Meromictic lake

As meromiktisches waters are referred to in the physical limnology - and Oceanography - standing bodies of water, in which the vertical circulation of water does not take place over the entire depth profile. The phenomenon is called meromixis.

Basics

The oxis system ( mixing ) is a system of classification of standing waters. Here the meromictic waters (Greek meros, part ') are between the holomiktischen ( by mixing ) and amiktischen ( not mixed ) types. In them, mixing takes place only in specific, separated regions. This zones are formed from different old water. This differs meromiktische waters of other layered water bodies, such as dimictic or oligomiktischen types that at least once a year - former seasonally, temptation irregularly - completely mix.

The term was coined by the Austrian Ingo meromixis Findenegg in 1935 and substantially expanded by G. Evelyn Hutchinson, 1937.

Mechanisms

The upper layer is called regularly circulating Mixolimnion (, Mixing water '), the deep-water area monimolimnion. In Mixolimnion often forms a regular - permanent or intermittent - mixed layering with the epilimnion ( surface water), metalimnion ( thermocline ) and a hypolimnion (deep layer) of just overlaid them the monimolimnion, so that forms a characteristic separation layer.

There are mainly two causes in question.

• Temperature -induced processes: Cold water below 4 ° C ( temperature of the largest water density) accumulates, it creates a temperature stratification ( separation layer: thermocline ). This mechanism occurs primarily in temperate or temperate climates.
• Salinity and other solutions and mixtures of water: By -bound substances, water is heavier and accumulates (interface: chemocline ).

Meromiktische waters usually form a stable condition which can only be lifted by Ausnahmsereignisse. The Mixolimnion supplies the monimolimnion regularly with supplies of cold water or enriched. The two processes also occur in combination, on the other hand, can remain stable high aline Monimolimnia even in warm deep water. Favoring, for example, a small water surface in relation to the depth where by little surface for the wind is formed in sheltered position. A special case is about the Ödensee, Styria, the meromictic is cold underground cave inflows: flows from only the warm surface water, the depth remains chemically stratified.

The Mixolimnion loses constantly by sedimentation biomass and thus nutrients to the monimolimnion. This often form in the deep-water anaerobic ( oxygen-free ) conditions from. A well known example is the Black Sea, the largest pool meromiktische the earth, which is uninhabitable in depth for higher organisms. Also, methane, ammonia, hydrogen sulfide and other metabolic waste products can accumulate so.

An example of a stand-alone habitat, through a meromiktisches waters due, the Ongeim'l Tketau (Jelly Fish Lake ) in Palau, highly acidic a truncated marine rest with his Jellyfish and Seeanemonenpopulation in Mixolimnion, a floating mat of Chromatium bacterium to the thin, chemocline and anoxic deep water.

In addition to natural processes and anthropogenic events for training can lead meromiktischer lakes. Examples are the Lake Lugano and Lake Zug, two rand alpine Talungsseen by eutrophication from agriculture and wastewater since the 1950s, the mining lake Merseburg -Ost 1b ( Raßnitzer lake ), or the Traunsee in the Salzkammergut by centuries of industrial discharges from the salt flats of Bad Ischl and Ebensee. In such cases, current research question, whether and how to cancel the " unnatural " meromixis.

It can come in normally oligomiktischen lakes also meromictic episodes. Thus, on Lake Hallstatt in the Salzkammergut by chlorides from the salt mining ( through mine afflictions ) phases interrupted mixing from the years 1971-1975, 1981-1988 and studied since 2006, and in Mono Lake, California, in the 1980s and 1990s by strong surface inflow of freshwater into the lake, which had by prior water extraction a highly enriched salt content.

In reservoirs - without bottom outlet - can often be found at Erststau by anaerobic decomposition of biomass accumulated over meromixis, which usually stabilizes after a decade or two. For this reason, if possible at least the forest topsoil is now felled, optimally removed. The same also takes place in the natural formation of lakes around by pinching off from Salt water and laid by mass movements Talungen.

Due to special circumstances, it may also lead to at least partial circulation processes come into the monimolimnion, causing potentially toxic breakdown products of microorganisms can lead to fish kills. It is also known carbon dioxide eruption of Crater Lake Nyos in Cameroon after earthquake (1986, 1800 deaths ).

Meromixis moves increasingly into the focus of Palaeoclimatology, because in the depth of undisturbed sedimentation takes place, which preserved climate archives arise. Also it is assumed that many rich fossil deposits were formed under low-oxygen conditions meromiktischer freshwater lakes or lagoons.