Ponds fulfill a lot of roles in Georgia. They provide water for thirsty crops and livestock. They offer a cooling dunk for people and animals. And they (sometimes reluctantly) give up their fish for a summer fish fry.
It's no wonder they run short of something.
"During the dog days of summer, we see many ponds become oxygen-depleted," said George Lewis, an aquaculture and fisheries specialist with the University of Georgia Extension Service. "Pond owners may need to aerate (add oxygen to) their ponds to ensure the pond stays healthy."
Most bodies of water that cover less than 15 acres are ponds, Lewis said. Anything larger is a lake. Both can become oxygen depleted, but smaller ponds are more at risk.
"Ponds tend to be warm through their entire depth. Lakes probably have a layer of cooler water near the bottom that can hold enough oxygen to support the life in it."
Ponds likely to become oxygen-starved include those near areas where a lot of organic material is on or in the ground or where fertilizer is used heavily.
This includes not only pastures with livestock and many farm fields, but areas around golf courses or manicured suburbs, too.
"Animals that graze around a pond add a huge amount of organic waste to the water," he said. That waste can deplete the oxygen faster.
Ponds with lots of fish are good candidates for oxygen depletion, too. Fish use a surprisingly large amount of oxygen.
Lewis said the oxygen isn't depleted in just one or two days. Hot, hazy days in late July and August are the most likely time the problem will start. The water is warm, so it holds less oxygen. The sun isn't shining directly into the pond so phytoplankton aren't producing as much oxygen. And the bacteria and fish in the water need more oxygen.
"Everything is right for less oxygen to be produced," Lewis said, "when in fact, the pond needs more oxygen."
Oxygen depletion usually happens slowly over four or five days before obvious symptoms appear. The first symptom is usually the water changing color to a bright green. Another sign is a fairly sudden increase in fish deaths.
Whatever symptoms appear, Lewis said, the visibility in the water shows the oxygen level. Put something white or shiny under the water. The oxygen is probably about right if you can see the item 12 to 18 inches deep.
"Visibility of less than 12 inches points to potential oxygen depletion," he said. "You may need to aerate the water before your fish start dying."
Lewis said night aeration provides the best oxygen source for ponds. During the day phytoplankton, microscopic plants in the water, produce oxygen through photosynthesis.
At night, the sun isn't providing the light the plants need to make oxygen, so the animals in the water slowly use it up.
"That's why you'll find the lowest oxygen levels in a pond just before sunrise," he said. "When the problem is getting serious, you'll see fish at the surface literally gulping air."
Pond owners use many methods to aerate their ponds. Lewis follows a rule of thumb for aerators of one horsepower per surface acre. Anyÿsystem that breaks up a water stream into small droplets falling into the pond should oxygenate well.
It's impossible to aerate a pond too much, he said. The oxygen dissolves in the water. When the water is holding as much as it can, no more will dissolve into it.
"Not all ponds will need aeration during the summer," Lewis said. "But if a pond needs oxygen, it needs it quickly."
