A dead zone is an area of water with oxygen levels too low to support most marine life. Fish, crabs, and shellfish either flee or die. The technical term is hypoxia. When oxygen drops below 2 milligrams per liter, most animals cannot survive. Below 0.5, the water is essentially lifeless.

Dead zones form when nutrients flood into coastal waters. Fertilizer from farms, sewage, and animal waste feed massive algal blooms. When the algae die, bacteria decompose them and consume oxygen. The result is a suffocating layer near the bottom. Warm, still water makes it worse because oxygen does not mix down.

The number of dead zones has grown. In the 1960s, scientists knew of about 50. Today, there are over 400, covering an area larger than the United Kingdom. The Gulf of Mexico dead zone is the size of New Jersey most summers. Reducing nutrient runoff is the fix, and it requires changes in farming practices across entire watersheds.

Common causes

• Agricultural fertilizer runoff
• Sewage discharge
• Livestock waste
• Warm water temperatures
• Stratified water layers