Many types of aeration equipment have been used to thermally de-stratify lakes, particularly lakes subject to low oxygen or undesirable algal blooms. Some dimictic lakes can turn into monomictic lakes, while some monomictic lakes might become meromictic, as a consequence of rising temperatures. Lake mixing regimes can shift in response to increasing air temperatures. It also poses a risk of oxygen sag as a long established hypolimnion can be anoxic or very low in oxygen. The mixing of the hypolimnium into the mixed water body of the lake recirculates nutrients, particularly phosphorus compounds, trapped in the hypolimnion during the warm weather. This is often referred to as "autumn turn-over". In temperate latitudes, many lakes that become stratified during the summer months de-stratify during cooler windier weather with surface mixing by wind being a significant driver in this process. The accumulation of dissolved carbon dioxide in three meromictic lakes in Africa ( Lake Nyos and Lake Monoun in Cameroon and Lake Kivu in Rwanda) is potentially dangerous because if one of these lakes is triggered into limnic eruption, a very large quantity of carbon dioxide can quickly leave the lake and displace the oxygen needed for life by people and animals in the surrounding area. Cryostratified lakes exhibit inverse stratification near the ice surface and have depth-averaged temperatures near 4☌, while cryomictic lakes have no under-ice thermocline and have depth-averaged winter temperatures closer to 0☌. Recent research suggests that seasonally ice-covered dimictic lakes may be described as "cryostratified" or "cryomictic" according to their wintertime stratification regimes. Heat waves can cause periods of stratification in otherwise mixed, shallow lakes, while mixing events, such as storms or large river discharge, can break down stratification. However, short-term events can influence lake stratification as well. polymictic, dimictic, meromictic) describes the yearly patterns of lake stratification that occur in most years. There is not a fixed depth that separates polymictic and stratifying lakes, as apart from depth, this is also influenced by turbidity, lake surface area, and climate. In shallow lakes, stratification into epilimnion, metalimnion, and hypolimnion often does not occur, as wind or cooling causes regular mixing throughout the year. If the stratification of water lasts for extended periods, the lake is meromictic. This process occurs more slowly in deeper water and as a result, a thermal bar may form. For example, in dimictic lakes the lake water turns over during the spring and the fall. In temperate regions where lake water warms up and cools through the seasons, a cyclical pattern of overturn occurs that is repeated from year to year as the cold dense water at the top of the lake sinks (see stable and unstable stratification). However, the temperature of maximum density for freshwater is 4 ☌. Cold water is denser than warm water and the epilimnion generally consists of water that is not as dense as the water in the hypolimnion. The thermal stratification of lakes refers to a change in the temperature at different depths in the lake, and is due to the density of water varying with temperature. Lake stratification is stable in summer and winter, becoming unstable in spring and fall when the surface waters cross the 4☌ mark. Typical mixing pattern for many lakes, caused by the fact that water is less dense at temperatures other 4☌ (the temperature where water is most dense). These changes can further alter the fish, zooplankton, and phytoplankton community composition, in addition to creating gradients that alter the availability of dissolved oxygen and nutrients. Rising air temperatures have the same effect on lake bodies as a physical shift in geographic location, with tropical zones being particularly sensitive. However, changes to human influences in the form of land use change, increases in temperature, and changes to weather patterns have been shown to alter the timing and intensity of stratification in lakes around the globe. Typically stratified lakes show three distinct layers: the epilimnion, comprising the top warm layer the thermocline (or metalimnion), the middle layer, whose depth may change throughout the day and the colder hypolimnion, extending to the floor of the lake.Įvery lake has a set mixing regime that is influenced by lake morphometry and environmental conditions. Lake stratification is the tendency of lakes to form separate and distinct thermal layers during warm weather.
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