The epilimnion is the upper layer of water in a stratified lake or reservoir. It is typically warm and well-mixed due to wind action, which helps distribute heat and oxygen. This layer is crucial for aquatic life, as it supports various organisms, including phytoplankton and fish. Beneath the epilimnion lies the thermocline, where temperature changes rapidly with depth. The epilimnion is important for understanding lake ecology and is often studied in relation to eutrophication and water quality. Its characteristics can influence the overall health of the aquatic ecosystem.