Dead Zones Doubling Every Decade
Submitted by LiveScience Staff
posted: 08 October 2009 03:04 pm ET
Global distribution of the more than 400 marine systems with dead zones caused by increased nutrient runoff. Their distribution matches the current human "footprint" in the northern hemisphere. In the southern hemisphere, dead zones have only been reported recently.
Earth's oceans currently have more than 400 dead zones, oxygen-starved areas that are hundreds or thousands of square miles and virtually devoid of life during summer months.
The tally is doubling every decade, according to the National Science Foundation.
Most dead zones, including one in the Gulf of Mexico, are caused by pollution that is dumped into oceans by rivers. It works like this:
Each year, spring runoff washes nitrogen-rich fertilizers from farms in the Mississippi River basin and carries them into the river and the streams that feed it. The nitrogen eventually empties out of the mouth of the Mississippi and into the Gulf of Mexico, where tiny phytoplankton feed off of it and spread into an enormous bloom.
When these creatures die, they sink to the ocean floor, and their decomposition strips the water of oxygen. This condition, called hypoxia, prevents animals that depend on oxygen, such as fish or shrimp, from living in those waters. In recent years, this so-called "dead zone" has grown to the size of New Jersey—about 20,000 square kilometers (7,700 square miles)—each summer.
But there's another emerging culprit, the NSF explains in a new special report. Every summer since 2002, the Pacific Northwest's coastal waters -- one of the U.S.'s most important fisheries -- has seen massive dead zones believed to be caused by an entirely different and surprising phenomena: changes in oceanic and atmospheric circulation that may, in turn, be caused by climate change.