Ameromictic lake is alake which 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]
The termmeromictic was coined by the AustrianIngo Findenegg in 1935, apparently based on the older wordholomictic. The concepts and terminology used in describing meromictic lakes were essentially complete following some additions byG. Evelyn Hutchinson in 1937.[3][4][5]
Typical mixing pattern for adimictic lake. This does not occur in meromictic lakes
Most lakes areholomictic: at least once per year, the surface and the deep waters mix. Inmonomictic lakes, the mixing occurs once per year; indimictic lakes, it occurs twice a year (typically spring and autumn), and inpolymictic lakes, the mixing occurs several times a year. In meromictic lakes, the layers of water can remain unmixed for years, decades, or centuries.
Meromictic lakes can usually be divided into three sections or layers. The bottom layer is themonimolimnion; the waters in this portion of the lake circulate little, and are generallyhypoxic and more saline than the rest of the lake. The top layer is themixolimnion, and essentially behaves like a holomictic lake. The area in between is thechemocline, or chemolimnion.[6]
The lack of mixing between layers creates radically different environments for life: the stratification, or stable layering, of lake waters means that the bottom layer receives little oxygen from the atmosphere, 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 ispurple sulfur bacteria. These bacteria, commonly found at the top of themonimolimnion in such lakes, usesulfur compounds such assulfides inphotosynthesis. These compounds are produced by decomposition of organic sediments in oxygen-poor environments. The monimolimnion is often rich inphosphorus andnitrogen. These factors combine to create an ideal environment for bacterial growth. The mixolimnion can have similar qualities. However, the types of bacteria that can grow at the surface are determined by the amount of light received at the surface.[8]
A meromictic lake may form because the basin is unusually deep and steep-sided compared to the lake's surface area, or because the lower layer of the lake is highlysaline and denser than the upper layers of water.[9] However, human influence can lead to cultural meromixis occurring.[10][11][12] The increased use of road salt as a deicing strategy, particularly in northern latitude regions, can disturb the natural mixing cycles in lakes by inhibiting mixing.[13][14] As salt is flushed into aquatic systems at high concentrations in late winter/early spring, it accumulates in the deepest layer of lakes leading to incomplete mixing.
Stratification in meromictic lakes can be eitherendogenic or ectogenic. Endogenic means the patterns seen in the lake are caused by internal events, such as organic matter accumulating in the sediments and decaying, whereas ectogenic means the patterns seen are caused by external causes, like an intrusion of saltwater settling in thehypolimnion, preventing it from mixing.[1]
The layers of sediment at the bottom of a meromictic lake remain relatively undisturbed because there is little physical mixing and few living organisms to agitate them. There is also 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, by examining trapped pollen grains and the types of sediments [seeProxy (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. 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,methane, or other dissolvedgases can build up relatively undisturbed in the lower layers of a meromictic lake. When the stratification is disturbed, as could happen from anearthquake, alimnic eruption may result. In 1986,a notable event of this type took place atLake Nyos inCameroon, causing nearly 1,800 deaths.[15][16][17] In the following decades after this disaster, active research and management has been done to mitigate gas buildup in the future through the Nyos Organ Pipes Program (NOPP).[18] The NOPP program placed large organ pipes intoLake Nyos, to reach the monimolimnion where harmful dissolved gases built up, that allow for gas release to the atmosphere, effectively degassing the monimolimnion.[18] Since 2019,Lake Nyos has successfully been degassed to a nonhazardous concentration of dissolved gas.[18] ParallelingLake Nyos,Lake Kivu is another lake that poses a potentially fatal threat to the community. Some management strategies have suggested taking a different approach, moving gases from the monimolimnion to the mixolimnion, rather than degassing to the atmosphere through organ pipes.[19]
While it is mainly lakes that are meromictic, the world's largest meromictic basin is theBlack Sea. The deep waters below 50 m (160 ft) do not mix with the upper layers that receive oxygen from the atmosphere. As a result, over 90% of the deeper Black Sea volume isanoxic water. TheCaspian Sea is anoxic below 100 m (330 ft). TheBaltic Sea is persistently stratified, with dense, highly saline water comprising the bottom layer, and large areas of hypoxic sediments (seeBaltic Sea hypoxia).
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.[20]
^Findenegg, Ingo (1935). "Limnologische Untersuchungen im Kärntner Seengebiete. Ein Beitrag zur Kenntnis des Stoffhaushaltes in Alpenseen".Internationale Revue der Gesamte Hydrobiologie (in German).32:369–423. As cited by Hakala (2004).
^Hutchinson, G. Evelyn (1937). "A contribution to the limnology of arid regions".Transactions of the Connecticut Academy of Arts and Sciences.33:47–132. As cited by Hakala (2004).
^Sanderson, B.; Perry, K. & Pedersen, T. (15 June 1986). "Vertical Diffusion in Meromictic Powell Lake, British Columbia".Journal of Geophysical Research.91 (C-6):7647–7655.Bibcode:1986JGR....91.7647S.doi:10.1029/JC091iC06p07647.
^Smol, John P.; Brown, S. R.; McNeely, R. N. (1983). "Cultural disturbances and trophic history of a small meromictic lake from central Canada".Paleolimnology. pp. 125–130.doi:10.1007/978-94-009-7290-2_20.ISBN978-94-009-7292-6.
