Science Junction


Water What-ifs

Teacher Notes,
Lesson Extensions and
Alternative Assessment Ideas

Teacher Notes, Lesson Extensions and Alternative Assessment Ideas

Lesson 1-Introductory Lesson-What is dissolved oxygen and how is it important to an aquatic ecosystem?

1. Dissolved oxygen test kits may be purchased from a biological supply company or LaMotte. You may request a LaMotte catalog from their web site located at:

2. Pond or stream water samples should be fresh and no more than an hour old. If samples are collected the day before, aeration is necessary as some of the aquatic organisms are extremely sensitive to environmental changes. As the dissolved oxygen levels decrease, the organisms will rapidly die off. The dissolved oxygen levels begin to decrease almost immediately after the sample is taken from the water.

3. Fish tank pumps, tubing and regulators may be purchased from a pet supply store or a department store and are fairly inexpensively.

    Lesson 2-How does organic material break down and effect the oxygen levels of an aquatic ecosystem?

1. 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. Bacteria use oxygen as part of the decomposition process. Methylene blue indicator solution changes from blue to colorless in response to low oxygen levels and is an indirect indication of how much organic material is present in a water sample. The faster the color changes, the less oxygen is present, and the more carbon dioxide and bacteria.

2. If it is impractical to complete this lesson in a one hour time frame, the test tubes may be prepared and examined during the next class period.

Lesson 3-Design an experiment to test the effects of increases or decreases in dissolved oxygen levels on plants and animals.

Note: Students should have completed Lesson 1 before beginning Lesson 3. This will familiarize them with the dissolved oxygen test kits and also give them a basic understanding of how dissolved oxygen effects aquatic organisms.

1. The materials listed are the basics needed to begin the lesson. Students will provide a detailed list of materials as they design their experiments.

2. Based on the example questions, students should develop their own question to be investigated.

3. Students will develop a step by step plan on how the experiment will be carried out. This should take the form of a procedure. Have the students look at a previous laboratory procedure as a model for writing the procedure for this lesson.

4. A trial run should be performed before a data collection is attempted. This gives the students an opportunity to correct any problems before they begin the actual data collection.

5. The data collected should be presented in table form. Have the students look at a previous lab's data table as a model for developing the data table for this lesson.

6. The laboratory report should be in the standard format and should include the question which was investigated and the hypothesis.

7. The oral presentation should be a brief presentation of the written report.

Lesson Extensions

1. Research different types of aquatic plants and animals and make a list based on their dissolved oxygen requirements.

2. Have each group use a different type of macroinvertebrate and apply the same levels of air. A determination can be made on the dissolved oxygen levels necessary for each type to grow and develop.

Alternative Assessment Ideas

1. Set up several different aquatic ecosystems. Using the air pump with a regulator, add differing a levels of air. Have the students keep a daily journal of the changes which take place over a period of time.

2. Have student assess the quality of several different water samples by determining the levels of dissolved oxygen in each sample.

3. Teach students to perform other chemical tests such as a nitrate and phosphate determination and pH. Assess their ability to conduct these tests accurately by providing known samples for them to analyze.

Water What-ifs Home | Dissolved Oxygen Lessons

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©1998 April J. Cleveland for Science Junction, NC State University.
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page design by Lisa Leonor Grable and April J. Cleveland
Last Modified: 7/27/00

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