REASONS FOR NATURAL VARIATIONS IN DISSOLVED OXYGEN LEVELS

Transcription

1 Period Date LAB. THE PHYSICAL PROPERTIES OF WATER: DISSOLVED OXYGEN In an aquatic environment, oxygen must be in a solution in a free state (O 2 ) before it is available for use by organisms (bioavailable). Its concentration and distribution in the aquatic environment are directly dependent on chemical and physical factors and are greatly affected by biological processes. In the atmosphere, there is an abundance of oxygen, with about 200mL of oxygen/1l air. In an aquatic environment, there are about 5-10mL O 2 /1L water. The concentration of the oxygen in aquatic environments is a very important component of water quality. Like terrestrial animals, fish and other aquatic organisms need oxygen to live. As water moves past their gills (or other breathing apparatus), microscopic bubbles of oxygen gas in the water, called dissolved oxygen (DO), diffuse from the water to their blood. Like any other gas diffusion process, the transfer is efficient only above certain concentrations. In other words, oxygen can be present in the water, but at too low a concentration to sustain aquatic life. Oxygen also is needed by virtually all algae and all aquatic plants, and for many chemical reactions that are important to lake functioning. REASONS FOR NATURAL VARIATIONS IN DISSOLVED OXYGEN LEVELS A physical process that affects DO concentrations is the relationship between water temperature and gas saturation. Cold water can hold more of any gas, in this case oxygen, than warmer water. Warmer water becomes "saturated" more easily with oxygen. As water becomes warmer it can hold less and less DO. So, during the summer months in the warmer top portion of a lake, the total amount of oxygen present may be limited by temperature. If the water becomes too warm, even if 100% saturated, O 2 levels may be suboptimal for many species of trout. This is the factor that we will investigate in this lab. Other abiotic sources of oxygen include the air and inflowing streams. Oxygen concentrations are much higher in air, which is about 21% oxygen, than in water, which is a tiny fraction of 1 percent oxygen. Where the air and water meet, this tremendous difference in concentration causes oxygen molecules in the air to dissolve into the water. More oxygen dissolves into water when wind stirs the water; as the waves create more surface area, more diffusion can occur. A similar process happens when you add sugar to a cup of coffee the sugar dissolves. It dissolves more quickly, however, when you stir the coffee. Dissolved oxygen concentrations may change dramatically with lake depth. Oxygen production occurs in the top portion of a lake, where sunlight drives the engines of photosynthesis. Oxygen consumption is greatest near the bottom of a lake, where sunken organic matter accumulates and decomposes. In deeper, stratified, lakes, this difference may be dramatic plenty of oxygen near the top but practically none near the bottom. If the lake is shallow and easily mixed by wind, the DO concentration may be fairly consistent throughout the water column as long as it is windy. When calm, a pronounced decline with depth may be observed. The biotic source of oxygen is photosynthesis. Oxygen is produced during photosynthesis and consumed during respiration and decomposition. Because it requires light, photosynthesis occurs only during daylight hours. Respiration and decomposition, on the other hand, occur 24 hours a day. This difference alone can account for large daily variations in DO concentrations. During the night, when photosynthesis cannot counterbalance the loss of oxygen through respiration and decomposition, DO concentration may steadily decline. It is lowest just before dawn, when photosynthesis resumes. We will explore this interaction later in the year once we study both cellular respiration and photosynthesis. 1 of 6

2 Seasonal changes also affect dissolved oxygen concentrations and bring a number of these factors to bear. Warmer temperatures during summer speed up the rates of photosynthesis and decomposition. When all the plants die at the end of the growing season, their decomposition results in heavy oxygen consumption. Other seasonal events, such as changes in lake water levels, volume of inflows and outflows, and presence of ice cover, also cause natural variation in DO concentrations. PROCEDURE Safety: Wear personal protection equipment: nitrile rubber gloves and chemical safety goggles. 1. Your instructor will assign one or more water temperatures for your sample: 0, 20, or 30 C. You may want to verify the temperature of your sample to ensure that it has reached, and remains at, the desired temperature. 2. Label three BOD sampling bottles. 3. Fill each sampling bottle when you are ready to test it. The most important aspects of this process are (1) not to trap any air in the bottle and (2) to avoid turbulence which will introduce more air into the sample and falsely increase the DO levels. Your instructor will guide you on the best method to achieve this. 4. Seal the bottle with a cap so that no air pockets are created and excess water is removed. For the rest of this procedure have the bottle on white paper so it is easy to see color changes. 5. Uncap the bottle. Add eight drops of manganous sulfate to the sample bottle. Be sure no air is added. 6. Add eight drops of alkaline potassium iodide azide to the sample bottle. Be sure no air is added. Note that the precipitate manganous hydroxide is produced immediately. 7. Cap the bottle and mix by inverting it several times. 8. Set the bottle on the lab bench and allow the manganous hydroxide precipitate to settle until it is below the shoulder of the bottle. 9. Carefully add one gram scoop of sulfamic acid to the sample bottle 10. Cap the bottle and mix by inverting the bottle several times until the precipitate completely dissolves. The sample should turn a clear yellow as free iodine is formed. The sample is now fixed and can be stored while you prepare your next sample. 2 of 6

3 11. After fixing the oxygen in each bottle, you will determine the amount of dissolved oxygen in each sample. Carefully measure out 20ml of a sample into a titration tube. Be accurate! Variations in filling from group to group and from bottle to bottle will result in inconsistent data. 12. Titrate with sodium thiosulfate into titration tube one drop at a time. You must accurately measure volume of sodium thiosulfate added to the solution on the syringe! Swirl the sample after each drop until the sample becomes a faint yellow color. 13. Add eight drops of starch indicatore (Lugol s iodine) to the 20ml sample. The starch indicator will change the solutions's color from yellow to purple. 14. Now continue the sodium thiosulfate titration. Add one drop at a time and swirl between each drop until the blue color disappears. This is the titration endpoint, when all free iodine has been converted to sodium iodide by the addition of sodium thiosulfate and where you should stop. You must now record the total amount of sodium thiosulfate added to the solution (total of both before and after adding starch indicator. Total volume of sodium thiosulfate added to your vial: Convert volume of sodium thiosulfate to ppm O 2 (.56 ml = 5.6 ppm); 15. Repeat these titration steps for each sample. 16. You will be provided with a nomograph for oxygen saturation. Use a straight edge or ruler to determine the percent DO in each of your temperature samples. (To use a nomograph, you line up the edge of a ruler with the temperature of the water on the top scale and the ppm DO on the bottom scale, and read the percent saturation on the middle scale.) Figure 1. The effect of temperature on dissolved oxygen in three different tap water samples. Temperature Lab Group DO Class Mean DO Lab Group % DO saturation Class Mean % DO Saturation 3 of 6

4 17. Graph both the lab group data and class means for percent saturation as a function of temperature. Please attach your graph to the end of this lab. Make sure it is properly labeled and neatly done. NOMOGRAPH OF OXYGEN SATURATION DETERMINING PERCENT SATURATION THE "QUICK AND EASY" METHOD For a quick and easy determination of the percent saturation value for dissolved oxygen at a given temperature, use the saturation chart above. Pair up the ppm of dissolved oxygen you measured and the temperature of the water in degrees C. Draw a straight line between the water temperature and the mg/l of dissolved oxygen. The percent saturation is the value where the line intercepts the saturation scale. Streams with a saturation value of 90% or above are considered healthy. 4 of 6

5 SUMMARY QUESTIONS 1. What is the relationship between temperature and solubility of oxygen in water? 2. Make an inference as to what causes this relationship. 3. Why is the amount of dissolved oxygen in water biologically important? 4. Trout are predatory fish and they are fast swimmers. Why are trout only found in the coldest streams? 5. A mammal uses only 1 to 2 percent of its energy in ventilation (breathing air in and out) while a fish must spend about 15 percent of its energy to move water over its gills. Explain this huge difference in their efforts to collect oxygen, i.e., why is it necessary for the fish to invest so much more energy in the oxygen acquisition process? 5 of 6

6 6. Would you expect the DO in water taken from a stream entering a lake to be higher or lower than the DO taken from the lake itself? Explain. 7. In the fall, would you expect the DO concentrations of water samples taken from a lake at 7:00 am to be higher or lower than samples taken at 5:00 pm? Explain. 8. Why do nuclear power plants occasionally pose a threat to some aquatic life? (any answer relating to the release of radioactive materials is not acceptable). Hint: how do power plants dissipate energy from nuclear reactions? 9. In this lab, we tested the DO content in freshwater, but much of the world s aquatic life lives in the ocean. Wouldn t it be interesting to investigate the effect of salinity on dissolved oxygen levels. Design an experiment: 6 of 6