At the same time, the incoming river water forms a layer on the surface of the gulf, preventing new oxygen from dissolving and mixing with the depleted waters below. Fish that do not flee suffocate, as do all stationary species and plant life.

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The size of the Gulf of Mexico dead zone fluctuates annually, but increased precipitation this year amplified runoff. The result is an oxygen-depleted area almost 50 percent larger than what has been seen on average for the past five years.

Dead zones occur throughout the world and persist through the summer until plunging water temperatures — and often hurricanes — mix oxygen back into the depths each fall. The hundreds of dead zones throughout the world cover nearly 100,000 square miles, with one in the Baltic Sea spanning more than 23,000 square miles several years ago. Collectively, nearly 10 million tons of biomass either moves from or dies in dead zones every year.

But these areas are preventable. Although oxygen depletion occurs naturally in some parts of the ocean, such as fjords and deep basins, the Gulf of Mexico’s dead zone is caused by humans. The nutrients these algal blooms feed on come from synthetic fertilizers and animal manure, human and industrial waste.

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And though the oxygen loss is temporary, the effects can be permanent. “We have good evidence it is chronically affecting the reproduction of certain species,” said Robert Magnien, an algal bloom expert with the National Oceanic and Atmospheric Administration. Models predicting what will happen if the dead zone persists show that species are likely to decline, added Alan Lewitus, who also studies nutrient pollution at the agency.

But dead zones can be reversed. After the collapse of the Soviet Union, nutrient runoff was reduced threefold to fourfold, eliminating a 15,000-square-mile dead zone off the northwest continental shelf of the Black Sea.

A dead zone in the Chesapeake Bay also has shrunk in recent years, Magnien said, because of major advances in wastewater treatment, sediment and storm water controls, soil management practices, and more selective and precise applications of fertilizer.

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Matt Liebman, who studies cropping systems at Iowa State University, is hopeful that a shift in agricultural practices in the Midwest could make a big difference for the Gulf of Mexico. In a study published this year, his team showed that runoff could be reduced by 60 percent if farmers growing corn and soybean rotated in one or two more crops.

Half of the nitrogen and a quarter of the phosphorous that runs into the gulf comes from corn and soybean fields in the Midwest, he said. In Iowa alone, those crops cover 65 percent of the state. Both plants have shallow roots and are green for only part of the year, unlike the native prairie grasses that once colonized the corn belt.

Liebman’s group compared two different management systems. In the first, corn and soybean were grown in alternating years with fertilizers applied at typical levels. In the second, alfalfa and oats were rotated in, manure was applied once, and fertilizer was used restrictively as needed. Despite using nearly 90 percent less fertilizer, corn and soybean yields increased, soil quality improved, and soil erosion decreased by 25 percent, all without decreasing profitability.

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In addition to rotating crops, Liebman found, “less productive areas could be converted into crop and non-crop vegetation that provides conservation benefits.” A previous Iowa State study showed that converting just 10 percent of farmland back to native prairie grass could reduce nitrogen and phosphorous runoff by nearly 90 percent.

“I’m optimistic that we have a lot of technical answers,” he said, but “we need a market pull and a policy push.”

Such policy changes could include regulations, financial incentives for farmers to adopt conservation practices, and technical assistance to help farmers incorporate the changes. Yet even with a dramatic shift toward reducing runoff, Magnien warns that it could be 10 to 20 years before the gulf shows visible improvement.