These two layers are separated by a layer of water which rapidly changes temperature with depth. The three distinct layers of water, each with a different temperature or range of temperatures, is an excellent example of thermal stratification within a lake system. Figure 2 below shows how the depth of the epilimnion increases through the spring and into the early summer. During the summer the epilimnion will reach a maximum depth and stratification will be maintained for the remainder of the summer. The warm water, abundant sunlight, and nutrients brought up from the lake bottom during spring overturn provide an ideal environment for algae growth within the epilimnion.
Algal blooms tend to give the epilimnion a greenish hue.
Stratification during the summer acts as a deterrent to complete lake mixing. Wind circulates the surface water, but the warm water of the epilimnion is unable to drive through the cold, dense water of the hypolimnion. Without mixing to provide dissolved oxygen, the lake bottom, lacking enough light for photosynthesis to occur, tends to have a very limited supply of oxygen during the summer. Respiration by animals and bacteria can deplete the dissolved oxygen at the bottom of the lake. A stratified lake of this nature is said to be in summer stagnation.