Dissolved Oxygen

Background Information
Dissolved oxygen (DO) 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. 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. 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.

Water temperature also affects DO levels. Colder water can hold more oxygen in it than warmer water. A difference in DO levels may be seen 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 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.

Species such as sludge worms, blackfly larvae, and leeches are more tolerant of low dissolved oxygen levels so those species are more likely to be found in warm waters. Species that need high levels of dissolved oxygen include pike, trout, bass, salmon, mayfly nymphs, stonefly nymphs, and caddisfly larvae so those will be more likely found in colder waters.

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.

Dissolved oxygen levels are often measured in parts per million (ppm) but sometimes are given in terms of Percent Saturation. Percent Saturation is the amount of oxygen dissolved in the water sample compared to the maximum amount that could be present at the same temperature. For example, water is said to be 100 % saturated if contains maximum amount of oxygen at that temperature. A water sample that is 50 % saturated only has half the amount of oxygen that it could potentially hold at that temperature. For this project, results will be reported in ppm but if you wish to determine the Percent Saturation, you can use this DO Percent Saturation chart. Sometimes water can become supersaturated with oxygen because of rapidly tumbling water. This usually lasts for a short period of time but can be harmful to fish and other aquatic organisms.

Test Procedure
To perform the dissolved oxygen test, use a dissolved oxygen test kit. Follow the instructions provided with the kit. Record results in parts per million (ppm). Some important points to remember are:

Try to 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
At 20 ^oC (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 20 ^oC, there may be more oxygen dissolved in the sample. Generally 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).

DO Percent Saturation values of 80-120 % are considered to be excellent and values less than 60% or over 125% are considered to be poor.

This project is developed and managed by the Center for Improved Engineering and Science Education (CIESE)