Final answer:
The amount of dissolved oxygen in water commonly decreases due to an increase in organic matter, which leads to more oxygen-consuming bacteria, and also decreases with higher water temperatures, resulting in hypoxic conditions.
Step-by-step explanation:
The amount of dissolved oxygen in water may decrease due to increase in organic matter in the water. When there is an increase in organic material, such as from dead plants and animals, the number of bacteria that decompose this material also increases. These bacteria use oxygen for the decomposition process, thus reducing the oxygen available for other organisms. Furthermore, the solubility of oxygen is inversely related to the temperature of the water. As water temperature increases, it can hold less dissolved oxygen, leading to conditions that may be hypoxic (low oxygen) or even anoxic (no oxygen), with serious implications for aquatic life.