika in Burundi, The DRC, Tanzania and Zambia
- Pantai Keracut (Keracut Beach) Lake, Penang National Park, northwest Penang island, Malaysia
- Jellyfish Lake (Ongeim'l Tketau), on Eil Malk in Palau
- Zigetangcuo Lake, a crenogenic lake in Nagqu Prefecture, Tibet. It is the meromictic lake located at a highest altitude.
- Kärntner Seen (Alpine lakes in the Austrian province of Carinthia; studied by Ingo Findenegg in the 1930s).
- Alatsee (small alpine lake in Germany's State of Bavaria, near the City of Füssen and Neuschwanstein Palace)
- Lake Vähä-Pitkusta in Finland.
- Lake Pakasaivo (Finland)
- Salsvatnet, Kilevann, Tronstadvatn, Birkelandsvatn, Rørholtfjorden, Botnvatn, Rørhopvatn and Strandvatn lakes in Norway.
- Centro Biologia Alpina).
- Lac Pavin and Lac du Bourget in France
- The Black Sea is also considered to be meromictic.
- Ballston Lake, 30 km NNW of Albany, New York
- Crawford Lake near Milton, Ontario
- Green Lake and Round Lake in Green Lakes State Park near Syracuse, New York
- Glacier Lake in Clark Reservation State Park near Syracuse, New York
- Great Salt Lake near Salt Lake City, Utah
- Hot Lake in Okanogan County, Washington 
- Irondequoit Bay near Rochester, New York is also considered meromictic; use of road salt has been cited as the main reason for its change
- Lower Mystic Lake in Arlington and Medford, Massachusetts
- McGinnis Lake in Petroglyphs Provincial Park, Ontario
- Mahoney Lake in the Okanagan Valley, British Columbia
- Powell Lake in the town of Powell River, British Columbia 
- Pink Lake in Gatineau Park, Quebec
- Redoubt Lake near Sitka, Alaska; one of North America's largest meromictic lakes.
- Soap Lake in Washington
- Sunfish Lake near Waterloo, Ontario
- Devil's Bathtub near Rochester, New York in Mendon Ponds Park
- Blackcat Lake near Dorset, Ontario in Frost Centre
- Chapel Lake, in Pictured Rocks National Lakeshore, near Munising, Michigan 
- Lakes A and C1 on Ellesmere Island, (Nunavut)
- Lake Mary, in the Northwest corner of Vilas County, Wisconsin 
- Big Soda Lake, Nevada 
- Knaack Lake, Wisconsin 
- Hutchinson, G. Evelyn (1957). A Treatise on Limnology. Volume I: Geography, Physics and Chemistry, (Wiley, New York). ISBN 978-0-471-42570-0.
- Hakala, Anu (2005). Paleoenvironmental and paleoclimatic studies on the sediments of Lake Vähä-Pitkusta and observations of meromixis, University of Helsinki doctoral dissertation.
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