Stratification - Aquatic ecosystems
Aquatic ecosystems also exhibit marked stratification. In lake and ocean ecosystems light penetration, temperature and availability of oxygen varies with depth. In summer, a well stratified lake has a layer of freely circulating surface water with uniform temperature known as the epilimnion.
A second layer called metalimnion which is characterised by a thermocline (a very steep and rapid decline in temperature). The third layer - the hypolimnion, is a deep, cold layer of dense water, often low in oxygen; and a layer of bottom mud. In terms of availability of light, a water body is divided into two layers: an upper lighted zone which is dominated by phytoplanktons and where photosynthesis occurs vigorously and a lower layer in which decomposition is most active. The lower layer roughly corresponds to the hypolimnion and the bottom mud.