Ecologists fear floodwaters will dramatically increase “dead zone” in Gulf of Mexico

Mississippi River floodwaters from heavy rains in May and June have scientists fearing a dramatically increased dead zone in the Gulf of Mexico, covering an area the size of Maryland.

Before the flooding, scientists had predicted that the Gulf this summer would see its largest-ever dead zone — more than 10,000 square miles. Now its likely to be even bigger.

Excess nutrients, especially nitrates from farm fertilizer runoff, cause algae blooms. After the algae dies, bacteria on the bottom feast on the remains, removing crucial oxygen from the water. The dead zone in the Gulf of Mexico forms in early summer and lasts through early fall.

The trillions of gallons of floodwater also help trap the oxygen-depleted water near the gulf floor. The fresh water, which stays at the surface because it is less dense, forms a physical barrier that keeps oxygen in the air from mixing with the water covering the dead zone.