Dissolved Oxygen, Productivity, and B.O.D. Lab

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Maurice Baldwin
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1 Dissolved Oxygen, Productivity, and B.O.D. Lab Most living organisms, including aquatic organisms, require certain levels of oxygen to carry out normal metabolic processes. They are thus aerobic organisms. The D.O. (dissolved oxygen) of a healthy aquatic ecosystem typically ranges from about 4 to 8 ppm (mg/liter). In general, a D.O. of below 4 ppm (mg/liter) in a river or lake represents a very unhealthy situation for fish and other organisms. The maximum amount of D.O. that a given aquatic ecosystem can hold depends on atmospheric pressure and water temperature. Water that is agitated comes in contact with the air, allowing it to be saturated with oxygen. The amount of D.O. in the system can also depend on the amount of organic material present. The B.O.D. (biochemical oxygen demand) is a measure of the amount of aerobic respiration in an aquatic ecosystem. Precisely defined, the B.O.D. is the amount of oxygen (mg/liter) consumed by microorganisms in a sample of water kept at 20 C (about room temperature) over a 5-day period. Sterile water would have no B.O.D. A water sample with healthy algae, bacteria, and other microorganisms will have a moderate B.O.D. Raw (untreated) sewage usually has a B.O.D. of 100 to 200. One of the greatest challenges to the health of an aquatic community is the addition of large amounts of organic matter, such as sewage, garbage, or plant and animal wastes. Although these pollutants are highly biodegradable, (capable of being broken down by normal biological processes), a healthy aquatic ecosystem can handle only so much of them before it becomes overloaded. Organic material is oxygen demanding waste, which means that decomposer bacteria require oxygen to break it down. When a body of water becomes overloaded with oxygendemanding waste, oxygen-using bacteria can deplete the D.O. content of the water below the level needed to support the diversity of organisms characteristic of healthy ecosystems. Besides increasing B.O.D. directly, organic wastes can also indirectly raise the demand for oxygen. Organic wastes generally contain high concentrations of nitrogen and phosphorus, substances that act as limiting nutrients for plants. Because nitrogen and phosphorus are usually present in ecosystems only in very small concentrations they act as fertilizers when added to lakes and streams in larger amounts. Nitrogen and phosphorus pollution can cause unnatural blooms of algae and other aquatic plants. When these plants die and begin to decay, aquatic microorganisms consume large amounts of oxygen in the decomposition process. The resulting abrupt decrease in D.O. is called an oxygen crash and can, in turn, bring about massive fish dieoffs. Ultimately, this kind of pollution can reduce a healthy ecosystem to a smelly, virtually lifeless sewer inhabited by select bacteria or other organisms suited to anaerobic conditions. Continued on next page

2 The D.O. Test and Solubility 1. You will test a beaker of tap water with the dissolved oxygen meter. First make certain that the meter has been calibrated. Gently place the probe into the sample bottle, being certain not to agitate the water, for that motion could increase the D.O. Record the D.O. in the table. 2. Determine the percent oxygen saturation of the sample. Since the amount of oxygen that can be dissolved in water depends upon the temperature, you will compare the D.O. with the temperature to determine percent oxygen saturation using a chart called a Rawson s nomogram. Place dots on the diagram to mark the temperature and the D.O. of a sample and, using a ruler, draw a line and connect the two dots. The point at which this line crosses the % Saturation line gives you the percent saturation of your sample. 3. Place your data in the data table. On the next line add 5 o C to the temperature, and using the same DO, determine the % dissolved oxygen.

3 4. On the third line, subtract 5 o C from your temperature reading and again using the same DO, determine the % dissolved oxygen. Water Temperature D.O. % saturation Part 1 D.O. conclusions 1. What is the effect of temperature upon dissolved oxygen? 2. If a sample of hot water and cold water have the same D.O., which sample is more likely to be saturated? 3. What happens to an open can of soda that is heated up? Why? 4. Based upon your D.O. results, which aquatic ecosystem could support a greater number of animals, arctic or tropical waters. Why? Part 2: The B.O.D. Test You will perform a simplified B.O.D. test on two bottles of water by comparing the D.O. of a water sample before and after 3 days (rather than the standard 5 days).

4 1. Fill two B.O.D. bottles with your water sample. Use the oxygen meter, and determine the D.O. of the samples. There should be no more than a 0.5 ppm difference between the samples, since they come from the same source. 2. Make certain that there are no air bubbles in the B.O.D. bottle. Fill the bottle until it is overflowing, then firmly put on the stopper. Invert the bottle. If a bubble floats up, try again. Do the same for the second bottle. 3. Cover one bottle with aluminum foil, and leave the other uncovered. Place both bottles in the light of the environmental chamber. Sample: Initial D.O. Final D.O. Light Dark 1. B.O.D. can be determined by Initial D.O. - Final D.O. of dark bottle The B.O.D. reflects the amount of aerobic respiration in the sample. Usually a high B.O.D. reflects the amount of organic waste being consumed by aerobic bacteria. Algae also carry on aerobic respiration. In addition to aerobic respiration, algae can photosynthesize. A sample with a lot of algae may have a high B.O.D., yet still be very productive. Productivity is a measurement of the number of carbon atoms being fixed by photosynthesis. 2. The gross productivity can be determined by using the final D.O. of light bottle final D.O. of dark bottle. Determine the gross productivity of your samples. Your sample source B.O.D. gross productivity Class Data sample sources B.O.D. gross productivity Part 2 B.O.D. and Productivity Conclusions 1. Discuss the differences in B.O.D. of the each of the samples and suggest sources of microorganisms or oxygen-demanding wastes in the sources with a high B.O.D. 2. Discuss the differences in productivity of each of the samples and suggest reasons for these differences. 3. Does any of the data make no sense in terms of productivity or BOD? If so, what sources of error do you think are responsible? Part 3 Fecal Coliform Test 1. Using a sterile pipette, add 1mL of sample water to the fecal coliform test vial (it s filled with either pink or purple liquid).

5 Wait 24 hours. If the color changes to yellow, there are fecal coliform bacteria present in the sample.

AP ENVIRONMENTAL SCIENCE

AP ENVIRONMENTAL SCIENCE SYLLABUS COURSE DESCRIPTION COURSE DESCRIPTION AND OUTLINE OF TEST TOPICS The course is designed to acq~aint the student with the physical,ecological,social and political principle