Circulating and aerating water in stormwater ponds provides two critical benefits
1) oxygenation of the water and
2) mixing of water to prevent stratification.
All living organisms consume oxygen to live, including aquatic species. As aquatic organisms grow and multiply, their demand for oxygen from the water increases. Stormwater ponds are often extremely productive systems because they receive large loads of nutrients from lawn fertilizers, people feeding fish, turtles, and ducks, and pet waste in the community. These nutrients encourage microbes, invertebrates, and fish to grow and multiply, increasing the demand for oxygen. Under normal conditions plenty of oxygen is supplied by diffusion from the atmosphere and by photosynthesis of phytoplankton to cover the demand; however, when nutrients become excessive or when ponds stratify, the supply of oxygen may not compensate for the demand. When the supply cannot keep up with demand, then oxygen becomes depleted, leading to stagnancy and fish kills.
The primary source of oxygen in ponds is the atmosphere. Oxygen diffuses into the water from the air. As long as the water can be exposed to the air, then an ample supply of oxygen can enter the pond. One of the leading obstacles for the diffusion of oxygen into the water is stratification of the water column. Stratification occurs when the sun heats the surface water while deeper water remains cool. The warm and cool layers do not mix. This disrupts the circulation of oxygenated water from the surface to the bottom. While the warm surface water receives oxygen from the atmosphere as well as from photosynthesis of phytoplankton, the cooler bottom layer becomes depleted of oxygen. As the summer progresses, oxygen levels drop severely in the bottom layer. Stratification can be a significant threat to fish health in small ponds because it increases the potential for a "turnover."
What causes a turnover?
Turnovers occur naturally in the fall. The incremental drop in air temperature slowly decreases the surface water temperature, and the layers in the pond mix gradually. This gradual mixing rarely causes a fish kill. On the other hand, rapid turnover can cause a fish kill. Rapid turnovers can occur anytime during the warmer months of the year when ponds are stratified and they most often coincide with storm events or windy days. A cold rain or the blowing winds of a storm front can cause rapid mixing and result in a fish kill if the pond is stratified.
How do I prevent a turnover?
What is the best way to circulate the water?
The primary goal of a circulation system should be to bring deep water to the surface and expose it to the atmosphere. By far, diffusion systems circulate water most efficiently. Diffusion systems are a lot like airstones in an aquarium. They use an air compressor to pump air into the bottom of the pond. The diffuser breaks the air into tiny bubbles which expand as they rise. The rising of the bubbles pushes water and forms a current that lifts bottom water up to the surface. The resulting current mixes the entire water column and prevents stratification in the pond.
Are there other ways to circulate the water?
Yes. Water pumps such as centrifugal irrigation pumps and sump pumps can be used to push water but they must be designed correctly to work. Remember, the goal of a circulation system should be to prevent stratification. Irrigation pumps can be mounted on land as long as they draw water through pipes from near the bottom and shoot it over the surface to create a current. Unlike diffuser systems, which are self-cleaning, water pumps are prone to becoming clogged by vegetation and debris and require frequent maintenance, especially when submerged plant growth is not controlled. Water pumps also consume more energy than air compressors.
What about fountains... Do they work?
Surface fountains are the least efficient ways to circulate ponds, and they do very little to prevent stratification. They move water, but typically fountains are floating devices that draw surface water into a shallow pump and spray it above the surface. Functionally speaking, they are circulating surface water that is already oxygenated and are not disrupting stratification. As a result, fountains are often deployed more for aesthetic reasons than functional circulation and aeration. That being said, surface fountains do provide for limited circulation that is beneficial to fish and other aquatic species. The amount of circulation they create is dependent on the size of the motor and the shape of the sprayer.
Will circulation systems control aquatic weeds?
No, not necessarily. Circulations systems will not control existing aquatic weed problems, except for some blue-green algae (cyanobacteria) which thrive in still water. Some cyanobacteria require stagnant waters to grow large blooms. Circulation systems can disrupt these harmful algae blooms from growing out of control. Also, in ponds that are overloaded with nutrients (eutrophic) and consistently low in oxygen, circulation system may improve water chemistry and make dissolved nutrients less available to algae and weeds. By reducing the availability of nutrients, circulation systems potentially can slow but not prevent the growth of aquatic weeds. If the pond is consistently loaded with new nutrients, then a circulation system will have little or no effect on reducing weed growth.
Are there any other benefits to having a circulation system?
Yes. Oxygenating the sediments and organic matter that collect on the pond bottom helps reduce the production of Hydrogen Sulfide, the gas that makes the rotten egg odor that most folks associate with stagnant swamps and sewers. This gas is produced by anaerobic bacteria that thrive in sediments with little oxygen. Also, oxygenating organic sediments helps accelerate the microbial breakdown of the "muck" that collects in ponds as leaves and debris decompose. Last, oxygenating bottom sediments helps slow the release of nutrients from organic sediments that contribute to algae blooms.
Are there any problems caused by circulation systems?
Yes. Circulation systems can keep sediment suspended in the water and reduce water clarity. They can increase shoreline erosion especially in small ponds with no shoreline protection using native plants or hardscapes (See section on Shoreline Protection) or where the system is placed too close to the bank. Last, circulation systems require electricity, so they are an ongoing utility expense and may require additional hardware to run electricity to the areas where it is needed.
Do I really need a circulation system?
As mentioned previously, most stormwater ponds are shallow basins that do not develop a large volume of low oxygen water in the bottom. Also, they are designed to exchange water with each passing storm. On the other hand, some stormwater ponds seem to have chronic problems with fish kills and would benefit from a circulation system. Stormwater ponds are prone to excessive weed and algae growth. If large masses of these plants are killed using aquatic herbicides, the resulting decay will remove oxygen from the water and may cause a fish kill. Aeration can help prevent such a fish kill. Finally, if your community has stocked the pond with grass carp or tilapia to control aquatic vegetation or other fish for recreational benefits, having a circulation system is good protection for that investment. Circulation systems may not be necessary unless your pond develops chronic problems with fish kills or noxious odors. It is important for the community to document fish kills so that they can be tracked for frequency and timing. This will help the community when deciding whether or not to invest in a circulation system.
When is the best time to install a circulation system?
It is best to install and start circulation systems before ponds become stratified, which usually means late fall through early spring (April). If your community is considering installing a system in the warm summer months, you should be careful not to cause a man-made turnover when the system is turned on. If the system is installed in the summer it must be started in short increments (10 minutes per day) over the first two weeks in order to gradually mix the water. Consult with your system manufacturer for start-up recommendations. You may need to test oxygen levels during start-up.
How large does the system need to be?
A general rule is that one diffuser unit can circulate a one to two acre pond, but sizing diffuser systems has a lot to do with surface area, depth, and the shape of the pond. Diffusers are less efficient in shallow ponds because the bubbles travel a shorter distance and create less current, so more diffusers may be needed. Also, ponds with odd shapes, bottlenecks, and narrow canals may need several diffusers to adequately circulate water in each of the sections of the pond. The same goes for fountains and water pumps. For water pumps, it is suggested that 1 horsepower of pump be used for each surface acre of pond. You should always consult with the circulation system manufacturer or professional installer for guidance on sizing and designing your system.
Do I have to run the system all the time?
If circulation and preventing stratification is your goal, then, yes, you should run your system all day and night. Running the system intermittently may allow the pond to temporarily stratify. The longer the pond remains stratified (layered), the greater the chance that turning the circulation system on will actually cause a turnover and fish kill. You do not need to run it all year. Ponds naturally destratify (mix) in the fall when air temperatures drop. As the temperature of the pond surface cools and matches the temperature of the deeper water, the pond will mix gradually. Once this occurs, the pond will remain mixed until the following spring when air temperatures rise. Circulation systems can remain off from November through March across most of South Carolina.
Is there anything else I can do to prevent low oxygen conditions?
Yes. Reducing the amount of nutrients entering the pond is essential to reducing the demand for oxygen in the system. Here are the best practices for reducing nutrients in runoff...
1) test your soil before fertilizing (you might be applying too much)
2) do not fertilize grass on bank slope around pond
3) after fertilizing lawns, sweep or blow fertilizer off of roads and driveways into the grass (if it is on the road, it will wash to the pond in the next storm)
4) do not feed fish, turtles, ducks, or geese (food in their mouths is fertilizer in the pond)
5) wash vehicles with phosphate free soaps or take vehicles to car washes
6) do not throw yard waste in ponds
7) plant shoreline vegetation to absorb nutrients
8) prevent sediment from washing off of construction sites
Reducing the nutrients carried in runoff will also help reduce aquatic weed problems, turbidity of the water, noxious odors, and other unfavorable conditions in residential stormwater ponds.