Epilimnion
The epilimnion (epilimnial) is called in physical limnology the upper heated and strongly agitated water layer in a stratified standing body of water . The epilimnion is separated from the lower water layer , the hypolimnion , by the thermocline , the metalimnion .
In the epilimnion, through the absorption of sunlight (warming), the day builds up through its own temperature stratification. Simultaneous wind action can prevent such stratification and lead to a phase of homogeneous heating. With decreasing solar irradiation in the evening and the nightly cooling of the surface, convection currents occur within the epilimnion , which homogenize the entire epilimnion and under certain circumstances can even include the uppermost layers of the metalimnion in the mixing.
The epilimnion is not to be confused with the mixolimnion , which in meromictic lakes describes the upper layer, which is included in the annual cycle, as the opposite of the monimolimnion .
In spring, the stratifying power of the sun's rays increases and exceeds the daily average more and more the mixing power of the wind and the nightly cooling. In this phase, the thickness of the epilimnion decreases more and more in favor of the metalimnion. The heat balance becomes negative in autumn . The cooling supports the preponderance of the mixing processes. The epilimnion is getting colder and thicker. More and more water from the metalimnion is mixed in upwards. The end effect of this process is the autumn circulation .
The formation of layers of water occurs due to the density anomaly of the water .