Dissolved Oxygen Maslyn, AP Biology

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Dissolved Oxygen is Awesome!!

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?

That’s a LOT of Energy!!

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.

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

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.

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 0.536 milligrams of carbon being assimilated.

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

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

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

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

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

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?