We have included two governmental reports here concerning aeration.
These reports are intended to help inform you. We thank the state
of Kentucky and the United States Department of Agriculture. These
reports do not mean to imply an endorsement of our products. Rather,
we include the articles to keep you informed. Full citations are
listed below.
WKY-211 - LOW OXYGEN AND
POND AERATION
William A. Wurts, State Specialist for Aqua
culture Kentucky State University Cooperative Extension Program,
P.O. Box 469, Princeton, KY 42445
Water can hold a limited amount of oxygen. That
is determined by atmospheric pressure, temperature and salinity.
In a natural setting, oxygen is added to water by atmospheric
diffusion at the surface, by wind circulation (augmented surface
diffusion) and by photosynthesis (oxygen produced by phytoplankton
or algae).
Photosynthesis accounts for most of the oxygen in
water. The oxygen content of water increases with increasing atmospheric
pressure and decreasing temperature and salinity. The amount of
oxygen in water is measured as milligrams per liter (mg/l) dissolved
oxygen (DO).
Oxygen Depletion
A number of conditions may develop which result
in oxygen depletion, DO at levels insufficient (less than 3 mg/l)
to support aquatic life (e.g. fish). Oxygen depletions are typically
associated with:
1. Hot, cloudy, still weather is
common from the end of July to the beginning of September.
High water temperature (86o F or greater) reduces
oxygen holding capacity. Cloud cover limits available light, slowing
or halting photosynthetic oxygen production. No wind stops pond
circulation and restricts surface diffusion of atmospheric oxygen.
Warm water increases fish consumption of oxygen
by accelerating their metabolic rate. Fish are ectotherms (cold
blooded); therefore, body temperature and activities are regulated
by water temperature. Fish biomass (total weight in pond) and
oxygen needs are usually greatest during the hot months of late
summer.
2. Sudden death of phytoplankton
or algae bloom, "bloom crash", may result from insufficient
light (e.g. cloud cover) for photosynthesis, inadequate pond nutrients
(a bloom too dense to be supported by available nutrients and
oxygen) and/or bloom senescence (the plant cell line becomes too
old to continue reproduction). Oxygen is consumed or depleted
when dead phytoplankton/algae decay. During the nighttime hours,
a dense phytoplankton bloom can remove all oxygen from the water
for respiration (to breathe) alone. When a bloom crash occurs,
the water appears to have become "black" or clear overnight.
3. Pond stratification or turnover.
During summer months in deep ponds (8 feet or greater), the upper
4-6 feet of the water column warms quickly and becomes less dense
or lighter than deep water. Because the upper layer is warmer
and lighter, it does not mix with the cool, deep water. The cool
water near the bottom becomes stagnant; oxygen is depleted and
toxic compounds may be produced by bacteria and decaying organic
matter. The deep layer remains un oxygenated (anoxic) because
of stratification (layering). A sudden, heavy rain (2-3 inches
or greater) or a strong cold front ("Blue Northern")
can rapidly cool and/or mix (wind turbulence) the upper layer.
The now cooler or circulating upper layer sinks or mixes and causes
the deep anoxic layer to rise above or combine with the surface
water. That depletes or reduces oxygen in upper waters where fish
are being cultured.
4. Organic waste decomposition.
When fish biomass becomes large in commercial ponds (late summer),
waste and organic loads (ammonia, nitrite, feces and uneaten feed)
can become high. Wastes and organics will decompose. That requires
oxygen, often more than is available in pond water. Also, high
waste loads can stimulate an algal bloom too dense to be supported
by the pond (discussed above).
These situations can occur alone or in interrelated
combinations. As just discussed, conditions may develop which
remove oxygen from water faster than natural processes can replace
it. When they occur, emergency or supplemental aeration may be
required to bring oxygen back up to tolerable (3-5 mg/l) or safe
(5 mg/l or greater) levels.
Aeration Equipment
Electric or mechanical aeration is used to place
as much oxygen into contact with water as economically practical.
That is normally accomplished by mixing large quantities of water
(both volume and total surface area) with atmospheric oxygen.
Several aeration devices are commercially available. Most aeration
equipment requires electricity (preferably, three phase or 230
volt) or fuel powered engines (tractors or pumps) at the pond
bank. Boyd and Ahmad (Auburn University); and Engle (University
of Arkansas, Pine Bluff) and Hatch (Auburn University) have conducted
individual studies rating pond aerators for performance and economic
efficiencies, respectively. The following is a general summary
that rates existing aeration equipment from highest to lowest
efficiency, both performance and economics:
1. Electric paddle wheel aerators.
A paddle wheel similar to that of the old river boat circulates
and mixes pond water. An electric motor powers the device. These
aerators can supply 1.8-4.9 pounds of oxygen/horsepower-hour (lb
O2/hp-hr). Most models will supply from 3.5-4.9 lb O2/hp-hr. A
tractor operated paddle wheel is effective for emergency situations
but is not practical for supplemental or continuous operation.
2. Electric pump-sprayer aerators.
Large volumes of water are pumped or sprayed over the pond surface.
These aerators can supply 1.5-3.2lb O2/hp-hr. While tractor and
engine powered versions of these devices are effective, as above,
they are only practical for emergency aeration.
3. Electric propeller aspirator
pump aerators. Water is pumped to the surface and mixed with air
by propeller blades. These aerators can supply 2.1-3.1 lb O2/hp-hr.
4. Experimental aerators. A number
of experimental aerators are currently being developed; some of
which may or may not be cost effective. One promising design is
the airlift aerator. Air bubbles produced by a centrifugal air
blower (electric) act as pneumatic pistons; pushing or drawing
water up a pipe or stack as they rise and expand. Field studies
demonstrated that a 1 hp blower can pump approximately 750-820
gallons per minute to the surface using individual 3-4 inch diameter
PVC pipes.
Extensive field trials and economic analyses will
be needed to test the practicality of experimental equipment.
Aeration Methods
As a general rule, 1 hp of electric aeration should
be available for each surface acre (4 acre feet) of intensive
aqua culture production. Aeration equipment should be placed along
the longest pond bank. Aerators should be started before DO falls
below 3 mg/l. Oxygen levels are lowest just before sunrise each
morning. If affordable and as a preventive measure, aerators should
be operated at night during prolonged periods (2-3 days or longer)
of cloudy, hot or rainy weather; immediately preceding and throughout
a sudden cold front passage; and when dense phytoplankton or algal
blooms have developed.
1. Emergency -- aerators are operated
temporarily when oxygen falls to or below 3 mg/l, during a crisis.
Tractor powered paddle wheels or irrigation pumps are typically
used. Aeration is continued until oxygen levels have stabilized
at 5 mg/l or higher.
2. Supplemental -- aerators are
operated whenever conditions leading to oxygen depletion have
developed, or nightly during the last 2-3 months of the season.
Aerators are turned on between 10:00 pm-midnight and left running
until 10:00 am the next morning or until oxygen levels have stabilized
at 5 mg/l or higher. Supplemental aeration is recommended for
intensive production densities above 2,000 lbs/acre.
3. Continuous -- aeration equipment
is operated continuously (24 hours daily). Some producers manage
highly intensive fish farms (greater than 5,000 lbs/acre) and
run electric aerators continuously from July to the end of September
or until water temperatures have dropped to 68-65oF and are falling.
The economics of that practice should be carefully evaluated.
The best way to deal with low oxygen is to take
action before a problem develops, good management. If budget constraints
prohibit purchase of aeration equipment, no more than 2,000 fish
should be stocked per acre. As aerators become more efficient,
it may become economically feasible to aerate continuously --
24 hours daily. That might significantly increase the quantity
of fish commercially produced in 4 acre feet of water.
L-2413 SRAC Publication
No. 370
Southern Texas Regional
Agricultural Aqua culture
Extension Center Service
July, 1989
Pond Aeration - Gary L.
Jensen,* Joseph D. Bankston * and John W. Jensen **
Oxygen in ponds comes from two sources--photosynthesis and diffusion
from the air. The most important source is photosynthesis which
is the process plants use for manufacturing food. In the presence
of sunlight, plants (especially algae) add oxygen to water as a
by-product of photosynthesis. At night, no oxygen is produced, but
respiration of algae, fish and bacteria continues to remove oxygen
from the water. Most of the time there is a desirable balance between
how much oxygen is produced and how much is used, but under some
conditions, the balance can be upset, and the oxygen concentration
becomes low enough to stress or kill fish. The amount of oxygen
in pond water can vary considerably from pond to pond and from hour
to hour. Typically, however, oxygen concentrations are lowest at
dawn and highest during late afternoon. The amount of oxygen water
can hold is dependent upon atmospheric pressure, salinity and temperature.
Water can hold less oxygen as altitude increases. However, this
is usually of little importance in the Southeast. Salinity is not
important for most freshwater fish producers. * Louisiana Cooperative
Extension Service *
* Alabama Cooperative Extension Service * The most important
factor is water temperature. As temperature increases, water can
hold less oxygen. Most low oxygen problems occur from June through
September. The reasons for this are: water can hold less oxygen
as it becomes warmer; respiration rates of both plants and animals
increase with the warmer water, so more oxygen is used; summer’s
still, hazy or cloudy days may reduce the amount of oxygen produced;
and large amounts of feed given to fish at this time of year-result
in large quantities of fish waste which create a higher demand
for oxygen. Causes of oxygen depletion. The most common oxygen
problem occurs when consumption by respiration exceeds the amount
of oxygen produced through photosynthesis and diffusion from the
air. Algae grow in large quantities as a result of heavy fish
feeding. As the quantity of algae increases, it accumulates closer
and closer to the surface to gather sunlight and increasingly
shades the lower depths. As a result, most of the oxygen is produced
near the surface, leaving a large volume of water below the first
2 to 4 feet deficient in oxygen production. Eventually, oxygen
produced during the day is less than the demand for oxygen during
the night, resulting in possible death or undesirable stress on
fish. Another type of oxygen depletion occurs when algae die suddenly.
When algae die, not only does the pond lose its source of oxygen
but the decaying algae use considerable amounts of oxygen. All
causes of sudden algae die-offs are not fully understood, but
it is known that die-offs can occur after pond treatments with
certain chemicals and herbicides. Predicting natural algae die-offs
is difficult. However, they are often associated with surface
algae scum's and very heavy algal "blooms." When a die-off
occurs, the green water often becomes streaked with gray, black
or brown. The color of the water may eventually become totally
brown, gray, black, milky or clear. A distinct foul smell may
also be noticeable. The third and most serious kind of oxygen
depletion is referred to as a "turn-over." During hot
summer weather, surface water becomes less dense as it absorbs
heat and it floats over a cooler, more dense layer of water. All
the oxygen is produced in the warmer layer and the two layers
may not mix for weeks at a time, especially in deepwater ponds.
Eventually, all the oxygen is used up in the lower, cooler layer.
A cool snap or a thunderstorm with wind and hard rain can cool
the warm surface water making it heavy enough to sink and mix
with the oxygen-deficient bottom layer. The net result is a dilution
of the oxygen and an increase in the demand for oxygen from dissolved
minerals and decaying organic matter. To complicate these problems,
the algae usually die at the same time. "Turn-over's"
cause the most catastrophic fish kills in ponds. Measuring oxygen
concentrations Oxygen concentrations can be measured using inexpensive
chemical kits ($40) or electronic oxygen meters ($200-$1,000).
A chemical kit is suitable as a backup, as a check on electronic
devices, or when fewer than three ponds are involved. It takes
several minutes to test oxygen in a pond using the chemical test
and results are relatively difficult to read by artificial light
at night. Electronic measuring devices, on the other hand, allow
the fish farmer to quickly measure oxygen in many ponds during
the night. Monitoring dissolved oxygen. Ponds in which fish are
fed should be monitored for oxygen at least twice daily, at daybreak
and nightfall, during summer and early fall. Higher evening oxygen
readings and correspondingly lower morning oxygen readings from
day to day, or low evening readings, usually warn of future problems.
Keep a chart of daily oxygen readings to help detect developing
water quality problems. Oxygen monitoring may have to be done
continually during darkness throughout the summer if stocking
and feeding rates are high. No season of the year is totally immune
from low oxygen problems so oxygen should also be monitored periodically
during cooler months. Two methods have been used to >project oxygen
concentrations at sunrise. The first is based on the fact >that,
in general, dissolved oxygen concentrations decrease during the
night at approximately the same rate from dusk to dawn. The rate
may vary from night to night so readings taken one night don’t
necessarily apply to predictions for oxygen decline during following
nights. Here’s how to use the projection method of predicting
nighttime oxygen depletions:
1) Mark vertically on graph paper
a line representing dissolved oxygen concentrations from 0 to
20 parts per million (ppm). Horizontally mark a line with the
time of night beginning at dusk. Use one graph for each pond.
2) Measure the dissolved oxygen
in each pond around dusk and plot it on the graph at the time
the reading was taken.
3) Return to the same place in
the pond 2 to 3 hours later and plot the new oxygen reading on
the same graph.
4) Draw a line using a straight
edge between the two plotted readings, and extend the line to
a point that crosses a vertical line drawn from dawn. Where those
lines cross is the concentration of oxygen expected at dawn.
5) If the predicted oxygen concentration
at dawn is less than 3 ppm, then action can be taken to begin
emergency aeration before 3 ppm is reached.
This method is a tool which producers can use to
make management decisions; it doesn’t work in all situations.
The best example of when it does not work is the case of an algae
die-off. Algae die-offs will cause sudden, unpredictable oxygen
depletions and must be monitored closely to prevent fish kills.
Another method to predict low dissolved oxygen was developed from
actual records on fish farms. It has been shown that when dissolved
oxygen in a pond at dawn is 5 ppm or more and dissolved oxygen
at dusk is more than or the same as the day before, no oxygen
depletion occurs during the following night or morning. But when
dissolved oxygen at dawn is less than 5 ppm and dissolved oxygen
at dusk is less than the day before, an oxygen depletion can be
expected during the night. The techniques described above are
only tools to help predict oxygen problems that occur under "normal"
conditions. Neither method can be used to predict oxygen depletions
which are caused by algae die-offs, turn-over, or other unusual
circumstances.
Aeration requirements for ponds
Oxygen requirements in ponds can vary greatly. A commonly used
rule of thumb is to use 1 horsepower/surface acre with an aerator
rated at least 2.5 lbs O2/hp per hour. This assumes normal oxygen
cycles during the day and night, healthy fish, and maintenance
of minimum oxygen levels above 2 to 3 ppm. However, an excessive
algae buildup or sudden algae die-off can create a much greater
than normal demand for oxygen. Fish, standing crops and maximum
daily feeding rates also influence the aeration requirement. Sick
fish and high concentrations of nitrites, ammonia or carbon dioxide
can also increase the need for additional aeration. In these situations,
additional capacity can be provided by portable aerators to prevent
fish losses which may occur either immediately or as delayed deaths
caused by disease. Also, power outages or mechanical breakdowns
can shut down any unit. For these reasons, mobile aerators with
high standard oxygen transfer rate (SOTR) ratings are also needed
for emergency or stand-by use. For most farm situations, one portable
aerator for every three to four ponds is adequate. If aeration
requirements exceed that supplied by available equipment, then
ponds with the fastest falling oxygen levels should be aerated
first.
Placement of aerators. Recent research
has shown that the most effective placement for fixed electric
paddlewheel aerators is midway along the longest side of a pond
with the discharge of the aerator directed toward the middle of
the pond. In this position, the aerator directs water perpendicular
to the long side, developing circulation that reaches most areas
of the pond. Placement of this type aerator in a corner of a pond
and directing water diagonally across the pond provides poor circulation.
Locating fixed aerators in the middle of a pond levee will incur
a higher installation cost and may be inconvenient when aerators
are needed in conjunction with harvesting operations near water
supplies. Portable aerators should be used before fish are stressed
to the point that they cannot reach the aerated area. The best
location to place an aerator before they are seriously stressed
will usually be in the part of the pond with the highest oxygen
concentration because that is where the fish will be found. If
two aerators are needed to keep fish alive, then they should be
operated alongside each other so that if one aerator cuts off,
the other aerator can hold fish until the problem is remedied.
Fish will panic and move to other oxygen less areas of the pond
in search of more oxygen if a single aerator cuts off during a
severe oxygen depletion. If fish are severely stressed from low
oxygen and cover the surface of the pond, aerators should be placed
in the area of highest fish concentrations in an attempt to aerate
as much of the pond as possible and attract fish to the aerator.
Fish will usually go to the shallow end of a watershed pond in
search of higher oxygen and fresh water. Be prepared to operate
an aerator in shallow water to attract fish to this area. Fish
also will tend to move out onto the banks when oxygen is very
low. Bank washer aerators are effective in aerating along the
shoreline so that relief is quickly provided to fish near shore.
Maintenance and safety. All types
of equipment require occasional maintenance and repair to ensure
longer life and dependability. Information and instructions for
recommended maintenance should be obtained from the manufacturer
or dealer when an aerator is purchased. Check the warranty and
keep all records of purchase for later use. Several types of aerators
have pillow bearings that require frequent greasing. This is important
for normal operation of equipment and protection of the bearing.
Some bearings have a self-greasing device that reduces this task.
All engines and motors should be maintained as recommended by
the manufacturer. Electric motors should be protected from splashing,
and all drive shafts should be checked periodically for proper
alignment. Differentials in tractor-powered paddlewheel aerators
should be checked for water and drained and re-serviced if water
is found. Check water depth of paddlewheel blades and adjust so
that the unit is level and depth is proper. Tractors used to power
aerators should be fueled and ready for operation when needed.
A good maintenance program is important because the loss of a
tractor or aerator during an emergency can result in high fish
losses. Safety is another important consideration. Hazards include
tractor use on narrow levees, operating PTOs and moving gears,
and the use of electricity near areas of activity and water. Make
certain all safety guards on tractors are in place and in good
condition, especially on PTO shafts and stub gear. Perform any
inspection or service only after equipment is shut down. Refuel
only when engine is not operating and is cooled down. Make sure
that tractor operators are experienced, especially when equipment
is used on levees and around ponds. When using tractors to relocate
or place aeration equipment, set brakes securely and block wheels.
Handle fuel with caution: Do not smoke around fuel. Use a vent
on fuel storage tanks, and make sure that tanks are properly grounded.
A fuel filter should be located in the fuel line between the fuel
tank and engine.For electrical safety: Do not drive over live
wires. Make sure that power is shut off at the control box before
any maintenance work is done. Use qualified electricians to install
wiring. Place exposed wiring in conduits to prevent rodents from
damaging the wiring.
This publication was supported in part by a grant
from the United States Department of Agriculture, Number 87-CRSR-2-3218,
sponsored jointly by the
Cooperative State Research Service and the Extension
Service.
Educational programs conducted by the Texas Agricultural
Extension Service serve people of all ages regardless of socioeconomic
level, race, color, sex, religion, handicap or national origin.
Issued in furtherance of Cooperative Extension Work
in Agriculture and Home Economics, Acts of Congress of May 8,
1914, as amended, and June 30, 1914, in cooperation with the
United States Department of Agriculture. Zerle L.
Carpenter, Director, Texas Agricultural Extension Service, The
Texas A&M University System.
2M–5-90,Reprint FISH
Texas Agricultural Extension Service • Zerle L.
Carpenter, Director • The Texas A&M University System Ž College
StatIon, Texas
%20a.jpg)
