The upper layer of water in a lake. Comparehypolimnion. Seethermocline.
Geology and Earth Sciences
The upper, warm, circulating water in a thermally stratified lake in summer. Usually it forms a layer that is thin compared to the hypolimnion.
Geography
The upper layer of a body of water, penetrated by light, thus enabling photosynthesis. This zone is warmer, and contains more oxygen, than the layers below.