Some coastal woes begin far inland
Farm runoff creates ‘dead zones’ offshore, but no national authority is tasked to address them.
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“We don’t really have the legal authority in place for anyone to fix this,” says Catherine Kling, a professor of economics at Iowa State University, Ames. “And if you don’t have that, you have to rely on voluntary” measures.Skip to next paragraph
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Ethanol production is likely to enlarge the dead zone. A March study in the Proceedings of the National Academy of Sciences found that if the US meets its stated goal of 15 billion to 36 billion gallons of corn-based ethanol by 2022, nutrient flow into the northern Gulf will increase by 10 to 34 percent. Without drastic changes in agricultural practices, the paper concludes, the Action Plan’s goals are “practically impossible.”
Already, last year farmers devoted a nearly California-sized tract of land to corn cultivation, a 15 percent increase over the previous year, and a 60-year high. Last year, the dead zone reached the third-largest extent ever observed.
Heavy rainfall throughout the Midwest this year has increased the Mississippi’s discharge by 75 percent. While the river’s fertilizer concentration is lower than last year’s because of high water volume, it will dump 37 percent more nutrients into the Gulf. Loss of crops to floods may urge farmers to plant a second crop, sending more fertilizer downstream.
A study published earlier this year speculates that, after 30 years of excess nutrients, the Gulf ecosystem may be near a tipping point. The northern Gulf’s sediments have become so saturated, the authors say, that the ecosystem is showing less resilience. Compared with 30 years ago, it takes a smaller nutrient load to cause the same size dead zone.
Some fishermen are worried, too.
“It has the potential to affect fisheries,” says Ms. Rabalais. Fish can flee when an area turns hypoxic. But often the bottom-dwelling bivalves and worms that form an integral part of the ecosystem can’t. “It reduces biodiversity,” she says. And with each reduction, returning fish find the ecosystem less able to sustain them.
Others think the dead zone’s potential impact on fisheries is being oversold. How oxygen-depleted waters affect an ecosystem depends on the ecosystem itself, says James Cowan, a fisheries oceanographer at Louisiana State University, Baton Rouge.
In Chesapeake Bay, for example, originally dominated by bottom-dwelling organisms like oysters, increased nutrient influx has restructured the food web. Low-oxygen waters can occupy up to 40 percent of the Chesapeake in summer, suffocating crabs, fish, and worms. Centuries of harvesting oysters, which once filtered excess nutrients from the water and so defended the bay against hypoxia, may have sent the ecosystem past a tipping point. Few oysters remain.
But at the mouth of the Mississippi, the ecosystem is already adapted to a harsh environment that includes large pulses of sediment and fresh water. Midwater organisms accustomed to these conditions dominate the ecosystem. The greater threat here, Dr. Cowan says, is loss of coastal wetlands that serve as fish nurseries. “For my money, that’s the bigger concern,” he says.