Grade Level/Discipline
This activity is best for honors biology, advanced biology or advanced placement biology classes. It is o.k. for 9th grade biology with a focused class that has worked with some chemistry.

National Standards
Content standard C and D for grades 9-12

Pre-activity set-up
You should have Hach test kits for dissolved oxygen. The test involves doing a titration with a correlation between a color change and the amount of dissolved oxygen in the sample. If you are not familiar with this test, the teacher must do the test before having students work with the Hach test kit. Doing the test takes about 30 minutes and includes some wait time. Students test bottled water that I have on hand just to learn the test before starting the primary productivity activity. Remember that temperature is important when doing the DO test and students need to remember to take the temperature of the water if they wish to determine the % saturation of oxygen in the water sample. Each lab group will need three bottles for their water samples. For the first titration, students can use the bottle and stopper in the test kit. For the samples being kept for 24 hours, it is important that they be collected in the same manner with no air bubbles on top and covered with either an air-tight stopper or a screw top lid. Purchasing additional glass stoppered bottles is possible but can be quite expensive.

Materials

Time Frame
3 class periods Period 1 – Collect water sample and do the first titration to determine initial dissolved oxygen in mg/L Period 2 – After a 24-hour time, repeat the dissolved oxygen test on the dark and the light water samples Period 3- Share data with class, determine productivity and discuss results.

Engagement and Exploration (Student Inquiry Activity
Note: You will want data from a minimum of 5 lab groups

When this activity is being done as part of a larger unit on water quality testing or work on the concept of food webs or the energy flow through an ecosystem, discussion can begin with questions like the following; How can we measure how much carbon is being captured by plants through photosynthesis? Give students time (5-10 minutes) to talk with lab partners about possibilities and report to class. If the class has not yet studied photosynthesis and the class is studying water quality, then focus the discussion on dissolved gases in a liquid. Again ask students for examples of gas dissolved in liquids. (Carbonated soft drinks will come to mind immediately)

Ask students if the oxygen in H2O is available for aquatic organisms. (It is not) Have students discuss their ideas with lab partners and report ideas to class. Give students 5-10 minutes. You can play devils advocate depending on student responses. (The oxygen chemically bonded to hydrogen is NOT available for respiration.) Where does the oxygen come from then? (Oxygen dissolves from the contact of the body of water with the atmosphere)

After ample discussion, give students the background reading and discuss the content ideas about the testing, primary productivity and the reason for doing the test three times. Discuss why they will test a bottle kept in the dark for 24 hours. What will cause any change in the dissolved oxygen? (The bacteria and other organisms continue to respire and consume the oxygen) What will the light bottle tell you after 24 hours? (When the value of the dark bottle is subtracted from the light bottle, you have measured gross productivity)

Ask students to make a prediction about the productivity rate for the water being tested.

Explanation (Discussing)
Ask students to explain what each of the three D.O. tests will tell us.

What variables must be controlled to insure that the test results are valid?

(Remind students about the effect of temperature on the dissolved oxygen content of water)

Ask students why the first test must be done on the water immediately after the sample is taken in the field. (The oxygen must be fixed immediately due to the relationship of D.O. to the temperature of the water. This may be a good time to review gas laws if they have been presented previously or from a chemistry class)

Elaboration (Polar Applications)
Students should set up the experiment and collect the data.

When testing a pond or stream, students can select several sites and collect the water samples in teams from several sites. Students calculate the initial dissolved oxygen; the dark bottle dissolved oxygen

Exchange (Students Draw Conclusions)
Students will analyze their data and share their information in class during a discussion.

Evaluation (Assessing Student Performance)
Students can write up a lab report that is teacher evaluated or students can present and discuss their results in a seminar format to the class. Students must compare their results to their prediction made before doing the primary productivity test. Have students come to consensus.

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