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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 said the movement of rivers usually removes substances like nitrates. So he wondered why that natural filtration process does not remove nitrates from the Mississippi River. Professor Cardenas said 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 said the study found that there is so much nitrogen in the river system that it simply cannot filter all of it. Nitrogen in the water 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. Farmers use nitrogen fertilizer and other chemicals to grow their crops. And that nitrogen washes from the fields into streams and rivers. Storms in the next few months will mix the Gulf water, and the dead zone will disappear. But it will return next year. 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.