What is a dead zone?

"Dead zone" is a more common term for hypoxia, which refers to a reduced level of oxygen in the water.

animation of nutrient runoff into Gulf of Mexico

Top panel: At 6,952 square miles, the 2019 hypoxic zone in the Gulf of Mexico is the 8th largest ever measured in the 33-year record. The red area denotes two milligrams per liter of oxygen or lower, the level which is considered hypoxic, at the bottom of the seafloor. Bottom panel: The long-term measured size of the hypoxic zone, indicated with green bars, measured during ship surveys since 1985; black dashed lines indicate the target goal established by the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force and the five-year average measured size of the zone. Graphic credit: Louisiana Universities Marine Consortium.

Less oxygen dissolved in the water is often referred to as a “dead zone” because most marine life either dies, or, if they are mobile such as fish, leave the area. Habitats that would normally be teeming with life become, essentially, biological deserts.

Hypoxic zones can occur naturally, but scientists are concerned about the areas created or enhanced by human activity. There are many physical, chemical, and biological factors that combine to create dead zones, but nutrient pollution is the primary cause of those zones created by humans. Excess nutrients that run off land or are piped as wastewater into rivers and coasts can stimulate an overgrowth of algae, which then sinks and decomposes in the water. The decomposition process consumes oxygen and depletes the supply available to healthy marine life.

Dead zones occur in coastal areas around the nation and in the Great Lakes — no part of the country or the world is immune. The second largest dead zone in the world is located in the U.S., in the northern Gulf of Mexico.