Low dissolved oxygen levels in aquatic systems can lead to which ecological condition?

• A. Hypoxia
• B. Hyperoxia
• C. Acidity
• D. Salinity increase

Answer: (A)

Dissolved oxygen is what aquatic animals use to breathe. When oxygen is depleted faster than it is replenished, the water becomes hypoxic, meaning there isn’t enough oxygen to support normal life processes. This often happens in warm or stagnant waters or in systems polluted with nutrients, where algal blooms grow and then die; the decomposition of that biomass by microbes uses up large amounts of oxygen, especially in deeper or darker zones, leaving bottom waters with very little oxygen. Under hypoxia, fish, invertebrates, and other aerobic organisms stress or die, and ecosystems can develop “dead zones” where few oxygen-loving species can survive. The other conditions listed—too much oxygen (hyperoxia), increased acidity (lower pH), and higher salinity—aren’t caused by low dissolved oxygen, so hypoxia best describes the ecological consequence.