LaMotte Dissolved Oxygen Water Quality Test Kit - Manual (Page 5)

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WHERE DOES THE OXYGEN GO?
Once in the water, oxygen is used by the aquatic life. Fish and other aquatic animals
need oxygen to breathe or respire. Oxygen is also consumed by bacteria to decay, or
decompose, dead plants and animals.
Respiration
All animals, whether on land or underwater, need oxygen to respire, grow and survive.
Plants and animals respire throughout the night and day, consuming oxygen and
producing carbon dioxide, which is then used by plants during photosynthesis.
Decomposition
All plant and animal waste eventually decomposes, whether it is from living animals
or dead plants and animals. In the decomposition process, bacteria use oxygen to
oxidize, or chemically alter, the material to break it down to its component parts.
Some aquatic systems may undergo extreme amounts of oxidation, leaving no oxygen
for the living organisms, which eventually leave or suffocate.
PERCENT SATURATION
The oxygen level of a water system is not only dependant on production and consumption.
The potential dissolved oxygen capacity of water is limited by atmospheric pressure
(altitude), salinity, and temperature. These factors determine the highest DO level
possible. The percent saturation value expresses the quantity of dissolved oxygen in
the sample as a percent of the theoretical potential.
When water holds all of the dissolved oxygen that it can hold at a given altitude,
temperature, and salinity, it is said to be 100% saturated. If it holds a quarter as much as it
could possibly hold under those conditions it is 25% saturated. It is possible to get percent
saturation values over 100% when water becomes highly aerated by tumbling over
rapids and dams. It can also become supersaturated on a sunny day when dense areas
of plants or algae produce oxygen through photosynthesis.
Low atmospheric pressure found at higher altitudes slightly decreases the solubility of
oxygen in water so the dissolved oxygen value must be corrected for altitude.
The various minerals dissolved in water lower the capacity of the water to hold oxygen.
A correction factor can also be applied to dissolved oxygen measurements in saline
waters. In fresh water, where the salinity is very low, this effect is insignifi cant when
compared to the effect of temperature. Therefore, a correction for salinity is not
incorporated into the calculation.
Cold water can hold more oxygen than warm water. That is why fi sh that require higher
levels of oxygen, like trout, are found in cold water and dissolved oxygen concentrations
are usually higher in the winter than they are in the summer at the same location. The
percent saturation concentration can be corrected for water temperature.
Percent saturation levels from 80 to 120 percent are considered to be excellent. Levels
between 60 and 79 percent are adequate. Above 125 percent and below 60 percent
saturation, levels are poor. Fish and invertebrates that can move will leave areas with
low dissolved oxygen and move to areas with higher levels. Slow moving, trapped
or non-mobile aquatic animals may perish if levels become too low. Extremely high
dissolved oxygen concentrations are harmful to fi sh even for very short periods of
time. Gas bubble disease, which is characterized by the rupturing of capillaries in the
gills due to supersaturated water, is usually fatal.