epilimnion

The epilimnion is the uppermost layer of water in a lake, situated above the thermocline. This layer is warmed by the sun and is actively mixed by wind action, leading to relatively uniform temperatures within the epilimnion itself.12

In temperate regions, deep lakes often exhibit thermal stratification during the summer. The epilimnion, being warmer and less dense, remains separate from the colder, denser hypolimnion below. This stratification significantly impacts the lake's ecology, influencing the distribution of dissolved oxygen and carbon dioxide, which in turn affects aquatic life. For instance, the epilimnion may have high oxygen concentrations, while the hypolimnion has low concentrations.3

The epilimnion is part of the limnetic zone, which is the open water area of a lake where light penetrates sufficiently for photosynthesis. Seasonal changes, such as spring and fall overturns, can disrupt this stratification, leading to mixing of the water layers.43