The epilimnion or surface layer is the top-most layer in a thermally stratified lake, occurring above the deeper hypolimnion. It is warmer and typically has a higher pH and higher dissolved oxygen concentration than the hypolimnion.
Being exposed at the surface, it typically becomes turbulently mixed as a result of surface wind-mixing. It is also free to exchange dissolved gases such as O2 and CO2 with the atmosphere. Because this layer receives the most sunlight it contains the most phytoplankton. As they grow and reproduce they absorb nutrients from the water, when they die they sink into the hypolimnion resulting in the epilimnion becoming depleted of nutrients.
References
Epilimnion Wikipedia(Text) CC BY-SA