JEFFERSON, Iowa — Because of rising demand for ethanol, American farmers are growing more corn than at any time since World War II. And sea life in the Gulf of Mexico is paying the price.

The nation’s corn crop is fertilized with millions of pounds of nitrogen-based fertilizer. And when that nitrogen runs off fields in Corn Belt states, it makes its way to the Mississippi River and eventually pours into the Gulf, where it contributes to a growing “dead zone” — a 7,900-square-mile patch so depleted of oxygen that fish, crabs and shrimp suffocate. The dead zone was discovered in 1985 and has grown fairly steadily since then, forcing fishermen to venture farther and farther out to sea to find their catch. For decades, fertilizer has been considered the prime cause of the lifeless spot.

With demand for corn booming, some researchers fear the dead zone will expand rapidly, with devastating consequences.

“We might be coming close to a tipping point,” said Matt Rota, director of the water resources program for the New Orleans-based Gulf Restoration Network, an environmental group. “The ecosystem might change or collapse as opposed to being just impacted.” Environmentalists had hoped to cut nitrogen runoff by encouraging farmers to apply less fertilizer and establish buffers along waterways. But the demand for the corn-based fuel additive ethanol has driven up the price for the crop, which is selling for about $4 a bushel, up from a little more than $2 in 2002.

That enticed American farmers — mostly in Iowa, Illinois, Minnesota, North Dakota, and South Dakota — to plant more than 93 million acres of corn in 2007, the most since 1944. They substituted corn for other crops, or made use of land not previously in cultivation.