Researchers found these particular dead zones in tropical North Atlantic waters, hundreds of miles from West Africa. Some of them were spotted north of Cape Verde's islands.
"It is not unlikely that an open-ocean dead zone will hit the island at some point," lead author Johannes Karstensen, a researcher at GEOMAR Helmholtz Centre for Ocean Research Kiel in Germany said in a statement. "This could cause the coast to be flooded with low-oxygen water, which may put severe stress on the coastal ecosystems and may even provoke fish kills and the die-off of other marine life."
The dead zones were formed inside of massive whirlpools called eddies. Some of the eddies that researchers observed were hundreds of feet tall and had rotating diameters of 60 to 94 miles. The associated dead zones stretched close to 650 feet.
Karstensen said that the minimum concentration of oxygen in the Atlantic's open waters was thought to be about one millimeter per liter of seawater. They found dead zones with oxygen levels 20 times less than that, meaning these open water areas had virtually no oxygen. Zooplankton -- an important part of the marine food chain -- remained at the surface of the eddies during the day, rather than going into deeper waters to feed.
Dead zones occur across the globe. According to the National Oceanic and Atmospheric Administration, "there is no part of the [United States] or the world that is immune" to them. A massive dead zone, which can be as large as the state of Massachusetts, crops up seasonally in the Gulf of Mexico. The Baltic Sea has seven of the 10 largest dead zones in the world.
The dead zones of greatest concern form near coastlines, primarily from nutrient pollution caused by human activity. Fertilizers and chemicals that run into the water trigger algae blooms, which then use up a lot of oxygen when they eventually decompose.
Unlike those dead zones, researchers looked to eddies that cropped up in a current coming from West Africa's coast, and then slowly moved westward as the earth rotated. The circulation of the water creates a thin layer on the top "that supports intense plant growth" Karstensen said. That plant growth, plus the rotation, causes water to lose more and more oxygen.
Scientists have been warning that dead zones -- where little to no marine life can survive -- across the globe could expand with warming waters. A 2014 Smithsonian report published in Global Change Biology found that 94 percent of the ocean's dead zones are in areas expected to warm up by at least 2 degrees Celsius by the end of the century.
The number of dead zones, now more than 400, has doubled every decade since 1960, a 2008 study published in the journal Science found. As of that study, they covered about 95,000 square miles.