Oxygen Saturation vs Temperature
Figure 1
The earth's basic air envelope is made up of about
78% Nitrogen, 21% Oxygen, and 0.03% Carbon Dioxide.
There are also traces of several other elemental and
molecular gasses but they will be ignored since they
have no known effects within the pond environment.
Concentrations of these gases within water is a whole
different story. The concentrations are much smaller
and are measured in milligrams per liter (mg/l) or
somewhat equivalently, in parts per million (ppm).
A typical pond at a temperature of 70° F. will have
concentrations of about 13 mg/l Nitrogen, 9 mg/l Oxygen,
and 35 mg/l Carbon Dioxide. As the air components dissolve
into the water, a point is reached where no more can
be added. This point is called saturation. The saturation
points are different for each of the gases and are
dependent upon several different factors but temperature
is the most important. As the temperature increases,
the water simply cannot hold as much of each type of
gas. For oxygen, the approximate saturation level at
50° F. is 11.5 mg/l, at 70° F., 9 mg/l, and at 90°
F., 7.5 mg/l. (See Figure 1) Impurities added to the
water (i.e. salt) further decrease these saturation
levels. Four pounds of salt per hundred gallons of
water (5 ppt) will decrease the oxygen saturation levels
about 1 mg/l.
Fish are remarkably well adapted for extracting oxygen
from the very low concentrations found in water. The
rate of oxygen consumption by Koi is closely related
to the water temperature. Koi are "cold blooded",
that is, their body temperature is essentially that
of their environment. Their metabolic activities are
basically enzyme-catalyzed chemical reactions that
are temperature dependent. The metabolism and activity
increase with temperature which increases their oxygen
demand. There is both an optimum and maximum temperature
at which the Koi live and function. At optimum temperature,
oxygen consumption is high because of rapid growth
and significant activity. Above this optimum temperature,
the fish start to experience stress. This stress triggers
their warning and defense systems which require a very
high oxygen consumption. Unfortunately, as we saw
above, the amount of oxygen available in the water
also decreases with temperature. The combination of
these two events normally limit the maximum temperature
at which the Koi can survive.
The minimum limiting oxygen concentrations for a fish
is dependent upon its genetic makeup, water temperature,
level of activity, long term acclimation, and stress
tolerance. Water with an oxygen concentration of less
than 3 mg/l will generally not support fish. When concentrations
fall to about 3-4 mg/L, fish start gasping for air
at the surface or huddle around the water fall (higher
concentration points). Bio-converter bacteria may
start to die off dumping toxins into the water compounding
the lack of oxygen to the fish. Levels between 3 and
5 mg/l can normally be tolerated for short periods.
Young Koi are less tolerant of low oxygen than the
older, larger ones. Above 5 mg/l, almost all aquatic
organisms can survive indefinitely, provided other
environmental parameters are within allowable limits.
Whereas the fish are reasonably comfortable and healthy
at 5-6 mg/L concentrations, many people consider the
efficiency of the bio-converter to be at maximum only
when the water entering the bio-converter media is
near oxygen saturation. Ideally, our ponds should
be at or near oxygen saturation at all times.
Pill, powder, and droplet (or combination) test kits
are available. Most involve three steps and a final
colormetric chart. Recommended test kit range 0 -
15 mg/L. Note: Some test kits can show false readings
if various chemical treatments are in the water. Electronic
Dissolved Oxygen meters are also available. These
are accurate and convenient, but quite expensive. A
Dissolved Oxygen test kit is considered nice to have
but not required for the average pond.
Return to Pond Water Chemistry.
CAUTION: Make sure that the transport tank's air supply
cannot be contaminated with the vehicle's exhaust.
Carbon Monoxide is very soluble in water and can be
even more deadly to the fish than to you.
Return to Pond Water Chemistry.
Return to Koi Ponds and Filters
Copyright © 1996, Norm Meck, Revised August 26, 1996
Whenever air is in contact with the water, whether through
natural or artificial means, a transfer of oxygen from
the air to the water takes place until the water becomes
saturated. Plants under light, convert carbon dioxide
to oxygen in the water. Fish, plants at night, and
aerobic bacterial action consume the oxygen.
Treatment:
It is not difficult to get all the air into the water
that the fish need. Oxygen is continually transferred
into the water at the surface of the pond and normally
only a small water fall will bring the pond water to
or near to saturation. Heavily populated ponds may
need supplemental air and ponds with a large amount
of algae may need supplemental air at night when the
plants are not making oxygen but consuming it. It
is very important that sufficient circulation is provided
within the pond so that all areas have proper oxygenation.
Almost all of the oxygen dissolved into the water from
an air bubble occurs when the bubble is being formed.
Only a negligible amount occurs during the bubbles
transit to the surface of the water. This is why an
aeration process that makes many small bubbles is better
than one that makes fewer larger ones. The breaking
up of larger bubbles into smaller ones also repeats
this formation and transfer process.
A "sheet" type waterfall can provide more
dissolved oxygen in a pond than the "cascade"
type waterfall whose velocity is low when the water
finally enters the pond. Although the cascade type
waterfall provides better aeration of the water that
is entering the pond, the sheet type provides better
aeration of the water that is already in the pond.
The sheet of water tends to shear the larger bubbles
of air formed at surface entry into smaller ones below
the surface. This action can occur at depths of up
to three feet or more and result in oxygen transfer
to a much larger amount of water than just that which
is entering the pond. For most situations, the amount
of water flow is determined by filtration requirements
and either type will be more than sufficient to maintain
the pond oxygen levels at or near saturation.
A common method of providing additional oxygen to the
water is through the use of an eductor type air jet
(sometimes called a venturi). An added advantage of
this device is that it can simultaneously provide improved
circulation of the pond water.
Air stones or similar bubble forming devices driven
by an air pump can also be used to provide supplemental
air. A single air stone can supply sufficient air for
up to a 1000 gallon pond although pond water circulation
problems may still exist. It is recommended that a
backup air pump with tubing and air stones (size and
quantity depending on pond size) be kept on hand in
case of main water pump malfunctions. This could also
be used to supply air to an isolation tank if needed.
In an emergency, just splashing the water by hand or
with a bucket can probably add enough oxygen to sustain
the fish until the problem is corrected.
When a power loss or other malfunction causes water
flow to stop and hence most aeration to also cease,
several problems develop. The oxygen concentration
drops and ammonia starts building up. The size and
population density of the pond will determine how long
before this becomes a problem but the bacteria in the
bio-converter will start dying off at about the 4 hour
point. After about 4 hours, it is important that before
circulation through the bio-converter is restored,
that it be drained to remove any toxins released by
the dying bacteria. The ammonia levels and nitrite
levels should then be monitored closely for a couple
of days.
Oxygenation During Transport
When plastic bagging fish for transport, use only enough
water to just cover the dorsal fin. Squeeze out the
current air, add 5-10 times the amount of oxygen as
water. This is normally sufficient oxygen for up to
6 hours (if oxygen is not available, just plain air
in the bag is sufficient for an hour or two).
Ammonia build up and temperature control then become
the major problems. Based on controlled experiments,
(the experiments were conducted using bass, trout,
and carp but it is assumed that the results also apply
to Koi) it was found that floating the transport bags
in the pond for 30 minutes prior to release decreased
the mortality rate due to temperature shock, particularly
for small fish. This test was conducted with the fish
bagged for one hour. For fish that had been bagged
for four hours, it was found that the mortality rate
increased for all sizes of the fish if the bag was
floated for 30 minutes. My recommendation is that if
the fish have been bagged for two or more hours, it
is better to release them immediately than to subject
the fish to the "bad" water in the bag for
an additional half-hour. Thirty minutes of floating
will prevent a sudden shock if the temperature difference
is large, but it will not acclimatize the fish to the
new temperature. Actual temperature acclimation of
a fish takes several days, similar to us dealing with
jet lag. It is not only the temperature the fish needs
to be accustomed to but also the pH, hardness, alkalinity,
"the taste", etc. of it's new surroundings.
If a transport tank is being used for moving fish, an
air stone or aeration column can be used. A venturi
(air ejector) is not recommended since the strong currents
induced make the fish have to "work" harder
which increases both the oxygen consumption and, of
more importance, the ammonia waste products in the
small tank. An air stone can be fed directly from bottled
oxygen or from a small air pump. An aeration column
can be fed from a small submersible water pump ideally
located at the opposite corner or end from the aeration
column.