Hypoxia in aquatic ecosystems is defined as dissolved oxygen levels dropping below what value?

• A. Below 2 mg/L.
• B. Below 3 mg/L.
• C. Below 5 mg/L.
• D. Below 7 mg/L.

Answer: (B)

Hypoxia means dissolved oxygen is too low to support normal aquatic life. In many freshwater guidelines, a practical cutoff for hypoxia is when DO falls below about 3 mg/L. At or below this level, fish and other aquatic organisms experience stress, slowed metabolism, and reduced activity, and the risk of mortality increases if conditions persist, especially in warmer water. Levels above 5 or 7 mg/L easily sustain most aquatic life, so they’re not considered hypoxic. While very low DO (around 2 mg/L or less) is often labeled severe hypoxia, the threshold used in this context is just under 3 mg/L, which is why this option is the best choice.