Temperature

Background Information

The water temperature of a river or stream is very important for water quality. Temperature influences the amount of oxygen that can dissolve in the water, the rate of photosynthesis by algae and other larger aquatic plants, and the metabolic rates of aquatic organisms.

The amount of oxygen dissolved in the water (DO) depends on temperature. Cold water holds more oxygen than warm water. The more oxygen in the water, the more living organisms it can support.

Temperature also influences the rate of photosynthesis by algae and other aquatic plants. As water temperature rises, the rate of photosynthesis increases. The faster plants grow, the faster plants die. As plants die, they are decomposed by bacteria that consume, or use up, oxygen. Therefore, when the rate of photosynthesis is increased, the demand for oxygen by the bacteria in the water (BOD) is also increased.

Temperature influences the metabolic rates of aquatic organisms. Metabolic rate is the speed at which cells conduct the chemical processes of life. Most aquatic animals are cold-blooded. Their metabolic rate is slower in cold water but faster in warm. Therefore, they need more food and oxygen in warm water. They will also release more wastes.

Temperature also influences the sensitivity of organisms to toxic wastes, parasites, and diseases. Bacteria and other disease-causing organisms grow faster in warm water.

Human-caused changes in temperature may come from thermal pollution. Thermal pollution is an increase in water temperature caused by adding relatively warm water to a body of water. Industries such as power plants may be responsible for discharging warmer water into a river. In this case, there would most likely be a difference in temperature between the point at which the water is discharged and a point farther downstream.

Test Procedure

If possible, measure the water temperature at the same level that the sample for the dissolved oxygen test is taken. That way, you may be able to see a correlation 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 stablilized at one point (usually after a couple of minutes). Record the temperature in degrees Celsius (C). If you measure the temperature in degrees Fahrenheit (F), convert to Celsius.

To convert temperatures from Fahrenheit to Celsius you can use this conversion chart (chart reading is a good skill to learn!!)

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.

References