Final answer:
With increased levels of oxidizable materials, the biochemical oxygen demand (BOD) will increase. BOD reflects the amount of oxygen consumed by bacteria decomposing organic matter, and higher levels of organic matter lead to a higher demand for oxygen.
Step-by-step explanation:
When there are increased levels of oxidizable materials, such as organic matter from sources like sewage, grass, and leaves, the biochemical oxygen demand (BOD) will increase. This is because BOD measures the amount of oxygen required by aerobic bacteria to decompose the organic matter in the water. With more organic matter present, more oxygen is consumed in the process, leading to an increased BOD. High BOD is indicative of a water body that is polluted, as seen in stagnant urban lakes with limited circulation, leading to oxygen depletion. In contrast, water bodies with low BOD, like turbulent rivers through natural forests, have better oxygen restoration due to continual contact with the atmosphere.