Pond Turnover:

Pond turnover is the mixing of the stagnant, stratified layers of water in a pond.  The upper level of water is known as the epilimnion.  This is where the water warm and has a high oxygen content.  This is also where fish are found.  The middle layer is known as the thermocline.  This is where the water temperature and oxygen level will gradually decline with increasing depth.  The lowest layer is the hypolimnion.  This is the layer that is the coldest and is the most oxygen deprived. 

Pond turnover occurs when weather conditions favor the mixing of these different layers.  In the Fall, the upper level is cooled by the colder air.  This makes the water more dense and it will sink to the bottom.  This will cause the bottom layer to then rise to the top. The opposite occurs after the winter when the water warms-up in the Spring.  Pond turnover can also be caused by cold rains or high winds during the summer.  This event can cause fish kills because the deoxygenated water is brought to the surface where the fish dwell, and can suffocate them.