Epilimnion

The epilimnion or surface layer is the top-most layer in a thermally stratified lake. The epilimnion is the layer that is most affected by sunlight, its thermal energy heating the surface, thereby making it warmer and less dense. As a result, the epilimnion sits above the deeper metalimnion and hypolimnion, which are colder and denser. Additionally, the epilimnion is typically has a higher pH and higher dissolved oxygen concentration than the hypolimnion. In the water column, the epilimnion sits above all other layers. The epilimnion is only present in stratified lakes. On the topside of the epilimnion it is in contact with air, which leaves it open to wind action, which allows the water to experience turbulence. Turbulence and convection work together to make waves which increases aeration. On the bottom side of the epilimnion is the metalimnion, which contains the thermocline. The thermocline is created because of the difference in temperature between the epilimnion and the metalimnion. This is due to the fact that since the epilimnion is in contact with air and is above everything, it interacts with the sun and heat more, making it warmer than the layers below. In certain areas during the winter, the epilimnion will freeze over, cutting off the lake from being aerated directly. Because of the epilimnion's susceptibility to air temperature change, it is often used to monitor warming trends. In most stratified lakes, seasonal changes in the spring and fall air temperature cause the epilimnion to warm up or cool down. During these seasonal changes stratified lakes may experience a lake turnover. During this, the epilimnion and hypolimnion mix together and the lake generally becomes un-stratified, meaning it has a constant temperature throughout, and the nutrients are even throughout the lake. There are different names for these turnovers based on how many times the lake does it in a year. Monomictic lakes flip only once, dimictic flip twice, and polymictic lakes flip more than twice.