1. Dissolved Oxygen
Maslyn, AP Biology

2. Dissolved Oxygen: D.O. significance Why measure DO?
Determining DO: Winkler Method
Winkler Method: Chemistry
A lot of different methods for doing DO

3. Dissolved Oxygen
DO is used to indicate how healthy a body of water such as a lake or stream is
The higher the DO level, the lower the pollution and the higher the productivity of the body of water
DO is also used to measure the amount of biomass a freshwater system can take

4. Dissolved Oxygen Significance
To ensure the life span of fish and aquatic life in salt and fresh water
Helps aerobic decomposition of organic matter in polluted waters

5. Why we measure DO Real world application:
Water treatment plants: measure DO to make sure the water is safe.
i.e. monitor aerobic waste and prevent corrosion in treatment systems

6. Effects on DO
The lower the partial pressure of oxygen on the outside of the water, the lower the DO in the water
Organisms such as trout and perch fish need at least 8 mg/L to survive
Below 2 mg/L DO, only sludge worms and mosquito larvae survive
Surfactant- soap like substance on avioli in lungs
Drive out dissolved oxygen- heat the water or increase the pressure

7. How do we measure DO? The Winkler Method
The Winkler Method uses titration to measure dissolved oxygen in water samples

8. The Winkler Method
The dissolved oxygen of a sample of water is “fixed” by added a series of reagents
Manganous Sulfate, Alkaline Iodide-Azide, and Sulfamic Acid
The reagents form an acid compound that is titrated with a neutralizing compound (Starch) that results in a color change
The color change is the end point which coincides with the DO in the sample

9. The Winkler Method Based on:
Reactions: releasing iodine= oxygen present in the sample
Titration of liberated iodine

10. Winkler Method Determine the presence of O2
Oxidize iodide until it is a pale straw color
Change color to blue
Titrate until the solution is colorless
Note the final reading

11. Winkler Method: Step 1 Add Manganous Sulfate & alkali iodide reagents
If no oxygen is present:
Mn++ + 2OH- = Mn(OH)2
White Manganese hydroxide
If oxygen is present:
Mn++ + 2OH- + 1/2O2= MnO2 + H2O
Brown Manganese Dioxide

12. Winkler Method: Step 2 Add sulfamic acid.
MnO2 + 4H+ +2I- = Mn++ + I2 + 2H2O
Iodine is formed by oxidation of iodide

13. Winkler Method: Step 3
Titrate liberated iodine with sodium thio-sulfate standard solution
2 Na2S2O3 + I2 = Na2 + S4 + 2 Nal
The color changes to a pale straw colored yellow

14. Winkler Method
Step 4: Add Starch indicator- color changes to dark blue/black
Step 5: Continue titration until solution becomes colorless
Step 6: Note the final reading is the dissolved oxygen

15. Dissolved Oxygen Summary
Significance: monitor normal aquatic life
Solubility of Oxygen depends on partial pressure, temperature, and salinity of water
Liberated iodine equal to DO
Use a graph in lab handout to find saturation

16. Dissolved Oxygen is Awesome!!

17. Food for thought 
Only a small fraction of the sunlight that reaches the Earth’s surface is actually used in photosynthesis.
Much of the radiation hits things that do not photosynthesize and if they do hit photosynthetic organisms, only certain wavelengths are captured by the pigments
Knowing this, how much energy in the form of organic compounds do you think is produced by the Earth’s primary producers each year?

18. That’s a LOT of Energy!!

19. Primary Production
Primary Production- the amount of light energy converted to chemical energy by autotrophs during a given time period
These photosynthetic products are the starting point for most studies of ecosystem metabolism and energy flow.
Consumers require their organic fuels second or third hand through food.

20. Primary Productivity
Unlike primary production, Primary productivity is defined as the rate at which organic materials are used or stored.
Primary production= amount
Primary productivity= rate

21. Photosynthesis role
Green plants obtain Carbon for carbohydrate synthesis from the carbon dioxide in the water or the air according to photosynthesis:
6CO2 + 6H2O  C6H12O6 + 6O2
The rate of carbon dioxide ultilization, the rate of formation of organic compounds, or the rate of oxygen production can be used as a basis for measuring primary productivity.

22. Oxygen Production
The measure of oxygen production over time provides a means of calculating the amount of carbon that has been bound in organic compounds over a period of time.
Each milliliter of oxygen produced is approximately milligrams of carbon being assimilated.

23. How do we measure O2 Production?
Light and Dark Bottle Method
DO concentrations of water samples are compared before or after incubation in light or darkness.
The difference between the measurements of DO in the initial and dark bottles is an indication of the amount of O2 being consumed in respiration by the organisms in the bottle

24. Gross vs. Net Primary Production
Gross Primary Production (GPP)- the amount of energy from light converted to the chemical energy of organic molecules per unit time
Net Primary Production (NPP)- gross primary production minus the energy used by the primary producers for their autotrophic respiration (Ra)
NPP= GPP – Ra
To ecologists, NPP is key because it represents the storage of chemical energy that will be available to consumers in an ecosystem

25. Net Productivity
In the bottles exposed to light, photosynthesis and respiration are occuring
Net Productivity- The change over time in DO concentration from the initial concentrations

26. How does this apply to life?
Think- Pair- Share Activity 
At what time of day would gross productivity be the highest in a eutrophic lake?
Justify your answer

27. Summary
DO can be a measure of productivity and health of a body of water
The higher the DO, the higher the productivity of a body of water
Photosynthesis raises the amount of organic compounds being reduced therefore raising the DO levels

28. Exit Slip
Knowing what you know about photosynthesis, DO levels and productivity, predict what the graph of productivity levels may look like on a cloudy day
Justify your answer
How would it compare to a sunny day?
What factors are involved?