Water circulation
Water circulations in the sense of limnology are phases of mixing in still waters ( lakes , lagoons , etc.). These transition phases lie between phases with stable temperature stratification . They play an important role in the lake ecosystem .
Stagnation and circulation
In a typical, sufficiently deep lake in the temperate zone, the following sequence occurs over the course of a year (dimictic circulation type ):
The upper layer ( epilimnion ) heats up due to solar radiation . It is separated from the lowest layer, the hypolimnion with its only 3.98 ° C (see density anomaly of water), by a thermocline with a steep temperature gradient ( metalimnion ). This temperature-related density stratification of the lake prevents the exchange of water between the depths.
In the epilimnion and parts of the metalimnion (in the trophogenic zone ), oxygen is produced by photosynthesis during this summer stagnation and, if it is undersaturated, it is also introduced from the air. In the hypolimnion only oxygen-consuming processes take place ( tropholytic zone ), so that the oxygen stored here can be reduced and in extreme cases even completely consumed.
As a result of the autumn cooling on the surface, a mixing begins to work its way downwards, driven by convection and wind. If a uniform temperature and finally a temperature of 3.98 ° C has been reached on the surface, there is no longer any density obstacle to water exchange in the entire body of water. This condition is called full circulation or, depending on the season, autumn circulation .
As a result of further cooling, colder and therefore less dense water is deposited on the surface and thus forms a cold-stable layer that can freeze over from the surface. This is winter stagnation .
A renewed warming in spring destroys this weak stratification and leads to spring circulation , which is followed by a new summer stagnation.
Ecological importance
In the course of the progressive mixing and the subsequent full circulation , nutrient-poor, but oxygen-rich surface water descends, while oxygen-poor, but nutrient-rich deep water reaches the surface. In this way, oxygen returns to the depths where it was previously consumed in the tropholytic zone, and the nutrients released there are again distributed to the surface, where they are again available for primary production in the trophogenic zone .
Circulation types
Waters of different depths and in different climatic zones can be classified according to circulation types. The annual number of circulations and the temperature distribution between the layers serve as systematic characteristics.
These groups, also known as mix types , are:
- Holomictic type: Still waters of this type experience a complete mixing at least once a year (e.g. in autumn or spring). They are divided into
- Monomictic type with the subtypes cold-monomictic and warm-monomictic : The water body in the monomictic water body only circulates once a year. This type occurs both in subpolar and polar areas (there a lake circulates in summer) and in the subtropics (there it circulates in winter).
- Dimictic type with the subtypes cold-dimictic and warm-dimictic : The water body in the dimictic water type circulates twice a year. Water bodies of this type are found mainly in the temperate zone , where the water body circulates in spring and autumn, while in summer and winter two layers of water, which are very different in temperature and density, are separated by a buffer zone and almost without exchange. There are other dimictic lakes in West Africa , for example , where the circulation depends on the rainy season .
- Oligomictic type: This type is found in tropical regions with consistently high temperatures during the day. The circulation of these waters is irregular and rather rare.
- Polymictic type with the sub-types moderate-polymictic , cold-polymictic and warm-polymictic : This type is found in shallow waters in the temperate latitudes and in lakes in tropical areas with strong nocturnal cooling, such as in the tropical high mountains. With them, the water circulates frequently or all year round.
- Atelomictic type: This type occurs in weakly stratified waters; they experience a short-term mixing during the nocturnal cooling phases.
- Meromictic type: Standing waters of this type do not experience complete mixing. This is usually due to an unfavorable relationship between surface and depth. In the deeper, unmixed body of water, the water of the greatest water density at 4 ° C is enriched with sedimentation and degradation products and thus becomes even denser. An oxygen-free or salt-enriched deep zone, the monimolimnion , is permanently formed . Meromictic lakes also arise when a mixed body of fresh water overlays a denser, exogenously fed salt water body, which is then not mixed.
- Amictic type: waters of this type do not have any full circulation and are chemically stable. This can occur, for example, through salinisation of the water body or through the formation of a permanent ice cover in highly polar regions.