The upper warm zone is called the epilimnion. Expressed more technically, the mere consists entirely of the epilimnion. In the absence of oxygen from the epilimnion, decomposition can cause hypoxia in the hypolimnion. The DCM of clear, stratified water is commonly found below the epilimnion. The warm layer is called the epilimnion and the cold layer is called the hypolimnion. During the colder fall season, heat is lost at the surface and the epilimnion cools.