Which layer of a lake is cool, poorly mixed, and anaerobic?

The layer of a lake that is cool, poorly mixed, and anaerobic is the hypolimnion. This layer is found below the thermocline and typically remains at a lower temperature compared to the warmer upper layer, known as the epilimnion. The hypolimnion is characterized by minimal mixing, especially during the stratified periods of warmer months, leading to stagnant conditions.

Due to its depth and isolation from the wind and surface currents that mix the upper layers, the hypolimnion often becomes depleted of dissolved oxygen. This makes it an anaerobic environment, meaning that oxygen levels are too low to support aerobic life forms. The lack of mixing also means that any organic matter that sinks to this layer decomposes anaerobically, leading to the accumulation of gases like methane and hydrogen sulfide.

In contrast, the epilimnion is warm, well-mixed, and supports a diverse range of aquatic life. The thermocline serves as a transitional layer between the epilimnion and the hypolimnion, where temperature changes rapidly with depth, but it does not share the same anaerobic characteristics as the hypolimnion. Therefore, the hypolimnion is distinctly recognized for its cooler temperatures, poor