The hypolimnion or under lake is the dense, bottom layer of water in a thermally-stratified lake.
Typically the hypolimnion is the coldest layer of a lake in summer, and the warmest layer during winter.
Oxygen dynamics
The deepest portions of the hypolimnion often have lower oxygen concentrations than the surface waters (i.e.,
epilimnion).
While oxygen can typically exchange between surface waters and the atmosphere (i.e., in the absence of ice cover), bottom waters are comparatively isolated from atmospheric replenishment of oxygen. In particular, during periods of thermal stratification, gas exchange between the epilimnion and hypolimnion is limited by the density difference between these two layers. Consequently,
decomposition of organic matter in the water column and sediments can cause oxygen concentrations to decline to the point of hypoxia (low oxygen) or anoxia (no oxygen).
In
Dimictic lake, eutrophic lakes, the hypolimnion is often
Anoxic waters throughout a majority of the stratified period.
However, hypolimnetic oxygen concentrations are replenished in the fall and early winter in many temperate lakes, as
lake turnover allows mixing of oxic surface waters and anoxic bottom waters.
Notably, Anoxic waters conditions in temperate lakes have the potential to create a positive feedback, whereby anoxia during a given year begets increasingly severe and frequent occurrences of anoxia in future years.
Hypolimnetic aeration
In eutrophic lakes where the hypolimnion is anoxic, hypolimnetic aeration 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.
See also
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