What defines the Gulf of Mexico Dead Zone?

• A. Coral reef decline
• B. Increased fish populations
• C. Oxygen levels rise
• D. Hypoxic conditions due to nutrient runoff

Answer: (D)

The main idea here is that this area is defined by low oxygen in coastal waters, a condition called hypoxia, triggered by nutrient pollution from land runoff. When fertilizers and other nutrients flow into the Gulf, they fuel large phytoplankton blooms. When these blooms die, their organic matter is decomposed by bacteria, which consumes a lot of dissolved oxygen in the water. The result is a band of water with oxygen levels too low to support many marine organisms, creating a “dead zone.” This pattern is tied to the Mississippi River watershed and tends to occur seasonally, especially in warmer months.

That’s why the option describing hypoxic conditions due to nutrient runoff best fits. It’s not about coral reef decline, which is a different issue, nor about increased fish populations or rising oxygen—those would contradict what defines the dead zone.