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.