As the weather cools into fall, one of the more dangerous periods of the year for Arkansas ponds begins, according to Scott Jones, small impoundment Extension specialist at the University of Arkansas at Pine Bluff.
Driven by the dissolution of thermal water layers that developed over summer, the fall turnover season can cause dissolved oxygen problems that in severe cases can cause significant fish kills. Pond owners need to be prepared by knowing the warning signs and take precautions while there is still time.
“Turnovers are a natural annual occurrence in most Arkansas ponds. They usually go unnoticed and cause no trouble. Water’s density changes relative to its temperature,” Jones said. “As surface water warms from increasing temperature and sunlight in spring, its density declines. The warmer surface water (the epilimnion) ‘floats’ on top of cooler, denser water near the bottom (the hypolimnion). Eventually, the difference in temperature and density between the layers is large enough that they will not mix again until fall.”
As the weather cools in fall, temperatures of the epilimnion decline. When the temperatures of the epilimnion match the hypolimnion, the whole pond volume can once again fully mix, he said. This period is known as the turnover. It can often be identified visually by a change in appearance from clear or green, to a cloudy, murky or muddier look than before.
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In some cases, a sulfurous (rotten egg) odor can be detected. While this odor is a sign of rich organic accumulation, and likely anoxic conditions near the pond bottom during summer, it is not a definite indicator of danger to fish.
“What makes turnovers dangerous, is that in most ponds most of the dissolved oxygen is located near the surface in the epilimnion,” Jones said. “As the pond mixes, the dissolved oxygen near the surface is diluted throughout the entire pond volume. Aerobic bacteria often go dormant during summer as the pond bottom becomes hypoxic. While these bacteria are dormant, organic muck is not broken down as quickly and it accumulates on the pond bottom.”
As oxygen reaches the pond bottom during the turnover, the aerobic bacteria reactivate, consuming the abundant organic matter and fresh oxygen quickly. This increases the demand on limited and diluted dissolved oxygen supplies and can lead to fish kills, he said. The disruption of dissolved oxygen, water chemistry and appearance of a pond following turnover can last a couple days to a couple weeks.
Certain combinations of weather patterns, pond design and management strategies can lead to more severe turnovers that can cause fish kills. Deeper ponds with less access to mixing forces tend to be at higher risk for oxygen problems during turnovers.
“A general guideline for Arkansas ponds is 12 feet max depth, though there are exceptions based on pond design and intended use. Ponds that are highly fertile, heavily stocked and accessed often by livestock are at higher risk of oxygen problems,” Jones said. “Large storm systems with heavy rain and significant temperature drops for several days in the mid-fall can initiate rapid and more dangerous turnovers.”
Turnovers can happen suddenly at any time of day when the conditions align. Monitoring water temperature profiles and weather patterns, one can roughly predict when their pond will turn, he said. However, once it occurs, and if oxygen problems develop, there may only be hours to react before significant fish losses occur.
“The only way to save fish during a dissolved oxygen emergency is supplemental aeration or flushing the pond with well-oxygenated water. The latter is usually impractical, leaving pond owners with aeration as their only option,” Jones said. “If a sufficiently sized aeration system is not already in place when a dissolved oxygen emergency begins, it is unlikely that a pond owner would be able to purchase and install one in time to save their fish.”
Commercial fish farms often have portable tractor PTO-driven paddlewheel aerators on hand to quickly drop into struggling ponds. Several companies offer surface aeration systems, paddle wheel or vertical propeller pumps, that can help during a dissolved oxygen emergency.
“Unfortunately, this type of equipment is not readily accessible, and they require electricity and installation before they can be safely operated,” Jones said. “All that to say, prevention is the most practical solution.”
While there is no control over weather, steps can be taken to reduce turnover risks. Below are some general guidelines to reduce turnover severity.
• Do not build in areas that are likely to receive highly nutrient-rich runoff.
• Do not build ponds deeper than about 12 feet.
• Fertilize only enough to develop and maintain a plankton bloom that reduces water clarity to between 18 and 24 inches of visibility.
• Feed only as much as fish can eat within 5 to 10 minutes.
• Do not over-stock the pond with fish.
• Keep aquatic weeds under control with a combination of plankton blooms, dyes, grass carp, physical removal and/or herbicides.
• Prevent livestock direct access to ponds with fencing and external watering structures; or use small, reinforced gravel-covered entry points to the pond.
• Install supplemental aeration systems.
Some strategies are not practical in every situation. Implementing even one can reduce a pond’s dissolved oxygen demand and lessen the harmful side effects of turnovers, he said. The most complete defense against turnover problems is a properly sized diffused aeration system. These systems completely mix the pond year-round preventing the thermal layers from ever developing. Without these thermal layers, there can be no true turnover.
“While turnover risks should be discussed, it must be emphasized that the vast majority of ponds, lakes and reservoirs turn over every fall with little to no disruption or evidence that they turned. It is the occasional spectacular disaster that occurs during severe turnovers that draws attention and makes this an anxious period for pond owners,” Jones said. “With wise design, management and maybe the purchase of ‘insurance’ in the form of diffused aeration systems, all pond owners can safely pass through turnover season without issue.”
For more information about pond turnovers, contact Scott Jones at (870) 575-8185 or joness@uapb.edu.
Debbie Archer is an extension associate for communications with the University of Arkansas at Pine Bluff School of Agriculture, Fisheries and Human Sciences.