URBANA, Ill. — The amount of oxygen in a pond plays a major role in fish survival. Low oxygen levels contribute to fish deaths.
Dennis Bowman, University of Illinois Extension interim assistant dean for ag and natural resources, said pond owners often call in late summer wanting recommendations for cleaning weedy or scummy ponds.
“The standard recommendation at this point of the year is to live with it,” Bowman said. “There are chemical options to clean up ponds and lakes, but treating now often results in a disaster.”
The plants living in the water photosynthesize during the day using light to turn water and carbon dioxide into sugar and oxygen. The aquatic plants and animals are also continually respiring, which uses oxygen to break down the stored sugars releasing energy for growth. As a result of this, oxygen levels in the water rise throughout the day and, as night falls, the levels drop.
“If you kill off the plants, not only do you have less oxygen being produced during the day, but the dead plants start to decompose,” Bowman said. “This decomposition is caused by bacteria and fungi, which will pull even more oxygen out of the water.”
Duane Friend, U of I Extension energy and environment educator, listed factors impacting oxygen levels in ponds.
“Little or no wind limits oxygen diffusion into water, as does warmer weather,” Friend said. “The capacity of water to hold dissolved oxygen decreases when water temperature exceeds 86 degrees, just when fish oxygen requirements are high.”
Higher temperatures also prompt the development of pond algae.
“When large amounts of algae die on a pond, their decomposition consumes large amounts of oxygen,” Friend said.
Algae may die from herbicide treatments over large areas or when algae become too dense to be supported by nutrients in the water.
“Fish kills are sometimes mistakenly attributed to herbicide poisoning,” Friend said, “but are actually caused by natural decomposition of organic material and the oxygen that is used in the process.”
As the top layer of water on a pond warms during the summer, the pond may become stratified, or layered. Colder, more dense water, stays close to the bottom and doesn’t mix with the water near the surface.
The cold layer loses its dissolved oxygen over time. A heavy rain may cause the pond to turn over, with oxygen-poor water rising to the surface where many fish may be living.
Fountain aerators or surface agitators may provide some relief from low-oxygen conditions. Friend recommends running aerators during the evening when dissolved oxygen levels are at their lowest. He recommends checking ponds regularly at sunrise for signs of stressed fish.
Living pond plants, including weeds, add oxygen to pond water during the daylight hours through photosynthesis. As pond plants die and decompose, oxygen is removed from the water as microbes decompose the material.
“If large amounts of plants die off at once, oxygen levels decrease dramatically, so do not use aquatic herbicides after July 1 to control excessive pond plants,” Friend said. “If fish are observed at the pond’s surface gulping for air, aerate the pond as soon as possible.”
In addition, check the pond during large algae blooms and after heavy rains. Larger fish are usually the first to be stressed and the first to die from low-dissolved oxygen levels.
Bowman said preventive care in the spring and early summer is the best approach. Proper identification of offensive plants is key to providing safe, effective, and economical control.
Is that pond algae toxic?
Algae is a natural component of most Midwestern farm ponds, and species of algae vary greatly. Though most pond algae is not toxic, high temperatures and stagnant water may increase chances for toxic algae to thrive.
Toxic algae may become dominant in a pond during periods of drought and high temperatures. Stagnant water with high nutrient levels may produce a bluish-green algae bloom resembling pea soup. Toxic algae bloom may also appear gray or chocolate in color.
“Rainfall helps flush nutrients and algae out of the pond, reducing the potential for blooms,” Friend said. “If an algae bloom occurs, do not allow livestock or pets to use the pond for drinking or cooling.”
Toxins are released naturally by algae as it dies off as the season progresses.
“If humans come in contact with the algae, they may experience skin irritation, or nausea and gastrointestinal issues if ingested,” Friend said.
In severe cases, the toxins can damage the liver.
U of I Extension does not recommended using algaecide to kill the algae, since this will prompt the release of high amounts of toxins at once.
Using pond dyes to reduce sunlight penetration into the ponds may reduce some algae production, Friend said. Although pond algaecides may be used in early to mid spring, no chemical applications should be applied to ponds after July 1.