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Shrimp cocktail is sure to be served at many New Year's cocktail parties. But environmental scientists say a so-called "dead zone" in the Gulf of Mexico may be threatening one of the shellfish's habitats. This ScienCentral News video has more.
Sucking the Life Out of the Sea
What's about the size of New Jersey, and could someday be responsible for the collapse of one of the United States' largest fisheries? It's the 5,800-square-mile "dead zone" currently stretching across the Gulf of Mexico.
Dead zones are ocean areas with very little oxygen— also known as hypoxic. "Most marine organisms require at least two milligrams per liter of oxygen in the water to survive," explains Don Scavia, director of the Michigan Sea Grant and an environmental scientist at the University of Michigan. "So when the concentration in this region gets below two milligrams per liter, those organisms, the fish, shellfish, that can swim, will leave the area." And if they can't swim, they die.
The main culprit appears to be the nitrogen in fertilizer that's used on farms miles away in the Midwest, near any river that flows into the Mississippi River. Duke University marine biologist Larry Crowder, who is working on a study of growth rates in gulf shrimp which suggests the dead zone is linked to a decrease in their size, explains: "Basically the nitrogen makes its way downstream with the fresh water in the Mississippi river drainage. The fresh water, when it comes into the Gulf of Mexico, because it's less dense than the seawater that it encounters there, flows over the top of the seawater. The nitrogen stimulates production of algae in the Gulf of Mexico, and to the degree that that algae isn't used up in the food web, it falls to the bottom and decomposes. While it's decomposing, bacteria…use oxygen in that decomposition process. So the nutrients enhance algal production, which subsequently leads to low oxygen in the bottom of the Gulf of Mexico."
Farms being asked to cooperate are within the Mississippi River watershed, which includes 31 states that cover about 40 percent of the country. image: USDA
Both Scavia and Crowder say that forcing the shrimp into deeper, colder waters (or into shallower, warmer waters) makes them grow more slowly. "If the shrimp are growing slower, and the population size is growing slower and we continue to fish at the rates that we've traditionally been fishing at, we may fish faster than they can actually grow," Scavia says. "The worst case scenario is if this dead zone is maintained into the future— or even worse, grew— that the fisheries could in fact collapse. I believe the shrimp fishery alone generates about a half a billion dollars per year. So there's significant resources and economic value coming from the gulf."
Scavia led a task force that in 2001 recommended decreasing nitrogen runoff by 30 percent in order to shrink the dead zone from around 6,000 to 2,000 square miles. But based on new computer models, Scavia published a more recent paper that predicts nitrogen runoff will need to decrease by 40 to 45 percent.
Crowder thinks Scavia's new analysis is on target. "What Don's paper is, is basically a warning to say, 'You know, if we head down this road— 30 percent nitrogen reduction, with the target of reducing the dead zone [to a quarter of its current size]— what we're going to find is that we're not going to get there'."
Still, both researchers agree that sticking with the 30 percent decrease would at least be a step in the right direction. Scavia's research was published in the June, 2004 issue of the journal Estuaries and was funded by the NOAA Coastal Ocean Program.
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