Meromictic Lake - Characteristics

Characteristics

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

Among the consequences of this stable layering (or stratification) of lake waters is that the deeper layer (the "monimolimnion") receives little oxygen from the atmosphere. The monimolimnion becomes depleted of oxygen. While the surface layer (the "mixolimnion") may have 10 mg/l or more dissolved oxygen in summer, the monimolimnion in a meromictic lake has less than 1 mg/l. Very few organisms can live in this oxygen-poor environment. One exception is purple sulfur bacteria. These bacteria, which are commonly found at the top of the monimolimnion in meromictic lakes, use sulfur compounds for photosynthesis; sulfur compounds are one of the products of sediment decomposition in "anoxic" (oxygen poor) environments.

This type of 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 higher levels 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, and very little oxygen or chemical decomposition. For this reason corings of the sediment at the bottom of meromictic lakes are important research tools in tracing climate history at the lake.

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 of a much smaller volume than the monimolimnion and therefore when they mix the oxygen concentration in mixolimnion will decrease dramatically. This may 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 due to 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.

While it is mainly lakes that are meromictic, the world’s largest meromictic basin is the Black Sea. Here 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 large hypoxic sediment areas below its halocline.