There are now more than 400 known dead zones in coastal waters worldwide, compared to 305 in the 1990s, according to study author Robert Diaz of the Virginia Institute of Marine Science.
Dead zones occur when excess nutrients—usually nitrogen and phosphorus—from agriculture or the burning of fossil fuels seep into the water system and fertilize blooms of algae along the coast.
As the microscopic plants die and sink to the ocean floor, they feed bacteria, which consume dissolved oxygen from surrounding waters. This limits oxygen availability for bottom-dwelling organisms and the fish that eat them.