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Temperature
Background Information
It is important to know the temperature of the water at the test site. The water temperature
has a direct influence on other water quality factors such as dissolved oxygen (DO) and
biological oxygen demand (BOD), as well as on the survival of some aquatic species.
Knowing the water temperature at the test site could help predict and/or confirm other
conditions of the water.
The amount of oxygen that can dissolve in water (DO) depends on temperature.
Colder water
will have a higher DO level than warmer water. A difference in DO
levels at the test site may be detected if tested early in the morning when the water is cool and then later in
the afternoon on a sunny day when the water temperature has risen. A difference in DO levels
may also be seen between winter water temperatures and summer water temperatures.
Similarly a difference in DO levels may be seen at different depths of the water if there is a
significant change in water temperature.
Also, warmer water usually will have a higher BOD level than colder water. As water
temperature increases, the rate of photosynthesis by algae and other plant life in the water
also increases. When this happens, plants grow faster and also die faster.
When the plants
die, they fall to the bottom where they are decomposed by bacteria. The bacteria require
oxygen for this process so the demand for oxygen (BOD) is high at this location.
Therefore,
increased water temperatures will speed up bacterial decomposition and result in higher BOD
levels.
Temperature also influences the survival of aquatic organisms. The metabolic rates of aquatic
organisms increases in warm water. Since metabolism requires oxygen, some species may
not survive if there is not enough oxygen in the water to meet their needs.
Also, water
temperature may affect the reproductive rates of some aquatic species; some species may
not be able to reproduce in warmer waters. Since bacteria and other disease causing
organisms grow faster in warm water, the susceptibility of aquatic organisms to disease in
warm water increases as well.
Sudden increases in temperature may be a result of thermal pollution which is the discharge of
large amounts of warm water from industrial plants. Sudden changes in water temperature
may cause thermal shock in some aquatic species and result in the death of that species.
Thermal pollution, even if gradual, may disrupt the ecosystem balance in such a way to
eliminate heat intolerant species from that area.
Test Procedure
If possible, measure the water temperature at the same level that the sample for the dissolved
oxygen test is taken. That way, a correlation may be made between DO level and
temperature (especially if you do more than one test). In any case, make sure the tip of the
thermometer is at least a few inches below the surface of the water and take a reading when
the temperature has stabilized (usually after a couple of minutes). Record the temperature in
degrees Celsius (C). If the temperature is measured in degrees Fahrenheit (F),
use a unit conversion calculator to convert to Celsius.
What to Expect
In colder water, the DO levels should be higher since oxygen can dissolve in colder water
more easily than in warmer water. If the DO levels are high, the water will be able to support
aquatic life (fish and plants) more readily.
In warmer waters, the DO levels may go down and the BOD levels may go up because of the
increased decomposition of plant matter (see above). The metabolic rate of some organisms may increase to a point at which it is impossible for them to survive.
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