The term dead zone refers to those areas in aquatic environments where there is a reduction in the amount of dissolved oxygen in the water. The condition is more appropriately called hypoxia or hypoxic waters or zones. Hypoxia in marine environments is determined when the dissolved oxygen content is less than 2–3 milligrams/liter. Five to eight milligrams/liter of dissolved oxygen is generally accepted as the normal level for most marine life to survive and reproduce. Dead zones can not only reduce the numbers of marine animals, but they can also change the nature of the ecosystem within the hypoxic zone.

The main cause of oxygen depletion in aquatic environments is eutrophication. This process is a chain of events that begins with runoff rich in nitrogen and phosphorus that makes its way into rivers that eventually discharge into estuaries and river deltas. This nutrient-laden water, combined...