Dissolved Oxygen

Background Information

Dissolved oxygen (DO) is essential to healthy streams and lakes. The dissolved oxygen level is 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. Water temperature also can affect dissolved oxygen levels. The higher the temperature, the less oxygen can dissolve in the water. Sometimes, factories or power plants discharge slightly warmer water into nearby streams or rivers. This is known as thermal pollution and it can greatly affect the fish populations in these waterways.

Test Procedure

To perform the dissolved oxygen test, use a dissolved oxygen test kit from LaMotte (or other supplier). Follow the instructions provided with the kit. Record results in parts per million (ppm). Try to sample the water away from the bank and well below the water surface level. When collecting the water sample it is important not to get any air bubbles in the water sample.

What to Expect

At 20 C (room temperature) the maximum amount of oxygen that can dissolve in water is 9 ppm. If the water temperature is less than that, it may actually have more oxygen dissolved in it. 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 (i.e. bass, trout, 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).

References