hypolimnion

The hypolimnion is the deepest layer of a stratified lake or reservoir, lying below the metalimnion (the thermocline) and the warmer epilimnion on top. During warm seasons in temperate regions, solar heating creates stable density stratification: a warm, well-mixed epilimnion at the surface, a rapid temperature gradient in the metalimnion, and a cold, dense hypolimnion at depth. The hypolimnion is typically cool relative to the upper layers and tends to be relatively uniform in temperature, often around 4°C, though the exact temperature can vary with depth, basin shape, and climatic conditions. In many lakes, especially those that are nutrient-rich, the hypolimnion becomes depleted of dissolved oxygen as microbes decompose sinking organic matter, leading to hypoxic or anoxic conditions by late summer.

Light penetration is minimal; mixing is limited, so chemical and biological processes unfold slowly. Under reducing conditions, phosphorus and other nutrients may be released from sediments, a phenomenon known as internal loading, which can influence productivity when mixing redistributes nutrients. Gas exchange with the atmosphere is limited, and dissolved gases such as carbon dioxide or methane can accumulate.

Ecologically, the hypolimnion supports cold-water organisms and may influence fish distribution and growth by shaping temperature and oxygen regimes. It also affects water quality and reservoir management; techniques such as hypolimnetic aeration or oxygenation aim to prevent severe oxygen depletion and control nutrient release during stratification. Compared with the epilimnion, the hypolimnion is colder, darker, more chemically stratified, and more prone to anoxia in warmer climates; it is a defining feature of a lake’s vertical structure during stratification and disappears during turnover when the layers mix.