The hypolimnion or under lake is the dense, bottom layer of water in a thermally- stratified lake. The word hypolimnion is derived from the Greek "limnos" meaning "lake". It is the layer that lies below the

thermocline A thermocline (also known as the thermal layer or the metalimnion in lakes) is a thin but distinct layer in a large body of fluid (e.g. water, as in an ocean or lake; or air, e.g. an atmosphere) in which temperature changes more drastically with ...

. Typically the hypolimnion is the coldest layer of a lake in summer, and the warmest layer during winter. In deep,

temperate In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout t ...

lakes, the bottom-most waters of the hypolimnion are typically close to 4 °C throughout the year. The hypolimnion may be much warmer in lakes at warmer latitudes. Being at depth, it is isolated from surface wind-mixing during summer, and usually receives insufficient irradiance (light) for

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...

to occur.

Oxygen dynamics

The deepest portions of the hypolimnion have low oxygen concentrations. In eutrophic lakes, the hypolimnion is often

anoxic The term anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts: * Anoxic waters, sea water, fresh water or groundwater that are depleted of diss ...

. Deep mixing of lakes during the fall and early winter allows oxygen to be transported from the

epilimnion The epilimnion or surface layer is the top-most layer in a thermally stratified lake. It sits above the deeper metalimnion and hypolimnion. It is typically warmer and has a higher pH and higher dissolved oxygen concentration than the hypolimnion ...

to the hypolimnion. The cooling of the epilimnion during the fall reduces lake stratification and allows for mixing to occur. The hypolimnion can be anoxic for up to half the year. Anoxia is more common in the hypolimnion during the summer when mixing does not occur. In the absence of oxygen from the epilimnion,

decomposition Decomposition or rot is the process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water, simple sugars and mineral salts. The process is a part of the nutrient cycle and is e ...

can cause hypoxia in the hypolimnion.

Hypolimnetic aeration

In eutrophic lakes where the hypolimnion is anoxic,

hypolimnetic aeration Via deep water aeration or hypolimnetic aeration, the oxygen demand of deep water is provided by oxygen from the atmosphere without destroying the lake’s natural stratification. Thus the deep water becomes aerobic, the dissolving of phosphates i ...

may be used to add oxygen to the hypolimnion. Adding oxygen to the system through aeration can be costly because it requires significant amounts of energy.

References

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External links

Water on the Web
Limnology Aquatic ecology

Oxygen dynamics

Hypolimnetic aeration

See also

References

External links