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U.S. scientists have determined that the Gulf of Mexico's annual "dead zone" — an area with low oxygen that can kill fish and marine life — is the fourth smallest since they started mapping the area in 1985. Scientists supported by the National Oceanic and Atmospheric Administration (NOAA) said in a report Tuesday that the area is only about 40 percent the average size predicted earlier this year based on nitrogen and other nutrients flowing down the Mississippi river. This year's dead zone off Louisiana is about 7,040 square kilometers, rather than the 15,000 square kilometers predicted by the NOAA. Every year the oxygen depletion begins as snow melt and spring rains bring fresh water to the gulf. Fresh water is lighter than salt water causing two layers to develop. Nitrogen and other nutrients in the fresh water feed a growth spurt of algae and microorganisms at the top. The microorganisms die and fall to the bottom, where their decay consumes oxygen from the bottom up, creating the dead zone. 美国科学家认定，墨西哥湾里由于含氧过低导致鱼和海洋生物死亡的所谓“死亡区”今年是1985年开始测量记录以来第四小的面积。 为美国国家海洋与大气管理局工作的科学家在星期二发布的一份报告中说，这片死亡区的面积只有今年早些时候预计的40%。对死亡区面积的预测基于密西西比河流入墨西哥湾水中的氮和其它营养素含量。 | 
