为广大考生分享2014年12月大学英语四级听力练习,希望对大家有所帮助,祝大家备考顺利!
An area known as a dead zone develops every spring in the Gulf of Mexico near the mouth of the Mississippi River. It can spread as much as 13,600 square kilometers, extending all the way to the eastern Texas coast. Scientists know what causes the dead zone -- too much nitrogen, but the solution might be hard to accept.
Bayani Cardenas is a professor of water studies at the University of Texas at Austin. He says the movement of rivers usually removes material like nitrates. So he wondered why that natural filtration process does not remove nitrates from the Mississippi River.
Dead pogies float in a fish kill in a pass near Bay Joe Wise on the Louisiana coast, Thursday, Sept. 16, 2010.
Professor Cardenas says his recent study shows that more than 99 percent of the rivers water does pass through the rivers sediment which is on the shore or bottom of the river. But he says the study found that there is so much nitrogen in the river system that is simply can not filter all of it.
The water that has nitrogen still in it supports the growth of algae. As the algae dies, it sinks to the bottom of the river where it breaks down or decomposes. It then takes oxygen from the water. This condition is called hypoxia and it is deadly to fish and shrimp.
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