LaMotte DO 4000 Dissolved Oxygen Meter - Manual (Page 5)

DISSOLVED OXYGEN BASICS
Oxygen, in the form of a gas, is a major component of air. Humans,
plants, and animals use this oxygen to respire, or breathe. Gaseous
oxygen also occurs in the water, where aquatic life uses it to respire.
Oxygen in water is often called dissolved oxygen.
Dissolved oxygen is vital to the survival of aquatic organisms. Naturally
present, dissolved oxygen enters water when plants photosynthesize or
through dispersion. When plants, particularly algae, photosynthesize,
sunlight is converted into needed chemical energy, releasing oxygen into
the water. Dispersion, the exchange of oxygen between the atmosphere
and water, can be aided by wind and wave action. Oxygen can also be
mechanically introduced into the water by use of an aerator.
Once in the water, oxygen is consumed in several ways, including the
respiration of plants and animals and the oxidation, or chemical
breakdown, of dead and decaying plants and animals. In extreme cases
all available oxygen may be used in the decomposition process, so no
oxygen is left for respiration, and the plants and animals eventually
suffocate.
Dissolved oxygen can be measured as the concentration of oxygen
present in the water (mg/L or ppm oxygen), or as the percent of oxygen
actually in the water as compared to the maximum amount the water
can hold. This is referred to as percent (%) saturation. The level at
which water is saturated is dependent upon several factors, but two of
the most important are temperature and salinity. As the temperature of
the water decreases, its ability to hold oxygen increases. Likewise, as
salinity levels decrease the saturation level increases.
Dissolved oxygen levels are vital in many areas, but they are of
particular concern to aquaculturists, environmental analysts and
wastewater treaters, who must constantly monitor levels in the water.
For aquaculturists, dissolved oxygen can be the difference between a
healthy, profitable crop of fish, and disaster. If sufficient levels of
dissolved oxygen are not maintained fish growth can be hindered, the
fish may be more susceptible to infectious diseases, or they may even
suffocate. Many aquaculture systems use aerators, especially at night
when DO levels are lower, to ensure sufficient oxygen for a healthy
crop.
Environmental analysts are also concerned with dissolved oxygen levels.
This test is one of the most important for determining the ability of a
pond, lake, or other body of water to support life. It is also used as part
of the biochemical oxygen demand (BOD) test to determine the
pollution level of the water.
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