Meromictic lake

A meromictic lake is a lake which has layers of water that do not intermix. In ordinary, holomictic lakes, at least once each year, there is a physical mixing of the surface and the deep waters.

The term meromictic was coined by the Austrian Ingo Findenegg in 1935, apparently based on the older word holomictic. The concepts and terminology used in describing meromictic lakes were essentially complete following some additions by G. Evelyn Hutchinson in 1937.

Most lakes are holomictic: at least once per year, the surface and the deep waters mix. In monomictic lakes, the mixing occurs once per year; in dimictic lakes, it 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 water can remain unmixed for years, decades, or centuries.

Meromictic lakes can usually be divided into three sections or layers. The bottom layer is the monimolimnion; the waters in this portion of the lake circulate little, and are generally hypoxic and more saline than the rest of the lake. The top layer is the mixolimnion, and essentially behaves like a holomictic lake. The area in between is the chemocline, or chemolimnion.

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. Very few organisms can live in such an oxygen-poor environment. One exception is purple sulfur bacteria. These bacteria, commonly found at the top of the monimolimnion in such lakes, use sulfur compounds such as sulfides in photosynthesis. These compounds are produced by decomposition of organic sediments in oxygen-poor environments. The monimolimnion is often rich in phosphorus and nitrogen. 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.

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 highly saline and denser than the upper layers of water. However, human influence can lead to cultural meromixis occurring. 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. 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 either endogenic 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 the hypolimnion, preventing it from mixing.

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 [see Proxy (climate)].