The dead zone in the Gulf of Mexico—a growing and shrinking region of oxygen-free water—could be the largest on record this summer, thanks to flooding and fertilizer runoff.

NOAA The dead zone, indicated in red, is filled with low to no-oxygen water, causing nearby fish to flee or die.

The near record-breaking Midwestern drought of 2012 shriveled corn crops and toasted pasture land. But it did have one positive side effect. The drought significantly reduced the size of the seasonal Gulf of Mexico dead zone. Less rain led to less fertilizer runoff—the dead zone is fed by a buildup of nitrogen-based fertilizer in the Gulf—which meant that the 2012 summer dead zone measured just 2,889 sq. miles. That’s still a zone the size of the state of Delaware, but it was the fourth-smallest dead zone on record, and less than half the size of the average between 1995 and 2012.

This year will be different. Heavy rainfall in the Midwest this spring has led to flood conditions, with states like Minnesota and Illinois experiencing some of the wettest spring seasons on record. And all that flooding means a lot more nitrogen-based fertilizer running off into the Gulf. According to an annual estimate from National Oceanic and Atmospheric Administration (NOAA) sponsored modelers at the University of Michigan, Louisiana State University and Louisiana Universities Marine Consortium, this year’s dead zone could be as large as 8,561 sq. miles—roughly the size of New Jersey. That would make it the biggest dead zone on record. And even the low end of the estimate would place this year among the top 10 biggest dead zones on record. Barring an unlikely change in the weather, much of the Gulf of Mexico could become an aquatic desert.



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The nitrogen nutrients that flow into the Gulf, especially during the rainy spring season, encourages the growth of explosive algal blooms, which feed on the nitrogen. Eventually those algae die and sink to the bottom, and bacteria there get to work decomposing the organic matter. The bacteria consume oxygen in the water as they do, resulting in low-oxygen (hypoxic) or oxygen-free (anoxic) regions in the bottom and near-bottom waters.

That’s what a dead zone—water, essentially, without air. Sealife—including the valuable shellfish popular in Gulf fisheries—either flee the area, much as you or I would if someone were to suck all the oxygen out of the room, or die. That’s why the dead zone matters—the larger it is, the greater the populations of fish that might be affected. With commercial fisheries in the Gulf worth $629 million as of 2009—and still recovering from the impact of the 2010 oil spill—the dead zone means business.

The major factor driving the size of the dead zone—beyond changing flooding patterns—is the use and overuse of fertilizers in America’s rich Midwestern corn belt. The U.S. Geological Survey estimated that 153,000 metric tons of nutrients flowed down the swollen Mississippi and Atchafalaya rivers during May—a 16% increase over the nutrient load average seen during the past 34 years. And as James Greiff of Bloomberg points out in a recent piece, those nutrients are used disproportionately to feed one particular crop:

The culprits behind the dead zone are many, but one deserves special attention: corn. Unlike, say, soybeans, which can grow without fertilizer, corn can’t grow without it. It takes 195 pounds of fertilizer to grow an acre of corn. And the U.S. grows a lot of corn — more than any other country. What’s more, 40 percent of the U.S. corn crop is devoted to making ethanol, which fuel companies must blend with gasoline under a congressional mandate. The Gulf dead zone is yet another reason for Congress to kill that mandate.

A state-federal task force was actually set up in 2008 with the aim of reducing the nutrient flow in the Mississippi by 45% by this year—but as the numbers demonstrate, there hasn’t been much success. Farmers could be encouraged to use fertilizer more efficiently—Greiff suggests ending the practice of applying fertilizer to fields in the fall after crops are harvested, and instead laying it down in the spring. They should also limit the amount of water running off their land, much of which ends up in the rivers and then the Gulf.

Of course, Midwestern farmers care chiefly about the crops in their own fields, not what might be happening in the Gulf of Mexico, hundreds of miles downstream. (And the American farmer is just a bit more politically powerful than Gulf fishermen, let alone environmentalists.) But that invisible telecoupling is what make today’s environmental threats—climate change, ocean acidification, the wildlife trade—so devilishly complex. Just thinking about it is enough to suck the oxygen right out of the room.

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