Nitrogen is a popular crop fertilizer that causes problems when it washes off fields into rivers on its way to the ocean.
In the Gulf of Mexico the nitrogen promotes algae growth. When the algae decays it soaks up oxygen, a condition known as hypoxia. Each year this process produces an area of oxygen deprived water in the Gulf called the dead zone, covering thousands of square miles.
Brent Aulenbach, with the U.S. Geological Survey, has just finished his annual measurement of the nitrogen in the Mississippi River basin entering the Gulf. He says his figures show nitrogen levels this year are 12 percent higher than the long-term average.
"Since the springtime nitrogen loads coming down the Mississippi River is one of the more important factors in determining the size of the hypoxic zone, I expect that above average nitrate flux from the Mississippi is going to cause above average hypoxia zone area this summer," says Aulenbach.
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Aulenbach says the amount of phosphorus entering the Gulf also increased. He says the levels of this farm fertilizer detected this spring were 15 percent above average.
Studies have shown that most of the farm fertilizers entering the Gulf comes from the Ohio River Basin and the Upper Mississippi River, including Minnesota.
That region is the nation's corn belt. Nitrogen is the most important fertilizer farmers use for corn. The number of acres planted to corn this spring were up nearly 20 percent over last year, mainly to meet ethanol demand.
Environmental groups have warned more corn means more fertilizer runoff. Donald Boesch says it's something worth studying. Boesch has researched the Gulf dead zone as head of the University of Maryland's Center for Environmental Science.
"Anytime you have one year's data all you can say is that it's an event rather than necessarily a trend," says Boesch. "One would want to know where the nutrients within the basin are coming from. Whether it's from the upper Mississippi, the Missouri and so on. That's consistent with any change in acreage, intensification of corn agriculture up in the basin."
Most farmers have resisted the charge that their fertilizers are the main contributors to the Gulf's dead zone. They say municipal waste water, lawn fertilizer and other sources also contribute.
Bruce Noel, who chairs the ethanol committee of the National Corn Growers Association, says farmers are careful how they apply nitrogen and phosphorus.
"People are going to point their fingers at corn growers. We've been taking the rap for everything from tortilla prices in Mexico to the hypoxia in the Gulf," says Noel. "But nitrogen is very expensive and we dribble it on, we don't just pour it on and then just sit back and watch it go."
Still ahead is a more detailed analysis of this year's hypoxia in the Gulf of Mexico. A team from Louisiana University is scheduled to be on the water during the last week of July to measure the size of the dead zone.