Key Concepts Oxygen levels in Puget Sound waters affect the kinds and numbers of living plants and animals. Dissolved oxygen concentrations are affected by factors such as temperature, salinity and dissolved organic matter. While oxygen is seldom in short supply for terrestrial organisms, the same is not true for aquatic organisms. Living things and the natural processes of decomposition compete for oxygen.
Aquatic systems can decompose reasonable amounts of organic wastes without harmful effects.
Excessive amounts of sewage wastes, on the other hand, reduce oxygen levels and productivity. The B.O.D. is calculated by subtracting the dissolved oxygen concentration in the sample after incubation from the dissolved oxygen concentration before incubation. In the sealed, darkened sample, organic matter is decomposed by the bacteria normally found in the water. The bacteria require oxygen for decomposition. Thus, the more organic matter present in the water sample, the more dissolved oxygen will be consumed by the bacteria over the five-day test period.