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Secret Agents of Dissolved Oxygen

Grade Level




Water chemistry is altered by the biological processes of phytoplankton (microscopic photosynthetic organisms). The first two activities begin with an inquiry into what some of these water chemistry changes are, and what influences these changes (type of water, exposure to light, etc.) and an opportunity for the students to design an activity based on experience gained from the first activity.

The last two activities explore the effect of greenhouse gases on heat absorption in the atmosphere and the effect of the biological processes of phytoplankton on these greenhouse gases. These two activities can be used in conjunction with the first two or as inquiry by themselves.

Phytoplankton have a key role in the production of oxygen and the resulting use of carbon dioxide in the atmosphere during photosynthesis. They alter the chemical characteristics of the water due to their biological processes. Because of the short season of light availability in Antarctica, the phytoplankton are profoundly more productive in each 24 hour period than are general oceanic phytoplankton. Therefore, the waters of Antarctica have an important role as a significant carbon dioxide sink. This may prove to be extremely important in light of our greenhouse gases problems.

Grade Level/Discipline
9-12, but may be adapted to advanced 8th grade classes


  • Students will determine the changes in different types of water in a sealed container over time.
  • Students will determine some causes of the changes in water in a sealed container over time.
  • Students will learn to measure dissolved oxygen, temperature, and carbon dioxide with calculator/ computer probe-ware or by other means.**
**See Teacher Background for alternatives to probe-ware measuring dissolved oxygen, temperature, and carbon dioxide

National Standards
Teaching Standards: A, B, D, E; Content Standards: A, C, F;
Program Standards: B; System Standards: A, B, F

Teacher Preparation for Activity
There is considerable pre-class set-up for this activity; however, this allows many variables to be tested in an inquiry-based manner with greater control and stronger comparisons . The different groups of students will be testing a variety of water types of unknown origin. By later learning all the different sources, students can draw conclusions about what affects the chemical characteristics of water without prejudice. The second part of this activity will draw on this knowledge base so the students can design their own experiment.

Pre-activity set-up

  • Water will need to be gathered from a variety of sources, but they should match in whether they are fresh or saline. Examples of water groups to consider are:

    Pond or freshwater aquarium water, sterile water, and water with algae bloom (See directions for algal culture below)
    OR Ocean or marine aquarium water, sterile saltwater, and saltwater w/ algae bloom

  • Take initial readings of pH, DO, and temperature so students will have initial readings
  • Each type of water needs to be placed into 6 bottles--3 covered with foil and three uncovered. These bottles should have screw lids and be filled to overflowing and carefully capped so that there are no air bubbles are present in the jar. Check the sides of the bottles and tap them gently to force air bubbles to rise if bubbles are present. Please see the chart below for how each bottle should be filled, treated (if necessary), and labeled. Labels are encrypted so that students can later be informed of the type of water they are testing and so that they will not let another group's data influence their DO and pH values. This is a set-up for three groups. For more groups, either repeat some of the tests for a given type of water, or add the dimension of marine and fresh being tested at the same time, too.
  • Examples of typical labels for bottles in group sets are in the chart that follows:

    BOTTLE TYPE    Group One        Group Two             Group Three
    Uncovered      UF-1 or UM-1     UFS-1 or UMS-1        UFB-1 or UMB-1
    Uncovered-     UF-2 or UM-2     UFS-2 or UMS-2        UFB-2 or UMB-2
    fixed O
    Covered        UF-3 or UM-3     UFS-3   etc.          UFB-3   etc.
    Covered-       UF-4 or UM-4     UFS-4                 UFB-4
    fixed O
    Sterilized     UF-5 or UM-5     UFS-5                 UFB-5
    Sterilized     UF-6 or UM-6     UFS-6                 UFB-6

  • Each class period doing the activity will serve as one of the trials for each type of water. If only one class is doing this activity, students can work in pairs so that at least three sets of each type of water are run.
  • The students will place the bottles in the aquarium bath so that the bottles have access to strong natural light. Artificial light may be used, but care must be given to avoid temperature increases. Placing the bottles in the aquarium during the incubation time buffers the temperature fluctuations while providing access to as much light as the water would normally have. Photosynthesis and respiration should occur leaving their signature chemical products (see Teacher Background) to be tested by the students.

*Directions: Add liquid plant food in the amount given for hydroponic growth to live (pond or aquarial) water. Expose solution to intense sunlight and/or sunlamp. Water should be green, indicating an algal bloom.


  • Bottles with tightly fitting lids (6 for each type of water) encrypted labeling by color code
  • Aluminum foil
  • Dissolved Oxygen (DO) probe and DO titration testing kit (or proper solution for fixing oxygen)
  • pH testing kit
  • Temperature probe or thermometer
  • UV light for sterilizing water (5 drops of chlorine bleach will kill the organisms, but may change chemistry of the water.)
  • Aquarium filled with water at same temperature as collected water to serve as temperature bath
  • Grow light if natural light is not present in sufficient quantities to simulate natural conditions
  • Large chart to record each classes data or overhead transparency with chart formatted
  • Appropriate markers for presentation format
Time Frame

Teaching Sequence
Engagement and Exploration (Student Inquiry Activity)
  1. Students should be given the sealed bottles of water. Do not tell the groups that they are different types of water.
  2. Hand out Student Reproducible Master - 1 (SM-1)
  3. Inquire of the students:
    • If there could be any changes that could happen to the water while the bottle is sealed
    • What physical characteristics of the water should be measured to determine those changes
    • What the could purpose of the foil possibly be
    • What could cause changes in the water
  4. Let students place bottles near a strong light source (natural is best, a grow-light will do)
  5. After a 24/48 hr. wait, students should test their respective water samples for dissolved oxygen, pH, and temperature and record their results on the data chart. (SM-1).
  6. Compare data between groups. There should be differences, not only between bottles, but between groups as the water came from different sources.
  7. Have students present or post results in a large format, such as a flipchart sheet or an overhead transparency chart or graph.
**TeacherTip** For more accurate results, combine the data from all your classes so that students may average multiple data sets for the same type water.

Explanation (Discussing)

  1. Discuss the differences between each of the results and let students brain-storm for possible reasons.
  2. Let the students know what the differences were in each of the groups water sources.
  3. Reexamine answers on explanations and correct inaccuracies.
  4. Determine the effects of light on dissolved oxygen, the effects of changes in dissolved oxygen on pH, and difference in pond/aquarial water before and after sterilization.
  5. Discuss possible aquatic inhabitants that might be present in one water type and not another.
  6. Discuss what possible processes phytoplankton and zooplankton undergo while living and what impact might both of those organisms have on the water chemistry and why.
  7. Discuss the differences in conditions that organisms in the water samples have versus the conditions in the water in Antarctica and what impact that might have on its productivity.
  8. Students should come to a consensus for the explanation. You may guide them with carefully planned questions, but it is important that the students can see the reasons behind the result on their own.
Elaboration (Polar Applications)
  1. Based upon the skills and information gathered during the first inquiry, students are to design and implement (optional) an experiment that determines if the time of day (exposure to sunlight) affects the amount of dissolved oxygen, CO2 and/or pH of the water.
  2. Hand out student activity sheet. SM - 2
  3. After the experiment design is implemented, discuss the results with student presenting their explanations for the results. Students should come to a consensus for the explanation. You may guide them with carefully planned questions, but it is important that the students can see the reasons behind the result on their own.
This is an important step in the inquiry process. DO NOT SKIP THIS STEP!

Exchange (Students Draw Conclusions)

Evaluation (Assessing Student Performance)

Besse Dawson
Pearland High School
3775 S. Main Street
Pearland, Texas 77581

Reasons for chemical changes in the water are that photosynthetic microorganisms (Cyanobacteria) in the water will react when light is intensified or removed by changing the rate of photosynthesis ( therefore oxygen production). The Cyanobacteria in Antarctica are extremely productive over the short summer light period afforded by the Earths tilt. The Antarctic Cyanobacteria actually have less yearly net activity versus the net yearly activity of those found in continually sunny climes, such as Hawaiian Pacific waters; however, Antarctic Cyanobacteria are more productive per diem than others.

The process of photosynthesis reduces the availability of hydrogen for ionization therefore reducing the acidity of the solution (pH goes up). When the organisms are removed, changes in dissolved oxygen and pH will no longer be observed because they are the agents of those changes. Chemical formula:

CO2 + H2O <=>CH2O + O2

where CH2O is the organic carbon. The H2 is tied up in that molecule, no longer able to disassociate from the O2 and therefore creating the H+ ions used to determine acidity. As O2 goes up, so does the pH in this system Conversely, as pH goes down, CO2 goes down.

Consultants: Dr. Dave Karl Principal Investigator/Research Mentor University of Hawaii dkarl@soest.hawaii.edu

Chris Carillo University of Hawaii

Biological Oceanographic Processes 3rd Edition; T.R. Parsons, M. Takashi, B. Hargrave; Butterworth-Heineman, 1995.

Student Reproducible Masters

Secret Agents of Dissolved Oxygen - Part I
Group Names ______________________________________________________


  1. Place your six bottles in the water bath, making sure each of the uncovered bottles has equal access to light.
  2. Wait at least 24 hours.
  3. Open ONE bottle at a time.
  4. Take measurements for temperature, pH, carbon dioxide, and dissolved oxygen and record data in table below.
  5. Get data from other groups or classes from teacher.
BOTTLE           1            2           3           4           5           6
             T PH CD DO   T PH CD DO  T PH CD DO  T PH CD DO  T PH CD DO  T PH CD DO

Trial 1

Trial 2

Trial 3

Trial 4

CD = Carbon Dioxide DO = Dissolved Oxygen

Average each bottle numbers trial value.

Graph the above data using color codes for each bottles data for initial and final value

1. What happened to the dissolved oxygen in each of the bottles?
2. What happened to the pH in each of the bottles?

1. Did the presence of light affect the DO? 2. Did the removal of light affect the DO? 3. Was the pH affected in either the uncovered or covered bottles? What happened to the pH and DO in bottles 5 & 6 versus bottles 1 & 3?

SM 1

Secret Agents of Dissolved Oxygen - Part II
Group Names ______________________________________________________

In the first activity, you tested water collected by your teacher that came from various sources. You tested this water for dissolved oxygen, pH, and temperature to determine the impact that natural processes, such as photosynthesis and respiration, had on the water chemistry.

You are to design an experiment that will determine if the time of day or the amount of time exposed to sunlight affect the amount of dissolved oxygen, CO2 and/or the pH of the water. Decide first what the overall plan is that will test only the one thing you want to be tested. Then select your materials and write up your procedure in a clear step-by step format. Have your plan approved by the teacher before proceeding any further. The teacher will let you know if you are to implement the test.

SM 2

We look forward to hearing from you! Please review this activity.

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