Oxygen Depletion in Ponds
George W. Lewis
Professor, Aquaculture and Fisheries
Warnell School of Forest Resources
The University of Georgia
From June to October, oxygen depletion is the most common cause of fish
kills in Georgia ponds. Depletion of oxygen in pond water will occur when
conditions create a greater demand for oxygen than the environment can
produce. When fish are observed dying because of oxygen depletion, it is
often too late to stop the kill. However, oxygen depletion can often be
anticipated and detected before fish begin to die and measures can be taken
to prevent a fish kill.
Causes of Oxygen Depletion
Oxygen depletion in a pond is the result of demand exceeding supply. Aquatic
animals, plants, and decaying organic matter consume oxygen. Aquatic plants
are primary producers of oxygen. Plants produce oxygen as a by-product
of photosynthesis. The rate of photosynthesis is dependent upon light.
Because of the effect of sunlight on photosynthesis, the amount of oxygen
in pond water fluctuates daily. Oxygen levels are usually highest at midday
and lowest just before sunrise. Fish kills usually occur when more oxygen
is consumed during the night hours than is produced during daylight hours.
Warm water does not contain as much oxygen as cold water. Thus, during
warm weather months, mismanagement of ponds by overstocking, overfeeding,
overfertilization, pollution from barns and feedlots, or chemical treatment
of aquatic weeds can result in oxygen depletion and fish kills.
During warm weather months, pond water stratifies into three layers
(Figure 1). The top layer is warmer and contains oxygen producing algae.
Wind action also contributes oxygen to the top layer. Most of the fish
are in the top layer. The middle layer is a relatively thin layer characterized
by a rapid drop in temperature and oxygen levels. The bottom layer is stagnant,
cool water, with little or no oxygen. It does not mix with the upper level
during warm weather. Fish rarely enter this area.
Oxygen depletion can occur when these layers mix during a "turnover."
Fish kills will occur if oxygen demand in the stagnant bottom layer of
water exceeds the oxygen available in the top layer. Turnover can be triggered
by heavy rains, strong winds blowing for long periods in one direction,
and rapid cooling in the fall.
Several calm, cloudy days during warm weather can also result in oxygen
depletion and fish kills. This is because the rate of photosynthesis and
subsequent oxygen production is dependent on the intensity of light. In
a pond with an excessive bloom of algae, several days of reduced light
and calm weather can result in not enough oxygen being produced to meet
the demands of respiration and decay.
The pond owner should be alert to possibility of oxygen depletion if
one or more of the following conditions exist:
If one of these conditions exist, a pond owner should inspect his pond
daily for signs of oxygen depletion.
After a heavy rain;
During periods of strong winds;
During periods of calm, cloudy days;
During the fall when air temperatures are rapidly cooling;
After chemical treatment of aquatic weeds.
Signs of an Oxygen Depletion
There are several signs of oxygen depletion:
Testing the oxygen level in a pond is an excellent way to detect a developing
problem. Oxygen test kits and meters are available for this purpose. Tests
of oxygen levels in ponds should be made at dawn or shortly thereafter.
If oxygen is less than three parts per million (ppm) in the top three feet
of water, immediate action should be taken.
Large numbers of fish swimming to the top and gulping air at night or early
in the morning. If disturbed they dive but quickly return to the surface.
If oxygen depletion has not reached a lethal level, fish are at the surface
in the early morning but return to deeper water as oxygen builds up during
the day. This may continue for several days. The pond owner should take
corrective action immediately.
Fish being fed suddenly stop eating.
This may be impractical for a farmer with a large number of ponds. One
can estimate the possibility of an oxygen depletion by measuring oxygen
in the early evening and again two or three hours later. Since the rate
of oxygen depletion is somewhat linear, these two values can be used to
extrapolate oxygen content versus time during the remaining hours of darkness.
Figure 2 illustrates this method.
The needs of fish for oxygen vary according to species, age and culture
conditions. Most warm water fish need oxygen dissolved in water at a rate
of at least one ppm for survival and more than three ppm for comfort. Oxygen
at five or more ppm provides the best growing conditions.
If signs of oxygen depletion are observed, emergency treatment must be
undertaken immediately to prevent loss. The following physical and chemical
emergency treatments are effective.
The most effective emergency treatment is mechanical aeration of the water.
There are a number of ways the water can be aerated mechanically.
Whichever method is used, the sooner it is applied and the larger the
volume of water sprayed or agitated per unit time and the sooner a current
is established the more effective the method will be. If mechanical aeration
is used, it is important not to disturb the bottom mud. Bottom mud contains
a large amount of organic material and decomposing bacteria that will contribute
to oxygen depletion problems if mixed with water.
Paddle Wheel Aerator. There are various designs for a paddle wheel
aerator. Essentially, it is constructed by mounting a used car or truck
differential on a trailer frame, constructing a paddle wheel on each end
of the differential, and attaching a power-take-off linkage to the differential.
The hub of the paddle wheel can be an 1 8-inch diameter metal cylinder.
Paddle blades are usually 8 to 12 inches long and the width of the drum.
Figure 3 is an example of the paddle wheel design. Ideally, the paddle
wheel should be backed into a pond until the bottomklades are just under
the water surface. The differential should be level with respect to the
water surface. The paddle wheel is efficiently operated at 1,500 to 1,800
rpm in large ponds; a reduced speed may be necessary in small ponds. Paddle
wheels should turn toward open water.
Large Volume Pumps. Any large volume water pump can be used to aerate
a pond. Ideally, the pump should set up so that it creates a current and
at the same time blows or sprays water across the pond surface.
Bushhog Mower. Bushhog mowers can also be used to aerate a pond.
Essentially, they are backed into the water and the mower blade is used
to circulate the water.
Outboard Motors. Outboard motors also produce current if run in
a fixed position. However, driving a boat in the pond with an outboard
in order to stir the water is practically useless.
Water Replacement. Water replacement is also very effective. Unfortunately,
most pond owners do not have an alternative water source with enough volume
to be effective.
Any one of the chemical treatments can be used alone, but the most effective
way is to use all three. Use potassium permanganate in the morning as soon
as signs of a depletion are observed. Treat with triple superphosphate
during midday and follow with builders or hydrated lime at dusk. It is
advisable to purchase the required amounts of these chemicals before oxygen
depletion problems are detected. Once oxygen depletion is a problem, immediate
treatment is essential. In some areas these chemicals are not readily available
and a fish kill can happen in a matter of hours. Storing these chemicals
near the pond will save time and help prevent a fish kill.
Add six to eight pounds of potassium permanganate per acre-foot.
oxidizes organic material in the water and reduces the demand for oxygen.
The water will become a purple color. If the color disappears within one
hour, repeat treatment at one-half the rate. This material can be toxic
to fish, so avoid overtreatment.
Add 50 to 100 pounds of triple superphosphate per surface acre.
This increases the production of oxygen by plants.
Add 50 pounds of builders or hydrated lime per surface acre. This
reduces carbon dioxide in the water and makes the remaining oxygen more
available to fish. This material can be toxic to fish so avoid overtreatment.
The key to preventing oxygen depletion is proper pond management. After
the emergency has passed, find and eliminate the cause of oxygen depletion.
Oxygen depletion can weaken and stress surviving fish making them more
susceptible to diseases and parasites. Surviving fish should be watched
closely during the few weeks following oxygen depletion to determine if
treatment is necessary.
If a high percentage of the fish have died as a result of oxygen depletion,
it may be necessary to drain and restock the pond. If there is any question
about the severity of a kill, have the population checked by a Department
of Natural Resources biologist the following summer. Biologists can be
contacted through your County Extension Agent or Conservation Ranger.
There are a number of aerators on the market designed to provide supplemental
aeration to a pond. Supplemental and emergency aeration methods should
not be confused. Supplemental methods are used to prevent oxygen depletion
from occurring while emergency methods are used to correct oxygen depletion
once it has occurred. Although emergency methods are effective, they are
usually too difficult and expensive to use continuously. Emergency methods
include adding chemicals to a pond such as potassium permanganate and hydrated
lime, pumping or agitating large volumes of water, or flushing the pond
with aerated water.
Most supplemental aerators pump or agitate water to oxygenate it or
inject air directly into the water. Supplemental aerators can be used in
emergency situations but most of them do not reoxygenate water as rapidly
as emergency aerators.
For a commercial fish farmer, the use of supplemental aeration can improve
quality and production and decrease losses. This is especially true if
adequate amounts of water are not available to periodically flush the pond
during the hot summer months. For the sportfishing pond owner, the positive
benefits of supplemental aeration are more questionable. Most oxygen depletions
in sportfishing ponds are the result of a management error by the owner.
Usually oxygen depletion in a sportfishing pond is the result of overstocking,
overfeeding, overfertilization, chemical treatment of aquatic weeds or
pollution from barns and feedlots. These management problems coupled with
hot summer weather, summer storms, cloudy weather or strong winds can cause
oxygen depletion. In the long run, it is probably more economical for the
sportfishing pond owner to correct the management problem rather than compensate
for the problem with supplemental aeration.