Dead Zones Pose Biggest Challenge for World’s Oceans

Large swaths of ocean deprived of oxygen, and thus devoid of fish and plant life — known as “dead zones” — are the primary threat to the world’s oceans in the 21st century, surpassing even overfishing, claimed experts at a meeting this week put on by the U.N. Environment Program. The main cause of dead zones is nitrogen, primarily from agricultural fertilizers that run off into rivers and streams and then into the ocean, where they cause massive algae blooms. The blooms use up all the available oxygen, starving other forms of life. Dead zones range in size from less than a square mile to 43,500 square miles and have doubled in number every decade since the 1960s — currently there are nearly 150. “Hundreds of millions of people depend on the marine environment for food, for their livelihoods, and for their cultural fulfillment,” said UNEP Executive Director Klaus Toepfer. “Unless urgent action is taken to tackle the sources of the problem, it is likely to escalate rapidly.”