Science Junction

Water What-ifs
Water Quality and Dissolved Oxygen

Dissolved Oxygen Reference Desk
 * Teacher Notes, Lesson Extensions and Alternative Assessment Ideas
 * Warning Signs of Pollution
 * Using Lamotte water quality test kits
 * Using CBL equipment

Dissolved oxygen is one of the best indicators of the health of a water ecosystem. Dissolved oxygen can range from 0-18 parts per million (ppm), but most natural water systems require 5-6 parts per million to support a diverse population.

Oxygen enters the water by direct absorption from the atmosphere or by plant photosynthesis. The oxygen is used by plants and animals for respiration and by the aerobic bacteria which consume oxygen during the process of decomposition. When organic matter such as animal waste or improperly treated wastewater enters a body of water, algae growth increases and the dissolved oxygen levels decrease as the plant material dies off and is decomposed through the action of the aerobic bacteria.

Decreases in the dissolved oxygen levels can cause changes in the types and numbers of aquatic macroinvertebrates which live in a water ecosystem. Species which cannot tolerate decreases in dissolved oxygen levels include mayfly nymphs, stonefly nymphs, caddisfly larvae and beetle larvae. As the dissolved oxygen levels decrease, these pollution-intolerant organisms are replaced by the pollution-tolerant worms and fly larvae.

Dissolved oxygen levels change and vary according to the time of day, the weather and the temperature. If yearly comparisons are made on dissolved oxygen levels, they should be done at the same time of day, during the same season and on a day with a temperature variation of only 10 degrees Celsius from the previous reading.

A decrease in the dissolved oxygen levels is usually an indication of an influx of some type of organic pollutant.

Dissolved Oxygen Lessons 1, 2, and 3 are intended for middle school and high school science students. These lessons focus on dissolved oxygen and its relation to organic materials and aquatic organisms. These lessons highlight several aspects of the following competencies:

Lesson1 What is dissolved oxygen and how is it important to an aquatic ecosystem?
Lesson 2 How does organic material break down and effect the oxygen levels of an aquatic ecosystem?
Lesson 3 Design an experiment to test the effects of increases or decreases in dissolved oxygen levels on aquatic plants and animals.

Download PDF version of all three Dissolved Oxygen lessons.


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Last Modified: 2/24/00

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