Dissolved oxygen refers to?

• A. Oxygen dissolved in air
• B. Oxygen dissolved in water that aquatic organisms need to survive
• C. Oxygen bound in minerals
• D. Oxygen produced by chemical reaction

Answer: (B)

Dissolved oxygen is the amount of oxygen gas that is physically dissolved in water and is available for aquatic life to breathe. It’s typically reported as mg/L or percent saturation and is influenced by temperature, salinity, and water movement. Oxygen enters water mainly from diffusion from the air and from photosynthesis by aquatic plants, while it is consumed by respiration, decomposition, and chemical reactions. If oxygen levels get too low, fish and other organisms can be stressed or die, making DO a key measure of water quality.

This description matches the idea of oxygen dissolved in water that aquatic organisms need to survive. Oxygen dissolved in air is not the same concept, oxygen bound in minerals isn’t readily available to life, and oxygen produced by a chemical reaction refers to a process rather than the amount dissolved in water.