What factor primarily affects the oxygen levels in water?

The number of microorganisms consuming organic waste is a primary factor that affects the oxygen levels in water. When organic waste decomposes, microorganisms break it down, a process known as decomposition. This process requires oxygen, which is drawn from the water, leading to a decrease in dissolved oxygen levels. If the decomposition is rapid and the demand for oxygen exceeds the supply, it can result in low levels of dissolved oxygen, which is detrimental to aquatic life.

Additionally, an abundance of microorganisms, often due to nutrient enrichment from runoff, can lead to increased oxygen consumption as they decompose excess organic matter. This relationship is critical because oxygen is essential for the survival of fish and other aquatic organisms, and lowered oxygen levels can cause stress or death to these organisms.