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.