墨西哥灣“死亡區”今年面積不到預測一半

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.美國科學家認定，墨西哥灣裏由於含氧過低導致魚和海洋生物死亡的所謂“死亡區”今年是1985年開始測量記錄以來第四小的面積。

爲美國國家海洋與大氣管理局工作的科學家在星期二發佈的一份報告中說，這片死亡區的面積只有今年早些時候預計的40%。對死亡區面積的預測基於密西西比河流入墨西哥灣水中的氮和其它營養素含量。

位於路易斯安那州沿海的死亡區今年的面積爲7040平方公里，小於美國國家海洋與大氣管理局早先預測的1萬5千平方公里。

每年春季融化的雪水以及雨水流入時，墨西哥灣水中的氧氣含量開始減少。流入的淡水比海水輕，水自然分成兩層。淡水中的氮和其它營養素促使水藻和微生物在上層快速生長。

微生物死亡後沉到水底，其腐爛過程由下至上地消耗氧氣，形成死亡區。