Meromictic waters

In physical limnology - and oceanography  - a body of stagnant water in which the vertical water circulation does not take place over the entire depth profile is referred to as a meromictic body of water . The phenomenon is called meromixis .

Basics

Holo mictic mixing of the water body of lakes of great depth

The Mixis system (thorough mixing) is a system of classification of still waters. Here the meromictic (partially mixing, from the Greek meros 'part') waters stand between the holomictic (completely mixing) and amictic (not mixing): In a meromictic water mixing takes place only in certain, separate zones, whereby zones of different ages are formed Forming water, while the water in differently stratified bodies of water, such as dimictic or oligomictic, at least once a year - in the former seasonally, in the latter irregularly - mixes completely.

The term Meromixis was coined by the Austrian Ingo Findeegg in 1935 and expanded significantly by George Evelyn Hutchinson in 1937.

Mechanisms

The regularly circulating upper layer is called mixolimnion ('mixing water'), and the deep water area is called monimolimnion . In the mixolimnion, a regular - permanent or intermittent - mixing stratification with epilimnion (surface water), metalimnion (thermocline) and also a hypolimnion (deep layer) is formed, only this overlays the monimolimnion so that a characteristic separating layer is formed.

There are two main causes.

• Temperature- induced processes: Cold water at 4 ° C ( temperature of the highest water density ) accumulates, a temperature layer is created (separating layer: thermocline ). This mechanism occurs primarily in temperate or temperate climates.
• Salinity and other solutions and mixtures of water: bound substances make water heavier and accumulate (separating layer: chemocline ).

Meromictic waters usually form a stable state that can only be reversed by exceptional events. The Mixolimnion regularly supplies the Monimolimnion with fresh or enriched water. The two processes also occur in combination, on the other hand, high saline Monimolimnia can remain stable even in warm deep water. For example, a small water surface in relation to the depth, where in a sheltered position there is hardly any contact surface for the wind, are beneficial. A special case is the Ödensee , Styria, which becomes meromictic due to cold underground cave inflows: only the warm surface water flows away, the depth remains chemically stratified.

The mixolimnion constantly loses biomass and thus nutrients to the monimolimnion through sedimentation . As a result, anaerobic (oxygen-free) conditions often develop in deep water . A well-known example is the Black Sea , the largest meromictic basin on earth, which is uninhabitable for higher living beings at depth. Also, methane , ammonia , hydrogen sulfide and similar metabolic waste can accumulate that.

An example of an independent biotope caused by a meromictic body of water is the Ongeim'l Tketau (Jellyfish Lake) in Palau, a cut off sea remnant with its jellyfish and sea anemone population in the Mixolimnion, a floating mat of the Chromatium bacterium on the thin, highly acidic one Chemoclines and the anoxic deep water.

In addition to natural processes, anthropogenic events can also lead to the formation of meromictic lakes. Examples are Lake Lugano and Lake Zug , two valley lakes at the edge of the valley, due to eutrophication from agriculture and sewage since the 1950s, the post-mining landscape of Merseburg-Ost ( Raßnitzer See ), or Lake Traunsee in the Salzkammergut due to centuries of industrial discharges from the Bad Ischl and Ebensee salt pans . In such cases, the question of current research is whether and how the "unnatural" Meromixis could be eliminated.

Ammonium distribution in Mono Lake (in micromoles according to water depth, 1982–1988)

Meromictic episodes can also occur in normally oligomictic lakes. At Lake Hallstatt in the Salzkammergut, phases of interrupted mixing from the years 1971–1975, 1981–1988 and from 2006 to 2011 have been investigated due to chlorides from salt mining (due to mine failure), or in Mono Lake , USA, in the 1980s and 1990s due to strong surface influx of fresh water into the lake, which had a highly enriched salt content due to previous water extraction.

In reservoirs - without a bottom outlet - Meromixis is often found during the initial damming due to anaerobic decomposition of the dammed biomass, which usually only stabilizes after one or two decades. For this reason, at least the forest is being felled today, ideally the topsoil is removed. The same thing also happens with the natural formation of new lakes, for example through constriction of salt water or in valleys relocated by mass movements .

Special conditions can also lead to at least partial circulation processes in the monimolimnion, which can lead to poisonous breakdown products of microorganisms to fish death . The carbon dioxide eruption of the Nyos crater lake in Cameroon after an earthquake (1986, 1,800 deaths) is also known.

Meromixis is also increasingly moving into the focus of paleoclimatology because undisturbed sedimentation takes place in the depths, resulting in well-preserved climate archives . It is also assumed that many rich fossil deposits were formed under the low-oxygen conditions of meromictic freshwater lakes or lagoons.

literature

• Anu Hakala: Meromixis as a part of lake evolution - observations and a revised classification of true meromictic lakes in Finland. In: Boreal Environmental Research Vol. 9, 2004, pp. 37-53, borenv.net (PDF).

Web links

Commons : Meromictic Lakes  - Collection of images, videos and audio files

Individual evidence

1. ^ Ingo Findeegg: Limnological investigations in the Carinthian lake areas. A contribution to the knowledge of the substance balance in alpine lakes. In: International Review of the Entire Hydrobiology . Volume 32, 1935, pp. 369-423.
2. ↑ Anu Hakala: Meromixis as a part of lake evolution - observations and a revised classification of true meromictic lakes in Finland. In: Boreal Environmental Research Vol. 9, pp. 37-53, 2004, borenv.net (PDF).
3. ↑ Honorary Professor Dr. Dr. hc Ingo Findeegg in memory . In: Carinthia II . 164./84. Year, 1974, p. 356 ( PDF on ZOBODAT [accessed on May 10, 2013]).
4. ^ G. Evelyn Hutchinson: A contribution to the limnology of arid regions. In: Transactions of the Connecticut Academy of Arts and Sciences Vol. 33, 1937, pp. 47-132; According to Hakala 2004.
5. ↑ Meromiktischer See . , Entry in GeoDataZone, geodz.com
6. ↑ Karl Stundl: Limnological investigations on some Styrian lakes. In: Communications of the natural science association for Styria. 83, 1953, section Ödensee (sea level 780 m, lake area 20 ha) , p. 171–184, here p. 175 ff. ( PDF on ZOBODAT ; there p. 5).
7. ↑ More detailed discussion in the Office of the Styrian Provincial Government, Technical Department Group Provincial Building Directorate: 1. Styrian Lake Report , University Library Graz, 2008, Der Ödensee , 7.2 Oxygen , p. 81 ( Umwelt.steiermark.at ( memento of the original from November 20, 2012 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF); literature.at ). 
8. ↑ John L. Ingraham, Roberto Kolter: March of the Microbes: Sighting the Unseen. New edition, Harvard University Press, 2012, ISBN 978-0-674-05403-5 , p. 28 ( limited preview in Google book search). That is why there are no natural deposits of eel in the Danube, as it lives as a deep-sea fish in its marine phase.
9. ↑ Christian Holzner: Investigation of deep water renewal in meromictic lakes using transient tracers and numerical modeling. Diploma thesis Swiss Federal Institute of Technology - Department of Environmental Sciences, Zurich 2001, library.eawag.ch ( Memento of the original dated December 3, 2013 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF). 
10. ↑ Harald Ficker, Hubert Gassner, Daniela Achleitner, Robert Schabetsberger: Limnological Effects of Brine Discharges on Lake Hallstatt Lecture. Report at the meeting of SIL AUSTRIA , 26. – 28. October 2009 in Salzburg, p. 14, protozoology.com (PDF).
11. ^ Ulrich Maniak: Hydrology and Water Management. Springer DE, 2005, ISBN 978-3-540-27839-9 , p. 534 ( limited preview in the Google book search).
12. ↑ Anu Hakala: Paleoenvironmental and paleoclimatic studies on the sediments of Lake Vähä-Pitkusta and observations of meromixis. Dissertation University of Helsinki, 2005, oa.doria.fi  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. (PDF).  
13. ^ David M Martill, Günter Bechly, Robert F. Loveridge: The Crato Fossil Beds of Brazil: Window Into an Ancient World . Cambridge University Press, 2007, ISBN 978-1-139-46776-6 , esp. P. 59 ( limited preview in Google book search).