Explain the connections between oxygen sag and biological oxygen demand (BOD). Use a drawing in your explanation and use appropriate labels.
What will be an ideal response?
BOD is one way to measure the amount of bacteria in a water sample. Organic wastes, including sewage, paper pulp, or food, are very rich in certain nutrients-especially nitrogen and phosphorus. Once in aquatic ecosystems, these nutrients stem the growth of decomposing bacteria. These bacteria consume large amounts of oxygen. The BOD can give us an approximation of the amount of organic waste in the water based on the oxygen content. The effects of oxygen-demanding wastes on bodies of water depend on the volume, flow, and temperature of the water. In general, a characteristic decline in oxygen is seen downstream from a point source (e.g., discharge pipe). This is called the oxygen sag. Farther downstream, levels of oxygen will eventually return to normal. A diagram similar to Fig. 18.7 on p. 402 in the text (showing the relationship between BOD and oxygen sag) should be drawn with appropriate labels. Zones, oxygen levels, and types of organisms may be included in this diagram.
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