the lake's surface area, or because the lower layer of the lake is highly saline and denser than the upper layers of water. 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)]. 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 much smaller in volume than the monimolimnion. When the layers mix, the oxygen concentration at the surface will decrease dramatically. This can result in the death of many organisms, such as fish, that require oxygen. Occasionally, carbon dioxide 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 from 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. The deep waters below 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 . The Baltic Sea is persistently stratified, with dense, highly saline water comprising the bottom layer, and large areas of hypoxic sediments (see Baltic Sea hypoxia). List of meromictic lakes There are meromictic lakes all over the world. The distribution appears to be clustered, but this may be due to incomplete investigations. Depending on the exact definition of "meromictic", the ratio between meromictic and holomictic lakes worldwide is around 1:1000. Africa Lake Nyos and Lake Monoun in Cameroon Lake Kivu in