Epilimnion

 

During the coldest times in the winter the ponds water temperature can fall to around 4°C. The water temperature remains fairly constant through it's depth.

As the days lengthen, and the warmth from the sun starts to gather strength, the upper layers will start to warm. The suns heat is only able to penetrate the upper layers since it's heat is quickly absorbed by the water. As warm water floats on cold, this upper layer starts to warm beyond those below. This layer is unstable to begin with and if a storm were to occur the waters would be quickly mixed. But slowly as the summer progresses this upper layer will become more stable and well defined.

This is the epilimnion, the area of warmest water within the pond and normally the area of most activity. Although well defined by mid-summer, it's by no means a static body. Wind blowing across the surface of the pond creates a circulation within the epilimnion and across the thermocline, allowing more of the waters to absorb the heat from the sun,slowly raising its temperature. This keeps the epilimnion thoroughly mixed by not only the circulation but the eddying and turbulence that will also occur within its structure.

From what has been said before it would seem as though the epilimnion is a constant depth across the pond's surface; this is not necessarily the case. During winds, the epilimnion can be moved bodily to the leeward side of the pond, causing a wedge deep at the leeward side, and shallow at the windward side of the pond. In extreme winds, this can even go so far, that there is no epilimnion at the windward end of the pond as the hypolimnion comes to the surface. When the wind drops, the epilimnion will roll back until it is level then overshoot and build upon the opposite bank. It will continue this movement until slowly it is once again stable.

The diagram to the left shows how this rocking motion of the epilimnion occurs until once again when conditions are calm it reaches rest and stability.

As autumn's shorter days start to take there toll, temperature will be lost through radiation especially on cold clear nights. This process will slowly break down the epilimnion, until a winter wind will totally obliterate it and it will become totally mixed with the hypolimnion and remain so until the warmer weather once again appears to repeat the process.