Science: Five Lake types

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Lac Pavin in France is a meromictic crater lake.

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1. Meromictic lake

A meromictic lake has layers of water that do not intermix. In normal lakes wind will make turbulence on the waters surface and cause the water near the surface and the water near the bottom to mix it may happen multiple times per year. But, wind is only effective at times of the year when the lake's deep waters are not much colder than its surface waters.

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 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.

Sources: Wkipedia