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Dissolved Oxygen and Aquatic Net Primary Productivity data | hook | main | background & resources | student Background 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 Resources and Reference Materials Back to: TEA Activities Page |