Dissolved Oxygen Investigation
I.B. Environmental Systems and Societies - Year 1
Your Name
Your Teacher
Period
Date
Question
How does the absence of light affect the concentration of dissolved oxygen in a closed aquatic system
containing algae?
Background
Oxygen is readily available in terrestrial ecosystems, but dissolved oxygen is scarce in aquatic
ecosystems. One source of dissolved oxygen is aquatic plants and algae. In the presence of light, aquatic
plants and algae carry out photosynthesis, resulting in an output of dissolved oxygen, as seen in the
following equation:
6 H2O + 6 CO2 + light energy  C6H12O6 + 6 O2
Since all organisms must carry out respiration in order to produce energy, dissolved oxygen is needed by
all aquatic and marine organisms. In the absence of light, aquatic plants and algae are not able to carry
out the process of photosynthesis, but are still able to respire, as shown in the equation:
C6H12O6 + 6 O2  6 H2O + 6 CO2 + energy
In aquatic ecosystems, if more photosynthesis than respiration is taking place, there will be an increase in
dissolved oxygen. If there is more respiration than photosynthesis, there will be a decrease in dissolved
oxygen.
Hypothesis
In a simulated closed aquatic ecosystem consisting of a bottle of water with algae, it is expected that as
light is removed, dissolved oxygen will decrease. This is due to the fact that as light decreases,
photosynthesis will also decrease, while respiration is still taking place.
Variables
Independent variable
Presence of light
Method of control
There will be 2 groups tested. The control group will consist of water
containing algae placed in clear test tubes, through which light can penetrate.
The light to be used will be from 60w light bulbs, which will be left on for 24
hours. The other group will consist of water containing algae placed in clear
test tubes, which will be covered in aluminum foil through which light cannot
penetrate.
Dependent variable
Dissolved oxygen
concentration
Method of control
The initial dissolved oxygen concentration will be measured in a single test
tube with water from the same source as all the other test tubes. The final
dissolved Oxygen will be measured from all 5 light and 5 dark test tubes at the
end of the 24 hours
Controlled variable
Test tubes
Volume of water
Type of water
Time
Method of control
All test tubes will be the same size (20mL)
Each test tube will be filled to the top.
Each test tube comes from the same source of pond water.
Each sample will be measured for dissolved oxygen after 24 hours.
Uncontrolled variable
Temperature
Method of minimizing influence
Although it will not be possible to maintain a constant temperature, all samples
will be stored in the same room for 24 hours so that all samples are exposed to
the same temperature variations.
Materials
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5 L pond water containing algae for one source
11 – 20mL test tubes
10 rubber stoppers
Aluminum foil
light source with 60w bulb
Vernier Dissolved Oxygen Probe (0 to 15 mg/L)
Vernier LabQuest 2
Method for collection of data
1. Cover five test tubes with a layer of aluminum foil so that no light may enter the test tubes. These
will be the dark test tubes. The remaining test tubes will be light test tubes.
2. Label each light test tube, 1-5 and each dark test tube, 1-5.
3. Calibrate the Vernier Dissolved Oxygen (DO) probe.
4. Fill 1 – 20 mL test tube with pond water to the top.
5. Using the calibrated DO sensor, measure the DO concentration in just one test tube. Be sure to
rinse the DO probe with distilled water in between measuring each sample.
6. This test may be rinsed and put aside
7. Fill each of the remaining 10 – 20 mL test tubes with pond water to the top and cap with rubber
stopper.
8. If necessary, add aluminum foil to the top of the dark bottles.
9. Store the bottles an equal distance from light source for 24 hours.
10. After 24 hours, use the Vernier DO probe to measure the DO concentration in each bottle.
11. (NOTE FROM YOUR TEACHERS: If this was YOUR procedure, you would need to list the steps you
would take to process your data)