Information about the Kentucky State University Cooperative Extension Program 2020
Forrest Wynne, State Extension Specialist for Aquaculture
During winter, reduced solar heat cools air temperatures which lower water temperatures in ponds. Except for periods of occasional ice cover, water in southern ponds are well mixed by wind and rain from the fall through the spring months. During that time, constant water mixing results in relatively uniform water temperatures throughout the water column.
Water stratification will occur in some ponds during the spring and summer. Sunlight begins the process by warming the waters near the pond's surface. With increasing depth, this warming is diminished from the loss of the sun's energy as light penetrates the water column and is absorbed. Cooler waters will accumulate throughout the summer months at depths where sunlight is absent. Depleted dissolved oxygen conditions may result in deep water where there is an absence of oxygen-producing algae or plants. Additionally, organic matter will accumulate and decompose near the pond bottom that will consume even more dissolved oxygen. These cooler waters will remain near the pond bottom until they are displaced by sinking surface waters made colder from lower air temperatures, wind or heavy rains.
Pond turnover is a term used to describe the sudden mixing of stratified water in a pond. The process will frequently occur during the summer or the first cool days and nights of fall. Water that has remained near the pond bottom throughout the warm summer months will be forced to the surface when surface waters become cooler than the deeper water. Dissolved oxygen concentrations may be depleted (below 3 mg/L) throughout the pond provided there is a large enough volume of oxygen-deficient water present to compromise the remaining volume. Sudden mixing of those waters may cause an algal bloom die off, a fish kill, or both.
deficient water present to compromise the remaining volume. Sudden mixing of those waters may cause an algal bloom die off, a fish kill, or both.
To maintain good health, warm-water fish require a concentration of 5 mg/L dissolved oxygen. For several weeks, they may be able to withstand concentrations as low as 2-3 mg/L. Physical attributes of certain ponds may contribute to turn-over events related to low dissolved oxygen that result in fish kills. These may include: excessive plant and algae growth, protection from prevailing winds, pond depth, organic material load and pond age.
In most cases, mechanically de-stratifying or aerating a pond during the warm season would increase the availability of dissolved oxygen to algae and fish. Other potential benefits may include the reduction of toxic chemical compounds building up near the pond bottom and reduced nutrient uptake by undesirable cyanobacteria (blue green algae). Increases in fish production may also result (Hargreaves 2003).
Brief descriptions of some basic pond and lake de-stratification and aeration systems are provided. The system best suited for a particular pond will depend on factors such as: average depth, wind exposure, and pond fertility.
In general, diffused air systems are used for water de-stratification in ponds and lakes deeper than 8 feet. De-stratification systems are run either continuously during the warm season, or year-round, to insure daily mixing of the water column. This helps prevent water stratification and low dissolved oxygen conditions from developing.