What term describes areas in oceans with insufficient oxygen levels?

• A. Harmful Algal Blooms
• B. Nutrient Runoff
• C. Oxygen-Depleted Zones
• D. Raw Sewage

Answer: (C)

Areas in oceans with insufficient oxygen levels are called oxygen-depleted zones, sometimes called hypoxic zones or dead zones. These pockets form when nutrient inputs from land trigger massive algal blooms, and as those blooms die and decompose, microbial activity uses up the oxygen in the water. In many coastal areas, the water is stratified and doesn’t mix well, so the depleted oxygen isn’t replenished, making it hard for fish and other aerobic organisms to survive.

Harmful algal blooms describe blooms that produce toxins or otherwise harm the ecosystem, which is related but not the name of the low-oxygen area itself. Nutrient runoff is a cause that leads to eutrophication and subsequent deoxygenation, but it’s not the term for the zones. Raw sewage can contribute to oxygen depletion, but again it’s a source, not the description of the zones.