Turnover in ponds and lakes is a natural phenomenon that is a result of the influence of thermal stratification on dissolved oxygen levels in a water body. Sometimes, pond turnovers can result in very low dissolved oxygen levels in a pond and result in a fish kill, which can be alarming to pond owners and raise concerns about pond health. Fortunately, there are ways to help prevent turnover from occurring that pond owners can incorporate into their pond management strategy.
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What Is Dissolved Oxygen?
Dissolved oxygen refers to the oxygen that is freely available in water to support aquatic life. Oxygen in water comes from two sources: diffusion from the atmosphere and aquatic plants and other photosynthetic organisms. The amount of dissolved oxygen can depend on many factors, such as temperature, time of day, amount of sunlight, presence of organic matter, the occurrence of algal blooms, wind movement, and the presence of suspended sediment. Fish and other aquatic life can become stressed, and a fish kill can occur when dissolved oxygen levels drop below three parts per million (ppm). As fish become stressed, a pond owner may notice larger fish “gulping” for air at the pond surface.
What Is Pond Turnover?
Pond turnover typically occurs in deeper ponds and lakes and is a common, seasonal cause of temporarily low dissolved oxygen levels. In summer months, a warm, oxygen-rich layer of water in the pond can form on top of a cold and dense deep layer of water with very low oxygen levels. This layering occurs because of the influence of sunlight on water temperature as well as photosynthesis. In the upper layer of the pond, algae can photosynthesize and help oxygenate the warm water, which is already oxygenated through diffusion with the atmosphere. At deeper depths, less sunlight is available for algae and photosynthesis, and the water is cooler and denser and unable to mix with warm water above. This lack of sunlight also reduces the metabolic activity of algae and, in turn, results in lower dissolved oxygen levels in this deep, cool layer.
When water temperatures uniformly cool throughout the pond, as with a seasonal cold front in the fall or even with a large flush of rainfall and runoff into a waterbody after a storm event, the pond can “turn over” and results in the cooling of the entire pond and rapid mixing of the low and high-oxygenated water. If the mixing occurs too quickly, it can cause the dissolved oxygen in the entire pond to quickly drop to very low levels, which can stress aquatic life and sometimes result in a fish kill.
Not all ponds will experience stratification and turnover. Pond depth is the most significant influence on pond stratification. Turnover is more of a concern in deeper ponds but can occur in ponds as shallow as six feet or less. If you have had a turnover event in the past, there is the risk that it could happen again.
Other factors to keep in mind that could reduce oxygen levels in a pond include:
- prolonged periods of cloudy days during warm weather, which can reduce photosynthesis and oxygen production,
- algal blooms, which can cause oxygen levels to crash with bloom die-off,
- dense tree cover, which can reduce the influence of mixing from wind,
- presence of large mats of floating or emergent plants, which can impede the diffusion of oxygen with the atmosphere,
- large amounts of organic matter in the pond from excess fish food or upland sources, including leaf litter, grass clippings, or other inputs, which can consume oxygen through decomposition, and
- excess suspended sediment in the pond, which impacts photosynthesis and oxygen production.
Measuring Dissolved Oxygen
To determine if you have a risk of pond turnover, you can use a dissolved oxygen meter, available through aquaculture or environmental monitoring suppliers, to determine if a thermocline is present and thermal stratification has occurred. A thermocline is the zone in the pond marked by a significant shift in temperature where warm, oxygenated water sits above cool, less oxygenated water. The presence of a thermocline in your pond can indicate the risk of a potential future turnover if rapid mixing occurs.
The thermocline is a zone in the pond, shown here in dark blue, where there is a dramatic shift in temperature between warm, oxygenated water above cool, less oxygenated water.
Lance Beecher, PhD, ©2022, Clemson University
To measure this, use the dissolved oxygen meter to take oxygen and temperature readings at different depths as you move the meter’s probe down the water column. Keep in mind that dissolved oxygen levels are at their lowest in the morning, and highest in the afternoon due to photosynthesis, so consider sampling several times during the day to get a better idea of fluctuations.
Dissolved oxygen meters require maintenance to keep their accuracy, so follow instructions for storing, calibration, and handling the meter. Alternatively, a pond owner can contact a pond management professional to assist with measurement.
Managing Ponds to Minimize Turnover Risk
The use of aeration systems, like vertical pump aerators or bottom diffusers, can help aerate pond systems. Vertical pump aerators are options for smaller, shallower ponds where surface aeration is needed. Vertical pump aerators include a submersed pump suspended underneath a small float. Water is pumped up through the float and a short distance into the air, agitating and, in turn, oxygenating the shallow water under and around the float. They will not aerate deeper areas of the pond and so may not prevent turnover in some pond systems. Vertical pump aerators differ from surface fountains, which are considered more of an aesthetic feature for a pond unless they are large enough to pump with enough force to aerate shallow surface water.
A vertical pump aerator is an option for shallower ponds. It forces agitation of the water’s surface, oxygenating the upper zone of the pond.
Photo source: Sean Fleming, The Lake Doctors, Inc.
Bottom diffusers, sometimes called bubblers, are best suited to prevent turnover in deeper pond systems. Bottom diffusers are installed on the bottom of a pond and use a compressor to pump air through a diffuser plate toward the pond’s surface, promoting vertical circulation and oxygenating the water column. Diffusers must be run continuously to prevent stratification in the pond; pond stratification can occur quickly, and if turned off and on, a diffuser will not be effective in prevention.
This pond uses two bottom diffusers to help prevent turnover in its deeper pond system. Diffusers, sometimes called bubblers, are placed on the pond bottom and force continual circulation of the pond water to maintain oxygen levels.
Guinn Wallover, ©2022, Clemson Extension.
Aeration system selection is dependent on many factors in a pond, including shape, depth, access, power supply, budget, and more. Pond owners should work with a pond or aeration management professional to assist with aeration system selection, sizing, and placement. Keep in mind access to a power source and the maintenance needs that will be specific to certain products.
In terms of timing, to prevent the formation of a thermocline, pond owners should continuously run their circulation system during the warm spring and summer months to prevent stratification from occurring. Starting and stopping the system in the evening and morning can still allow the thermocline to form during warm days. Make sure to follow all manufacturer recommendations for startup and seek professional assistance for help.
More information on pond management can be found at the Clemson Extension Home and Garden Information Series, Land Grant Press, or the Clemson University Stormwater Pond Management Conference.