Dead zones begin to form when excess nutrients, primarily nitrogen and phosphorus, enter coastal waters and help fertilize blooms of algae. Major nutrient sources include fertilizers, wastewater, and the burning of fossil fuels.
So, how do dead zones occur naturally?
Because most organisms need oxygen to live, few organisms can survive in hypoxic conditions. That is why these areas are called dead zones. Dead zones occur because of a process called eutrophication, which happens when a body of water gets too many nutrients, such as phosphorus and nitrogen.
What is a dead zone and how do they form?
"Dead zone" is a more common term for hypoxia, which refers to a reduced level of oxygen in the water. Hypoxic zones are areas in the ocean of such low oxygen concentration that animal life suffocates and dies, and as a result are sometimes called "dead zones."
How much of the ocean is a dead zone?
According to a new study in Science, the rest of the world fares no better—there are now 405 identified dead zones worldwide, up from 49 in the 1960s—and the world's largest dead zone remains the Baltic Sea, whose bottom waters now lack oxygen year-round. Click here to see a map of dead zones around the world.