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VOA慢速[农业]墨西哥湾死水区成因

2014-10-09    来源:普特英语听力网    【      美国外教 在线口语培训
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From VOA Learning English, this is the Agriculture Report.

An area known as 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.

Professor Cardenas says his recent study shows that more than 99 percent of the river's water does pass through the river's 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.

Where does all this nitrogen come from? The answer is farms. The Mississippi River system carries water from 33 American states and part of Canada to the gulf of Mexico. Along the way nitrogen and other chemicals used in farming enter the system. Farmers say these chemicals must be used to produce enough food for a growing world population. 

Aaron Packman is a professor of Civil and Environmental Engineering at Northwestern University. He says farmers may be able to better control the amount of nitrate fertilizer they put on fields.

"How much fertilizer do you need to give you good yields and then how much is maybe a marginal gain from adding lots more fertilizer? There is really a question here: can you maybe [reduce the amount] and get close to the same level of yield without having such a negative impact?" said Packman.

The water in the Mississippi River system should be able to clean itself naturally as it flows down stream. But flood controls and other human-made structures have hurt this filtering process.

Storms in the next few month will mix the gulf water and the dead zone will disappear, but it will return next year. And scientists say it will grow larger in years to come, if something is not done to reduce the amount of nitrogen in the Mississippi River.

And that's the VOA Learning English Agriculture Report. For more agriculture stories, go to our website learningenglish.voanews.com. I'm Caty Weaver.

本文来源于普特论坛[report整理] SENEWS-2014-07-01


这里是美国之音慢速英语农业报道。

墨西哥湾的密西西比河口附近每年春天都会变为死水区。它会扩散开至1.36万平方公里,一直延伸到德克萨斯东部海岸。科学家了解到,导致死水区的形成的原因是过量的氮。但是解决这个问题的方法可能难以被接受。

Bayani Cardenas是德克萨斯大学奥斯汀分校水资源研究的教授。他说,河水流动通常会过滤掉水里的某些成分,如硝酸盐。所以,他很困惑为什么大自然的过滤程序没有清除掉密西西比河里的硝酸盐成分。

Cardenas教授表示,他最近的研究显示,超过99%的河水确实会流经岸边或河底的沉积物。但他表示,该研究发现,河水的含氮量是很高的,根本无法被完全过滤掉。

含氮的河水帮助藻类植物的生长。随着藻类植物死亡,它便会下沉到河底发生分解,然后带走水中的氧气。这种情况被称为缺氧,对于鱼虾来说是致命的。

那么这些氮是从何而来呢?答案就是农场。密西西比河水系将美国33个州以及加拿大部分地区的水带到墨西哥湾。农场使用过的氮元素以及其他的一些化学成分在这一路上进入了该水系。农民表示,一定要使用这些化学成分,才能产出足够多的食物来满足不断增长的人口数量。

Aaron Packman美国西北大学土木与环境工程学的教授。他说,农民也许能够更好的控投放到田地里的氮肥总量。

Packman.说:“需要多少肥料才能保证高产?增加更多肥料的边际收益又是多少?问题就在于:是否能够在不产生如此多的负面影响的情况下尽可能获得同等的收益?

密西西比河水系的水本该在流向下游的过程中自净。但是防洪控制以及其他人造建筑破坏了这个程序。

未来几月的暴风雨将会翻动墨西哥湾的水,死水区也会消失,但在明年又会重现。科学家表示,如果不采取措施来减少密西西比河水里的含氮量,那么死水区的面积会越来越大。

以上就是本期美国之音慢速英语农业报道的全部内容。想了解更多的农业新闻,请访问我们的网站voalearningenglish.com。我是Caty Weaver。

 



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