When you look at the surface of a pond or reservoir, you may think that the water is uniform from top to bottom. This is not exactly true for many ponds and reservoirs. During the summer months, ponds and reservoirs are typically stratified into three layers. The water near the surface is warmer and contains an adequate level of dissolved oxygen. This upper layer is referred to as the epilimnion. The water at and near the bottom of the pond or reservoir is quite ifferent. It is much cooler and has very little dissolved oxygen. This lower layer is known as the hypolimnion. A thin transition layer is typically found between the warmer and cooler water. This middle layer is known as the thermocline.
As fall approaches, the upper water layer begins to cool down. This will reduce the density difference between the layers. A cold rain or strong wind can trigger sudden mixing of the layers, or turnover. If the stagnant layer (layer with little to no dissolved oxygen) is relatively large when compared to the upper layer, a fish kill can result from turnover.
Fish kills from pond turnover are caused by a lack of oxygen. They generally occur because turnover happens quickly and prematurely. If the pond water has the chance to begin to cool naturally, turnover will not be so sudden or catastrophic. To avoid problems related to turnover, aeration systems can be used and ponds should not be constructed too deep.
Ponds and reservoirs may also become stratified or layered during the winter. In this case, the cooler water is at the surface. The water near the bottom is warmer. As the weather warms in the spring and spring winds blow, the water in the pond or reservoir will be mixed, and the layers will disappear. The water will remain at a near uniform temperature until late spring or early summer, when it will once again separate into layers.
If you would like to learn more about farm pond management, please contact the ________ County Cooperative Extension Service.
NOTE TO AGENTS: The Kentucky Department of Fish and Wildlife has a booklet available titled "A Guide to the Management of Farm Ponds in Kentucky". This booklet provides a great deal of resource information.Prepared by Henry Duncan, Extension Water Quality Liaison, and Kimberly B. Henken, Extension Associate for Environmental and Natural Resource Issues