Dead zones are areas of water with extremely low dissolved oxygen where aquatic life cannot survive.

• A. Dead Zones
• B. Eutrophication
• C. Biochemical Oxygen Demand
• D. Sewage Treatment (Primary)

Answer: (A)

Areas with extremely low dissolved oxygen that cannot support aquatic life are called dead zones. This happens when nutrient pollution fuels algal blooms; as these blooms die and decompose, bacteria consume large amounts of oxygen, creating hypoxic conditions that most aquatic organisms can’t endure.

Eutrophication describes the nutrient-driven enrichment process that leads to blooms and oxygen depletion, but it refers to the overall process rather than the specific low-oxygen area itself. Biochemical Oxygen Demand measures how much oxygen will be consumed by microbial decomposition of organic matter, so it’s a metric, not the actual dead zone. Sewage treatment (primary) is a treatment stage that removes solids and is not about the dissolved-oxygen-depleted areas in natural waters.

So, the described situation is best identified as a dead zone.