Dead zones have been occurring in the gulf since the 1970s, and studies show that the main culprits are nitrogen and phosphorus from crop fertilizers and animal manure in river runoff. They settle in at the mouth of the gulf and fertilize algae, which prospers and eventually starves other living things of oxygen.
Government studies have traced a majority of those chemicals in the runoff to nine farming states, and yet today, decades after the dead zones began forming, there is still little political common ground on how to abate this perennial problem. Scientists who study dead zones predict that the affected area will increase significantly this year, breaking records for size and damage.
For years, environmentalists and advocates for a cleaner gulf have been calling for federal action in the form of regulation. Since 1998, the Environmental Protection Agency has been encouraging all states to place hard and fast numerical limits on the amount of those chemicals allowed in local waterways. Yet of the nine key farm states that feed the dead zone, only two, Illinois and Indiana, have acted, and only to cover lakes, not the rivers or streams that merge into the Mississippi.
The lack of formal action upstream has long been maddening to the downstream states most affected by the pollution, and the extreme flooding this year has only increased the tensions.
“Considering the current circumstances, it is extremely frustrating not seeing E.P.A. take more direct action,” said Matt Rota, director of science and water policy for the Gulf Restoration Network, an environmental advocacy group in New Orleans that has renewed its calls for federally enforced targets. “We have tried solely voluntary mechanisms to reduce this pollution for a decade and have only seen the dead zone get bigger.”
Environmental Protection Agency officials said they had no immediate plans to force the issue, but farmers in the Mississippi Basin are worried. That is because only six months ago, the agency stepped in at the Chesapeake Bay, another watershed with similar runoff issues, and set total maximum daily loads for those same pollutants in nearby waterways. If the states do not reduce enough pollution over time, the agency could penalize them in a variety of ways, including increasing federal oversight of state programs or denying new wastewater permitting rights, which could hamper development. The agency says it is too soon to evaluate their progress in reducing pollution.
Don Parish, senior director of regulatory relations for the American Farm Bureau Federation, a trade group, says behind that policy is the faulty assumption that farmers fertilize too much or too casually. Since 1980, he said, farmers have increased corn yields by 80 percent while at the same time reducing their nitrate use by 4 percent through precision farming.
“We are on the razor’s edge,” Mr. Parish said. “When you get to the point where you are taking more from the soil than you are putting in, then you have to worry about productivity.”
Dead zones are areas of the ocean where low oxygen levels can stress or kill bottom-dwelling organisms that cannot escape and cause fish to leave the area. Excess nutrients transported to the gulf each year during spring floods promote algal growth. As the algae die and decompose, oxygen is consumed, creating the dead zone. The largest dead zone was measured in 2002 at about 8,500 square miles, roughly the size of New Jersey. Shrimp fishermen complain of being hurt the most by the dead zones as shrimp are less able to relocate — but the precise impacts on species are still being studied.
The United States Geological Survey has found that nine states along the Mississippi contribute 75 percent of the nitrogen and phosphorus. The survey found that corn and soybean crops were the largest contributors to the nitrogen in the runoff, and manure was a large contributor to the amount of phosphorus.
There are many other factors, of course, that determine what elements make it from crops into river water, for example, whether watersheds are protected by wetlands or buffer strips of land.
John Downing, a biogeochemist and limnologist at Iowa State University, said structural issues were also to blame. Many farms in Iowa, he said, are built on former wetlands and have drains right under the crop roots that whisk water away before soils can absorb and hold on to at least some of the fertilizer.
Still, overapplication of fertilizers remains a key contributor, he said. “For farmers, the consequences of applying too little is much riskier than putting too much on.”
This article has been revised to reflect the following correction:
Correction: June 3, 2011
An earlier version of this article used an incorrect spelling of a chemical that runs off into rivers from crop fertilizers and animal manure. It is phosphorus, not phosphorous.
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