Parts of the world's oceans are running low on oxygen, a new study finds. Fertilizers and other chemical pollutants in river runoff fuel blooms of algae that cause oxygen levels to dip precipitously when they die. A review of research into these so-called "dead zones," detailed in the Aug. 15 issue of the journal Science, finds that the number of dead zones has roughly doubled every decade since the 1960's. The study authors, Robert Diaz of the Virginia Institute of Marine Science and Rutger Rosenberg of the University of Gothenburg in Sweden, tallied 405 dead zones in coastal waters worldwide today, affecting about 95,000 square miles (245,000 square kilometers) of ocean, an area about the size of New Zealand. While that may seem small compared to the total coverage of the oceans, the local effects can be devastating to marine ecosystems. These dead zones occur when fertilizer runoff dumps excess nutrients, such as nitrogen and phosphorous, into coastal waters, providing food for algae. When these microscopic plants die and sink to the ocean bottom, bacteria feed on them and subsequently consume all the oxygen dissolved in the water. This leaves fish and other bottom-dwelling sea creatures without enough oxygen to survive, causing mass die-offs and displacements. Typically, the researchers noted, these events aren't noticed until they threaten valuable fish stocks. The world's largest dead zone is in the Baltic Sea. The largest dead zone in the United States sits in the Gulf of Mexico at the mouth of the Mississippi River and is about the size of New Jersey. Scientists have predicted that the Gulf dead zone could grow larger than ever this summer. Diaz and Rosenburg said that dead zones now rank as one of "the key stressor[s] on marine ecosystems," along with over-fishing and habitat loss. "There is no other variable of such ecological importance to coastal marine systems that has changed so drastically over such a short time as dissolved oxygen," they wrote. With the possibility that climate change could exacerbate the situation through changes in ocean circulation, Diaz and Rosenburg recommend cutting back the amount of nitrogen-rich fertilizer that runs off into rivers.
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