These pages are
reprinted from www.gotalgae.com, courtesy of www.kascomarine.com.
of Dissolved Oxygen
Have you ever
been swimming in a pond or lake in late summer and noticed the top
few feet of water is like bath water and below that, the water is
cool and refreshing? This cooler water, although very refreshing
to dive into, can be very dangerous to your pond and its inhabitants.
ability of water to absorb and hold oxygen is vital to pond health
and the health of your fish and other pond inhabitants. Dissolved
Oxygen (DO) is the measure of the amount of oxygen dissolved in
a body of water as an indication of the degree of health of the
water and its ability to support a balanced aquatic ecosystem. The
warmer the water, the less dissolved oxygen it can hold. As the
water temperature increases during the summer months, the less oxygen
it can hold.
second factor for your pond to have oxygen is the ability to be
in contact with the air, or more specifically oxygen, in order for
the oxygen to dissolve into the water. Most oxygen is supplied naturally
by wind and the photosynthesis process. As the wind blows, waves
are created which mix the surface water and expose more surface
area of the water to the air and oxygen is diffused into the water
at the air/water interface. This process is more efficient on large
lakes because there is a large open area for the wind to blow across
the lake with less blockage from trees, hill, and buildings, and
the larger surface area allows the waves to travel further. Small
ponds that are surrounded by trees or other obstructions that block
the wind do not receive as much oxygen from wind.
is the process plants use to create energy. The aquatic plants will
use sunlight and carbon dioxide to produce energy in the form of
cellulose and release oxygen as a byproduct. This is described in
further detail in the Algae Solutions,
Aquatic Plant Solutions,
and Aeration pages, but more weeds is
not always a good thing for your pond even though they are producing
oxygen. During the daytime when the pond is receiving sunlight,
photosynthesis can release oxygen into the water for the fish and
bacteria to use. However, during the nighttime hours when there
is no sunlight, photosynthesis stops and plants actually start to
use some oxygen instead of producing it, and as the night progresses,
the oxygen levels steadily drop. Also, once the plants die off,
the extra decomposition taking place also uses up available oxygen
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to Temperature Problems
Now that we understand some basics
of dissolved oxygen, we can get back to the temperature problems
in your pond. Temperature problems are a seasonal issue. As the
sun exposure to the pond increases through spring and into summer,
the water warms. As the summer progresses, the water at the surface
continues to warm faster than the water below because not as much
sunlight penetrates the lower portions of the water column. The
area of the water column that receives sunlight is called the Euphotic
Zone. The layer of warm water is known as an epilimnion. Also, warm
water is less dense or lighter than cooler water, so warm water
stays at the surface and colder water sinks to the bottom.
As the summer continues, this temperature
difference expands. The surface water is very warm and the water
below much cooler. The cold water layer is known as the hypolimnion.
As stated earlier, the cooler the water the more oxygen it can hold.
However, if the cool water has no exposure to the air or oxygen,
it cannot hold the oxygen, no matter how cold it is. Therefore,
the cooler water near the bottom does not have a continuous oxygen
supply, and over time can turn anoxic or have all of its oxygen
used up. When this occurs, organisms such as fish and bacteria need
to move into areas of higher oxygen or end up dying. This limits
the area your fish have to live and also greatly limits the amount
of decomposition of organic matter at the pond bottom.
area between the warm water and the cold water is called a Thermocline.
The thermocline is very important to your pond health and can act
as a barrier between aerobic and anaerobic areas, and prevent water
mixing. For the organisms in your pond that require oxygen, the
thermocline can mean life of death. Basically, the lower down the
water column the thermocline is located, the better off they are.
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Pond Progression and Turnover
We'll start our seasonal progression
in winter. The water temperature is fairly consistent throughout
the entire water column. As stated earlier, warmer water is less
dense than cold water and will float to the top. However, as water
cools and gets gradually heavier, it is reversed at about 4 degrees
Celsius and continues until the water is frozen (this is why ice
floats). If you are in a northern climate that gets ice, there will
be warmer water near the bottom of the pond that is warmed by the
earth and will not float to the top because the surface water under
the ice is cold enough to be lighter.
As the ice melts in late winter
or early spring, that 4 degree Celsius less dense water warms to
above 4 degrees. When this happens it is more dense or heavier than
the warm bottom water. What then occurs is called Turnover. The
heavier water is on top and all of a sudden will to the bottom and
the warm water will do just the opposite and rise to the top. This
mixes up the entire water column and the pond will typically look
dirty with suspended debris. Spring turnover is much less severe
than fall turnover because there are not as much oxygen problems
in spring as in late summer.
As spring progresses, the water
at the surface warms and the thermocline is very near the surface.
As the water warms further down, the thermocline moves further down
the water column, still usually a few feet from the surface and
the cold water remains at the bottom because it is less dense even
though it is warming slowly.
With fall comes cooler temperatures.
The pond water will begin to cool throughout fall. The cooler temperatures
will continue to cool the water until the water above the thermocline
is actually colder than the bottom water. Now you have a situation
where turnover could occur. The quicker turnover occurs, the more
severe the effects. A cool fall rain or cold front with lots of
wind can quickly cool the surface water and cause a rapid turnover.
Once the cooler surface water becomes denser than the bottom water,
it will sink and displace the warm bottom water. Again, the water
column will be mixed and look dirty with suspended debris.
is much more severe than Spring Turnover. As discussed earlier,
the thermocline separates warm water from cold water and the bottom
water does not have as much dissolved oxygen because it has not
been in contact with the air all spring and summer. During turnover,
the closer the thermocline is to the surface, the worse the effects
will be on the pond. If there is a large volume of water below the
thermocline that has very little or no oxygen when turnover occurs,
the low oxygen water gets mixed with the rest of the pond, thus
decreasing the dissolved oxygen throughout the entire water column.
Also, the chances of turnover are greater in fall due to common
fall rains and cold fronts. This decrease can be drastic enough
to cause massive fish die offs as in the picture at the top.
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Turnover and Temperature Problems
effects of spring and especially fall turnover can be devastating
to your pond. Turnover may not happen every year, but if turnover
does occur, it could ruin a large investment in your pond. The best
ways to prevent turnover or at least minimize its effects is to
keep the water mixed and add supplemental aeration.
wind and wave action fails to sufficiently mix the pond water and
eliminate or push the thermocline further down in the water column,
it is advised to physically mix the water in another way. There
are three types of equipment the work best for mixing water, Horizontal
Aspirators, Horizontal Circulators, and Bottom Diffusers (other
methods of aeration described in the Aeration
Aspirators - This type of equipment uses a motor that
is mounted above the water level with a float, extended shaft, propeller,
and draft tube to suck in air. They can be adjusted to point the
propeller in several angels. They are good at causing directional
flow and can mix deeper pond water, plus by sucking in the air are
adding some oxygen. One drawback is the extended shaft leads to
premature failure of the unit.
Circulators - This type of equipment works similar
to the aspirators, but do not use the extended shaft or draft tube.
They can be angled in several positions and can successfully mix
deeper ponds and create directional flow, while providing some oxygen.
They can also be mounted in variable water depths and used to prevent
ice in winter. Kasco offers horizontal circulators known as Water
Diffusers - This type of equipment uses a pump on the
shore to push air through a weighted tube to an air stone or several
stones on the bottom of the pond. The air then bubbles through the
air stone and rises to the surface. The rising bubbles break through
the thermocline and help to mix the water. These work best in deep
ponds where a good circulation can be set up. In shallower ponds,
they do not operate as efficiently.
way to limit the effects of turnover is to add supplemental aeration.
Using an aeration device (discussed in more detail in the Aeration
page), you can help mix the water as discussed above, plus add vital
oxygen to the pond system. If an aerator is in place before turnover
occurs, even if the pond turns over, the added oxygen will help
to prevent a massive fish die off and create a buffer for major
problems associated with turnover.
Pond Aerator is a great option for adding oxygen to the water and
it will also help to push the thermocline down to about 5-10' depending
on size. The added oxygen will be great for your pond all season
and the mixing will help limit the amount of water moved during
turnover. If turnover still occurs, the aerator will help save your
fish and your pond. Typically, it is recommended to have about 1hp
of aerator per acre of water to properly aerate the pond.
also offers Aerating Fountains which will add some oxygen, mix some
water, and also provide a beautiful display in the pond. These models
do not add as much oxygen or move as much water as a Pond Aerator,
but they do offer a great display. Typically, it is recommended
to have about 1.5 to 2hp of aerating fountain per acre of water
to properly aerate the pond.
By adding equipment
to mix the water column and add vital oxygen to the pond, you can
create a better pond ecosystem throughout the year and eliminate
or limit the negative effects of spring and fall turnover, which
can be devastating to your pond.
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These pages are reprinted from www.gotalgae.com, courtesy