A dead zone is defined as what?

• A. A zone with moderate algae
• B. A zone with high oxygen
• C. An area of water with too little oxygen for most aquatic life to survive
• D. A zone with normal oxygen

Answer: (C)

The main idea here is oxygen depletion in water. A dead zone is an area where dissolved oxygen is so low that most aquatic life cannot survive. This often happens when nutrient pollution fuels algal blooms; when the algae die, bacteria break them down and use up a lot of oxygen in the process. If the water also becomes stratified and doesn’t mix, the oxygen from the surface can’t replenish the bottom layers, so the deeper water becomes hypoxic. Oxygen levels fall below what many species need (often very low, like under about 2 mg/L, though exact thresholds vary), causing fish and many invertebrates to die or leave. The other statements don’t fit because moderate algae describes a bloom but not the associated severe oxygen loss; high or normal oxygen would support life, not create a dead zone.