Meromictic

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See also: Lake stratification

A meromictic lake has layers of water which do not intermix.^[1] 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 Fall), 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 zoologist G. Evelyn Hutchinson in his 1957 Treatise on Limnology ^[2] to describe lakes for which mixing occurs less frequently than annually. 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.^[3] 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 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.^[4]

Contents 1 Various Meromictic Lakes on Earth 1.1 North America 1.2 Australia & New Zealand 1.3 Europe 1.4 Africa 1.5 Asia 2 References 3 External links

[edit] Various Meromictic Lakes on Earth

Sorted to the different geographical regions, here are examples of meromictic lakes:

[edit] North America

• Soap Lake in Washington.
• Green Lake and Round Lake, in Green Lakes State Park, 13km east of Syracuse, New York
• Ballston Lake, 30 km NNW of Albany, New York
• Crawford Lake near Milton, Ontario
• Pink Lake in Gatineau Park, Quebec
• McGinnis Lake in Petroglyphs Provincial Park, Ontario
• Mahoney Lake in the Okanagan Valley, British Columbia
• Irondequoit Bay (Rochester, NY) is also considered meromictic. Use of road salt has been cited as the main reason for its change.

[edit] Australia & New Zealand

• Lake Fidler, beside the Gordon River in Tasmania's Wilderness World Heritage Area, Australia.

[edit] Europe

• Salsvatnet, Kilevann, Tronstadvatn, Birkelandsvatn, Rørholtfjorden, Botnvatn, Rørhopvatn and Strandvatn lakes in Norway are meromictic.
• Lake Cadagno is a "crenogenic" meromictic lake in Switzerland, and the location of the Alpine Biology Center (Centro Biologia Alpina).

[edit] Africa

• Lake Nyos and Lake Monoun in Cameroon

• Lake Kivu in Rwanda

[edit] Asia

• Pantai Keracut (Keracut Beach) Lake, National Park of Penang, northwest Penang island, Malaysia
• The Black Sea is also considered to be meromictic.

[edit] References

1. ^ Wetzel, Robert G. (2001). Limnology: Lake and River Ecosystems (Third Edition) (Academic Press, New York). ISBN 978-0127447605.
2. ^ Hutchinson, G. Evelyn (1957). A Treatise on Limnology. Volume I: Geography, Physics and Chemistry, (Wiley, New York). ISBN 978-0471425700
3. ^ Lampert, Winfried and Sommer, Ulrich (1997). Limnoecology: The Ecology of Lakes and Streams (Oxford University Press, Oxford). Translation by James F. Haney. ISBN 978-0195095920.
4. ^ Krajick, Kevin (2003). "Defusing Africa's Killer Lakes," Smithsonian Magazine, September 2003 issue.

[edit] External links

• "Density Stratification", part of an educational website Water on the Web operated by the University of Minnesota, Duluth. Retrieved 11-March-2007.
• Lake Fidler revived