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Dissolved Oxygen and Aquatic Net Primary Productivity data | hook | main | background & resources | student Hook Oxygen is a requirement to sustaining most life forms. Land plants and animals take in oxygen from air and use it in the process of cellular respiration to provide energy for life functions. In the watershed, organisms living in the streams and ponds must take their oxygen from the water. Oxygen must be in solution in a free state (O2) before it can be used by organisms. The concentration and distribution of O2 is directly dependent on chemical and physical factors including such things as temperature, currents, wind, pH and salinity. Spatial distribution of O2 in water can be highly variable due to absence of mixing by currents, wind, tide or natural flows. 1 In our atmosphere there is about 200 mls of O2 in every liter of air. In aquatic environments, there is about 5 – 10 mls of dissolved oxygen (DO) in a liter of water. This is 20 – 40 times less than air. 2 Therefore the measurement of DO in aquatic ecosystems can be an important indicator of water quality. Because we live in air, it is hard to understand the variations that influence availability of oxygen in water. Oxygen diffused 300,000 times faster in air at 20o C than in water. Thus O2 is more uniform in the air we breathe. In a watershed, the oxygen available is influenced by the temperature variability through out the stream, the pH changes, the riffles which mix the water with air, and the surface area exposed to air. Also the partial pressure of oxygen in the air above the water affects the DO in water. Therefore watersheds at higher elevations with less oxygen in the air will have less DO in the water. At 4,000 meters in elevation (about 13,000 feet) the amount of DO is less than two-thirds of that at sea level. 2 Biological processes such as respiration and photosynthesis can significantly affect O2 concentrations. Photosynthesis usually increases the DO in water. Respiration that requires oxygen will decrease the DO. Measuring the DO concentration of a body of water is often used to determine whether biological activities requiring oxygen are occurring and therefore are an important indicator of pollution.2 DO concentration is considered an important indicator of organic pollution (sewage, algae blooms, etc.). For example, bacteria consume organic pollutants and consume excessive amounts of oxygen in the process, thus lowering DO levels in the water. Productivity is seen as a measure of how much photosynthesis occurs by the plants living in the water at any given time. The primary productivity of an ecosystem is defined as the rate at which sunlight is stored by plants in the form of organic compounds. Only those organisms possessing the pigment chlorophyll can use solar energy to make new organic compounds. The following is a basic equation for photosynthesis: 6CO2 + 6H2 O -------ý C6H12O6 + 6O2 We can use a measure of oxygen production over time as a basis for measuring primary productivity. We can calculate the amount of carbon that has been bound in organic compounds because for each milliliter of oxygen produced, approximately 0.536 milligrams of carbon has been captured.2 We will use the light and dark bottle method to measure the rate of oxygen production. We will measure the DO at the time of sampling. We will also collect two more water samples at the same time. These two bottles will be left for 24 hours, one in light and one in dark, then tested for DO. The difference between the initial and the dark bottle is a measure of the oxygen consumed by organisms in respiration in the water. The difference between the initial and the bottle in the light is a measure of both photosynthesis and respiration. The change in DO in this bottle is a measure of net productivity. The difference between the DO concentration in the light bottle and the dark bottle is the total oxygen production. This is an estimate of the gross productivity. _____________________________________________________________ 1. From "Dissolved Oxygen" by Valerie C. Chase in Carolina Tips, 5/1/88 2. Biology Lab Manual, College Entrance Examination Board, 19 Procedure 1. Fill three bottles with stream water. Make sure you allow the water to flow into the bottle under the surface of the stream. Fill the bottles to the top with no air spaces left. 2. Take the temperature of the stream water. 3. Fill the DO bottle with the water to be tested. Allow the water to overflow the bottle for two or three minutes. Avoid trapping air in the bottle by angling it slightly and quickly pushing the glass stopper in place. If bubbles become trapped in the bottle in this step or step 2 or 4, discard and start again. 4. Use clippers to open Dissolved Oxygen 1Reagent Powder Pillow and 1 Dissolved Oxygen 2Reagent Powder Pillow. Add the contents of each pillow to the bottle. Stopper the bottle carefully to exclude air bubbles. Grip the bottle and stopper firmly; shake vigorously to mix. A flocculant (floc) precipitate will be formed. If oxygen is present in the sample the precipitate will be brownish yellow in color. This will not affect the test results. 5. Allow the sample to stand until the floc has settled halfway in the bottle, leaving the upper half of the sample clear. Shake the bottle again. Again let it stand until the upper half of the sample is clear. Note the floc will not settle in samples with high concentrations of chloride, such as sea water. No interference with the test results will occur as long as the sample is allowed to stand four or five minutes. 6. Use the clippers to open one Dissolved Oxygen 3 Reagent Powder Pillow. Remove the stopper from the bottle and add the contents of the pillow. Carefully re-stopper the bottle and shake to mix. The floc will disappear and a yellow color will develop if oxygen is present. 7. Fill the plastic measuring tube level full of the sample prepared in the preceding steps. Pour the sample into the square mixing bottle. 8. Add Sodium Thiosulfate Standard Solution drop by drop to the mixing bottle, swirling to mix after each drop. Hold the dropper vertically above the bottle and count each drop as it is added. Continue to add drops until the sample changes from yellow to colorless. 9. Each drop used to bring the color change in step 8 is equal to 1mg/L of dissolved oxygen (DO) DAY ONE To determine aquatic primary productivity: 1. Do a DO test on the first sample immediately after the sample is collected. Record the results ______mg/L 2. Of the remaining two samples, label one DARK. Cover the DARK bottle with aluminum foil so that no light can get in. In this bottle no photosynthesis can occur. The only change in DO will result from respiration from all of the organisms in the sample. 3. Label the third bottle the LIGHT bottle. 4. Back at school place the two bottles in a location where they will be exposed to the natural daylight Leave the sample bottles for 24 hours. DAY TWO 1. Determine the DO on the two bottles that have been exposed to natural light. Record your results. 2. DARK bottle _____________mg/L LIGHT bottle ______________mg/L 3. Use the following formula to calculate respiration rate. I = Initial Bottle L – I = Net Productivity L = LIGHT Bottle I – D = Respiration D = DARK Bottle L – D = Gross Productivity 3. Richard J Patterson, Athens Academy, P.O. Box 6548, Athens, GA 4. Hach test kit for Dissolved Oxygen , model OX-2P, Cat No. 1469-00 5. Barb Schulz, Lakeside Upper School, Seattle, WA 98125 6. Project Green Data table:______________ Date______________
Determine the amount of carbon fixed using the constant .536 grams of carbon fixed per ml of dissolved oxygen. Discussions Questions/Extensions ...... Back to: TEA Activities Page data | hook | main | background & resources | student |