DO
Dissolved Oxygen, also known as Oxygen Saturation, is a measure of the amount
of gaseous oxygen that is present in natural bodies of waterponds, lakes, rivers,
streams – as well as stormwater runoff, wastewater, fish farms and aquariums. It gives
an indication of the health of the water and its ability to sustain a
balanced aquatic ecosystem. DO levels are also important in
drinking water and many other industries.*
MILWAUKEE INSTRUMENTS
2950 BUSINESS PARK DRIVE
ROCKY MOUNT, NC 27804
www.milwaukeeinstruments.com
sales@milwaukeeinstruments.com
All About Balance
Oxygen enters water by diffusion from
the atmosphere, plant photosynthesis,
and from the increased
surface area of churning
water. In turn, oxygen is
consumed by respiration
of aquatic animals (and
plants, too, at night),
decomposition of organic
matter and pollutants,
and various chemical
reactions. In a healthy
ecosystem, oxygen
creation and oxygen use
remain in balance.
However
DO levels do fluctuate over a
24-hour period, and seasonally.
They vary with the depth of water,
with water temperature, and with
altitude. Some imbalance can be
tolerated over short periods of time,
but significant imbalance can be
disastrous to aquatic life, making it
imperative to know and control
DO levels.
When To Measure
Unlike streams and rivers, still waters
in ponds and lakes experience little
churning; therefore,
photosynthesis plays
a larger role in the
production of dissolved
oxygen. DO levels are
at their highest in the
late afternoon of very
hot, sunny days. Once
the sun goes down,
aquatic plants stop
producing oxygen and
join withsh and other
organisms in the water
to consume oxygen through respiration.
If the demand for oxygen is too heavy
during the night, DO levels may get
dangerously low by early morning.
Levels below 1-2mg/L for several hours
can cause ash kill.
Know Your DO
Dissolved oxygen meters measure in
parts per million (equals mg/L) or as a
percentage of saturation, where
saturation is the maximum amount
of oxygen that
can be dissolved in
water at a given altitude and
temperature. Most meters also measure
temperature, in Celsius, and may
include an auto compensation feature.
The probe is lled with a salt solution
and has a selectively permeable
membrane that allows DO to pass from
the water being tested into the salt
solution. The DO that has diffused into
the salt solution changes the electrical
potential of the solution. This change is
sent from the probe to the meter where
it is displayed in mg/L and %.
DO Plays a Key Role in Pond Management
A common cause of fish kills is oxygen depletion. This condition
usually occurs during summer in very fertile ponds as a result of pond
turnover or the die-off of an algal bloom. During hot weather most
ponds have a layer of water near the bottom that contains little or
no dissolved oxygen. When high winds or cold rain cause this water
to mix with the upper pond water, oxygen levels often drop low
enough to kill fish. Oxygen depletion also occurs when dead algae or
other plants decay in the pond after herbicides have been applied to
control weeds.
Meter Calibration
Some meters feature auto calibration,
while less expensive meters must
be manually calibrated. This is an
easy, open-air calibration requiring
no calibration solution. You can find
open-air calibrations on the TECH TIPS
section of this website.
*Our drinking water tastes better with
high DO levels; however, industry prefers
low DO in its water supply as the oxygen
promotes corrosion in piping.