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Pond Water Aeration

Many homes and small farms in rural America have backyard ponds ranging in size from ¼ -acre to five acres or more. Three puzzling questions can arise in regard to these ponds: 1) Why did the fish suddenly die? 2) Why is the water pea green? And, 3) Why are there so many aquatic plants growing in the water?

All of these questions, and a host of others, point to a disruption in the ecosystem of the pond. The disruption usually can be traced to a drop in the level of dissolved oxygen.

Oxygen is an absolute key to a healthy pond. In a perfect world oxygen enters the pond at the surface of the water. Wave action at the surface and/or mild thermal currents carry the oxygen into the deeper depths of the water. Beneficial algae growth at various depths in the pond also produces dissolved oxygen during the daylight hours. Aerobic microorganisms (called aerobes) breathe this subsurface oxygen as they digest the organic nutrients in the water.

Several factors can disrupt this perfect world. For example, if the pond was created by damming up a flowing creek, then excessive amounts of nutrients can flow into the pond. These nutrients take the form of decaying leaves or other organic matter, and they can overwhelm the microbial population. If the microbes cannot digest the nutrients fast enough, then aquatic plants suddenly have access to these nutrients as a readily available food source. A viable, but expensive, option to correct this problem is to divert the creek around the pond.

There can be other sources of excessive nutrients in the pond. Cattle, for example, will often wade into a pond unless it is fenced. Not only do they roil up mud on the bottom, but cattle manure is high in nutrient loading. As the microbes ramp up in growth to consume these nutrients, they quickly deplete the naturally occurring dissolved oxygen. At this point the microbes begin to die off. As the nutrient load continues to grow, the pond becomes susceptible to the growth of harmful algae and/or aquatic plants.

Even fertilizers, pesticides, mown grass, or excessive amounts of fish food thrown into the pond can add to the nutrient load. To maintain a healthy pond, you need to control as many of these factors as possible. Even so, a good mechanical aerator is almost a must if you want to ensure that your pond stays healthy.

Another issue that few homeowners understand is the phenomenon known as “water turnover” in the pond. Water turnover occurs when the water near the surface begins to sink to the bottom, thus causing the water at the bottom to rise. This turnover normally occurs when colder weather arrives.

In the summer months large populations of microscopic algae create green “blooms” which block the sun from reaching lower depths in the pond. As a result the lower level of water remains cooler than the surface level. Since water becomes denser as it cools, a layering of the water occurs as the cooler water sinks and warmer water rises. This layering is called a thermocline (thermo = heat and cline = slope.)

If your pond is much more than six feet deep, then you can assume that the really deep layer has little or no dissolved oxygen. Such deep layers cannot benefit from the surface mixing of air and water. Plus, since sunlight usually cannot penetrate much more than six feet through the algae bloom, then there is no beneficial plant or algae growth in the deep zone to produce oxygen. This condition can spell disaster if the water turns over.

When winter arrives, the surface water cools dramatically. At 39.4 degrees Fahrenheit water reaches its maximum density. This cold heavy water then sinks, forcing the deeper oxygen-depleted water to the surface. As the warmer and colder waters mix, oxygen levels can fall below the threshold for fish to live. The result is a fish kill that leaves the land owner puzzled and frustrated.

Any sudden changes in air or water temperature can stress life within the pond. Water turnover is more likely to occur in ponds that are deeper than eight feet and which are sheltered from prevailing winds (which mix air with surface water.) Ponds with less organic matter are not as likely to suffer from water turnover.

Two types of aerators are most often used in farm ponds. One is some sort of air diffuser system. This type of aerator has a diffuser head that sits on or near the bottom of the pond. Air is forced through the diffuser by means of an air compressor or an air blower. The advantage of this system is that air can be introduced at depths greater than is possible with many other aerators. The challenge is that the air bubbles tend to rise straight to the surface, thus providing little mixing or dispersion throughout the pond.

The other common method of introducing oxygen into the pond is the fountain sprayer. These systems spray water into the surface air in the hope of mixing oxygen in the spray. The spray patterns can vary from fine delicate sprays to agitated boiling-type sprays. These systems can look impressive, yet they create very limited water circulation. Plus, their short contact time with the air means they encapsulate only limited amounts of oxygen.

While oxygen can dissolve into the water at the air/surface interface, much of the dissolved oxygen in the pond comes from beneficial algae. Microscopic algae “blooms” produce oxygen during the day but then consume this oxygen at night. In a perfect world there exists a balance between daylight production and night consumption of oxygen. However, on cloudy days the algae produce less oxygen than they consume at night. Plus, the constant decay of algae and other organic matter can further deplete oxygen levels. An extreme drop in oxygen levels can kill all fish, invertebrates, and much of the microscopic algae populations.

The above problems can be compounded in hot weather. Warm water holds less oxygen than cold water. Also, high altitudes and/or salinity will decrease the water’s ability to maintain dissolved oxygen levels. Normally, such drastic conditions as described above last only a few days, but the damage to the fish population can be swift.

As the algae bloom dies, the bright green color of the water can change to black or to gray with black streaks. During this process larger fish can begin to die. Smaller fish can be seen gasping (called piping) at the surface. Warm water fish such as bass or catfish need about 5 milligrams per liter (5 mg/L) of dissolved oxygen to live. They will die at 3 mg/L or less. Coldwater fish such as salmon or trout need about 7 mg/L. Concentrations of 5 mg/L or less will kill them. All species of fish can survive for short periods in low dissolved oxygen, but the added stress can lead to disease. If you are serious about pond management, you can buy a dissolved oxygen meter from any number of manufacturers. One quality company is YSI at https://www.ysi.com/index.php.

In colder climates snow covered ice on a pond can block sunlight from reaching the algae blooms and other oxygen producing plants. As these algae and plants die and then decay, oxygen is consumed. With no access to air at the surface, and with no oxygen being produced beneath the surface, the water becomes depleted of oxygen, possibly resulting in fish kills. Clearing the pond of its covering of snow can let sunlight penetrate the ice. Though limited in intensity, this light can help maintain algae populations. An aerator can reduce the possibility of ice covering the pond.

In nearly all cases a good aerator can aid your efforts at water quality management. Aeration can provide needed dissolved oxygen, create circulation, prevent ice formation, reduce the severity of water turnover, and reduce fish kills. A well constructed pond can also help reduce the severity of dissolved oxygen depletions. An ideal pond ranges in depth from three feet to eight feet. Such a pond is ideal for beneficial algae growth. It also will prevent the growth of unwanted aquatic plants and harmful algae.

In virtually every pond or lake a self-aspirating aerator is ideal for oxygen entrainment. The propeller-type aspirator can provide good circulation of oxygen throughout the water. However, in shallower ponds (5 to 6 feet) these aerators can roil the bottom too much. Some aquaculture pond owners complain that propeller driven aspirators create a raised, donut-shaped mound on the bottom of their aquaculture ponds. This donut is created by the severe agitation of the water at the bottom.

In contrast to a propeller-driven aspirator, the Turbine uses the engineering principles of precession and centrifugal force. The Turbine quickly entrains and then gently, but firmly disperses dissolved oxygen in a 360 degree radius. At the same time it forces a plume of dissolved oxygen straight down. The net result, as seen in the video footage, is an almost magical saturation of the surrounding water with an underwater cloud of dissolved oxygen. It is virtually impossible for the Turbine to clog with aquatic plants or debris. There is nothing to wear out except for the electric motor. Even here, the Turbine uses three phase motors with a time-delayed, slow-start motor starter to increase the service life of the motor. According to the European manufacturer, the Turbine is the most efficient, reliable, cost effective self-aspirating aerator in the global market.

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Call us with your questions on pond aeration.

(For more information on pond construction and management contact your local county extension agent, aquaculture agent, or regional fisheries biologist.)

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