Turnover is an event that occurs when pond water becomes stratified. Essentially stratification is when the water in your pond is not being mixed and the top layer becomes heated by the sun while the bottum layer stays cool. The problem with this is because oxygen levels become depleted on those cooler levels.
This is where turnovers come in. Turnovers are when the warm and cool layers of water rapidly mix which dilutes oxygen levels even further and usually result in fish kills. Although turnovers occur naturally in the fall ever year, they are usually much more gradual and don’t cause much harm. Rapid turnovers on the other hand typically occur during the warmer months and coincide with storms or windy days.
The best way to prevent stratification and turnovers is mechanical circulation, mainly through a well maintained aeration system. It is also important that the construction of your pond is at the proper ratios. The ratio of surface waters, called epilimnion, to deep waters, called hypolimnion, is a major factor to the amount and severity of turnovers you can expect.