墨西哥湾“死亡区”今年面积不到预测一半

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.

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。

位于路易斯安那州沿海的死亡区今年的面积为7040平方公里，小于美国国家海洋与大气管理局早先预测的1万5千平方公里。

每年春季融化的雪水以及雨水流入时，墨西哥湾水中的氧气含量开始减少。流入的淡水比海水轻，水自然分成两层。淡水中的氮和其它营养素促使水藻和微生物在上层快速生长。

微生物死亡后沉到水底，其腐烂过程由下至上地消耗氧气，形成死亡区。