Materials

Bottle with closeable lid Dissolved Oxygen test kit Data Collection Student Worksheet (optional)

Background Information
Dissolved oxygen (DO) is the amount of oxygen that is dissolved in water and is essential to healthy streams and lakes. The dissolved oxygen level can be an indication of how polluted the water is and how well the water can support aquatic plant and animal life. Generally, a higher dissolved oxygen level indicates better water quality. If dissolved oxygen levels are too low, some fish and other organisms may not be able to survive (see macroinvertebrates).

Much of the dissolved oxygen in water comes from oxygen in the air that has dissolved in the water. Some of the dissolved oxygen in the water is a result of photosynthesis of aquatic plants. Other factors also affect DO levels such as on sunny days high DO levels occur in areas of dense algae or plants due to photosynthesis. Stream turbulence may also increase DO levels because air is trapped under rapidly moving water and the oxygen from the air will dissolve in the water.

In addition, the amount of oxygen that can dissolve in water (DO) depends on temperature. Colder water can hold more oxygen in it than warmer water. A difference in DO levels may be detected at the test site 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 apparent at different depths of the water if there is a significant change in water temperature.

Dissolved oxygen levels typically can vary from 0 - 18 parts per million (ppm) although most rivers and streams require a minimum of 5 - 6 ppm to support a diverse aquatic life. Additionally, DO levels are sometimes given in terms of Percent Saturation. However for this project, the results will be reported in ppm (if you wish to determine the Percent Saturation, you can use this DO Percent Saturation chart).

Test Procedure

Take out the Dissolved Oxygen (DO) test kit. Closely follow the instructions provided with the kit. Don't forget to record your results in ppm.   When collecting your water sample, here are some important guidelines: Sample the water away from the bank and below the water surface level. Be careful not to get any air bubbles in the sample during collection; it may result in a false high reading. Allow the water to gently fill the DO sample bottle from bottom to top. Put a lid on the bottle while it is under water.   Test the DO level immediately. Biological activity in the sample and exposure to air can quickly change the DO level.   Repeat the DO test three times.

What to Expect

DO Level (in ppm)	Water Quality
0.0 - 4.0	Poor Some fish and macroinvertebrate populations will begin to decline.
4.1 - 7.9	Fair
8.0 - 12.0	Good
12.0 +	Retest Water maybe artificially aerated.

NOTE: At 20º C (room temperature) and standard atmospheric pressure (sea level), the maximum amount of oxygen that can dissolve in fresh water is 9 ppm. If the water temperature is below 20º C, there may be more oxygen dissolved in the sample. In general, a dissolved oxygen level of 9-10 ppm is considered very good.

At levels of 4 ppm or less, some fish and macroinvertebrate populations (e.g. bass, trout, salmon, mayfly nymphs, stonefly nymphs, caddisfly larvae) will begin to decline. Other organisms are more capable of surviving in water with low dissolved oxygen levels (i.e. sludge worms, leeches). Low DO levels may be found in areas where organic material (dead plant and animal matter) is decaying. Bacteria require oxygen to decompose organic waste, thus, deplete the water of oxygen. Areas near sewage discharges sometimes have low DO levels due to this effect. DO levels will also be low in warm, slow moving waters.

Print...