Meromictic lake

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Lac Pavin in France is a meromictic crater lake
McGinnis Lake is a meromictic lake near Peterborough, Ontario.

A meromictic lake has layers of water that 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.[2] 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.[3][4][5]

Characteristics[edit]

Lake zones
Littoral zone
Limnetic zone
Profundal zone
Benthic zone
Lake stratification
Epilimnion
Metalimnion
Hypolimnion
Destratification
Lake types
Holomictic lake
   Monomictic lake
   Dimictic lake
   Polymictic lake
Meromictic lake
Amictic lake
Aquatic ecosystems
Wild fisheries
Typical mixing pattern for a dimictic lake. This does not occur in meromictic lakes

Most lakes are holomictic; that is, at least once per year, physical mixing occurs between the surface and the deep waters. In so-called 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, however, the layers of the lake water can remain unmixed for years, decades, or centuries.

Meromictic lakes can usually be divided into three sections or layers. The bottom layer is known as the monimolimnion; the waters in this portion of the lake circulate little, and are generally hypoxic and saltier than the rest of the lake. The top layer is called the mixolimnion, and essentially behaves like a holomictic lake. The area in between is referred to as the chemocline. [6]

The lack of mixing between layers creates radically different environments for organisms to live in: among the consequences of this stratification, or stable layering, of lake waters is that the bottom layer receives little oxygen from the atmosphere, and hence becomes depleted of oxygen. While the surface layer may have 10 mg/L or more dissolved oxygen in summer, the depths of a meromictic lake can have less than 1 mg/L.[7] Very few organisms can live in such an oxygen-poor environment. One exception is purple sulfur bacteria. These bacteria, which are commonly found at the top of the monimolimnion in such lakes, use sulfur compounds such as sulfides in photosynthesis; these compounds are produced by decomposition of organic sediments in oxygen-poor environments. The monimolimnion is often also rich in phosphorus and nitrogen. These factors combine to create an ideal environment for bacteria to grow in. The mixolimnion can have similar qualities; however, the types of bacteria that can grow at the surface are determined by the amount of light that the surface receives. [8]

A meromictic 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 upper layers 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. There is also very little chemical decomposition. For this reason cores of the sediment at the bottom of meromictic lakes are important in tracing past changes in climate at the lake, through examining trapped pollen grains and the types of sediments themselves [see Proxy (climate)].

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 much smaller in volume than the monimolimnion; therefore, when the layers mix, the oxygen concentration at the surface will decrease dramatically. This can 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.[9]

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 dense, highly saline water comprising the bottom layer, and large areas of hypoxic sediments (see Baltic Sea hypoxia).

List of meromictic lakes[edit]

Strandvatnet in Nordland down to the left; only a small isthmus separates the lake from Ofotfjord.
Soapy foam on the shore of Soap Lake
Lac du Bourget is the largest and deepest lake in France
Green Lake is a meromictic lake near Syracuse, New York.
Sunfish Lake is a meromictic lake near Waterloo, Ontario.

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.[10]

Africa[edit]

Antarctica[edit]

Asia[edit]

Australia[edit]

Europe[edit]

North America[edit]

References[edit]

  1. ^ Wetzel, Robert G. (2001). Limnology: Lake and River Ecosystems (Third Edition) (Academic Press, New York). ISBN 978-0-12-744760-5.
  2. ^ Lewis, William M., Jr. (1983). "A revised classification of lakes based on mixing". Canadian Journal of Fisheries and Aquatic Sciences 40 (10): 1779–1787. doi:10.1139/f83-207. 
  3. ^ Hakala, Anu (2004). "Meromixis as a part of lake evolution - observations and a revised classification of true meromictic lakes in Finland," Boreal Environmental Research Vol. 9, pp. 37-53.
  4. ^ Findenegg, Ingo (1935). "Limnologische Untersuchungen im Kärntner Seengebiete. Ein Beitrag zur Kenntnis des Stoffhaushaltes in Alpenseen," Internationale Revue der Gesamte Hydrobiologie Vol. 32, pp. 369-423; as cited by Hakala (2004).
  5. ^ Hutchinson, G. Evelyn (1937). "A contribution to the limnology of arid regions," Transactions of the Connecticut Academy of Arts and Sciences Vol. 33, 47-132, as cited by Hakala (2004).
  6. ^ Walker, K. F. "The Stability of Meromictic Lakes in Central Washington." JSTOR. N.p., Mar. 1974. Web. 27 Sept. 2013.
  7. ^ Lampert, Winfried and Sommer, Ulrich (1997). Limnoecology: The Ecology of Lakes and Streams (Oxford University Press, Oxford). Translation by James F. Haney. ISBN 978-0-19-509592-0.
  8. ^ Fry, Brian. "Sources of Carbon and Sulfur Nutrition for Consumers in Three Meromictic Lakes of New York State." JSTOR. N.p., May 1986. Web. 27 Sept. 2013.
  9. ^ Krajick, Kevin (2003). "Defusing Africa's Killer Lakes," Smithsonian Magazine, September 2003 issue.
  10. ^ Hakala, Anu (2005). Paleoenvironmental and paleoclimatic studies on the sediments of Lake Vähä-Pitkusta and observations of meromixis, University of Helsinki doctoral dissertation.
  11. ^ Gene E. Likens (2010-05-17). Lake Ecosystem Ecology: A Global Perspective: a Derivative of Encyclopedia of Inland Waters. Academic Press. Retrieved 2012-07-22. 
  12. ^ Jacquet, Stéphan et al. (2003). "The proliferation of the toxic cyanobacterium Planktothrix rubescens following restoration of the largest natural French lake (Lac du Bourget)," Harmful Algae, 4:651-672.
  13. ^ Anderson, G.C. (1958). Some Limnological Features of a Shallow Saline Meromictic Lake, Limnology and Oceanography, 3(3), 259-270.
  14. ^ [Sanderson, B., Perry, K., Pedersen, T.: Vertical Diffusion in Meromictic Powell Lake, British Columbia, Journal of Geophysical Research, 91(C-6), 7647-7655, 1986]
  15. ^ Redoubt Lake Sockeye Salmon Enhancement and Monitoring, Tongass National Forest, US Forest Service
  16. ^ http://www.nps.gov/piro/naturescience/lakesandponds.htm
  17. ^ Weimar, Walter C., and G. Fred Lee. "Some Considerations of the Chemical Limnology of Meromictic Lake Mary." JSTOR. N.p., May 1973. Web. 27 Sept. 2013.
  18. ^ Cloern, James E., Brian E. Cole, and Ronald S. Oremland. "Autotrophic Processes in Meromictic Big Soda Lake, Nevada." JSTOR. N.p., May 1983. Web. 27 Sept. 2013.
  19. ^ The Role of Phototrophic Bacteria in the Sulfur Cycle of a Meromictic Lake

Bibliography[edit]

External links[edit]