The oxygen sag curve is best described as?

• A. A graph showing dissolved oxygen dropping after organic waste enters water, then recovering downstream.
• B. The amount of oxygen consumed by plants during photosynthesis.
• C. A measure of how cold the water is.
• D. The oxygen sag curve describes a graph showing dissolved oxygen dropping after organic waste enters water, then recovering downstream.

Answer: (D)

An oxygen sag curve shows how dissolved oxygen changes downstream after organic pollution enters water. When organic matter is introduced, microbes decompose it and consume oxygen, causing the dissolved oxygen level to fall downstream where the load is greatest. As the polluted water mixes and the waste is diluted, natural reaeration from the air and ongoing dilution gradually restore oxygen levels, so the curve dips and then recovers downstream. This dynamic between biological oxygen demand and reaeration/dilution is what the sag curve represents. It’s not about photosynthesis or temperature alone—photosynthesis would add oxygen, and temperature affects DO solubility but doesn’t describe the sag pattern itself. So the best description is a graph showing dissolved oxygen dropping after organic waste enters water, then recovering downstream.