Farm Runoff in Mississippi River Floodwater Fuels Dead Zone in Gulf

Jenny Marder -

A dead zone -- already the size of the state of New Jersey -- is growing in the Gulf of Mexico, fueled by nutrient runoff from the swollen Mississippi River.

This year, with floodwaters from the Birds Point levee breach and the Morganza and Bonnet Carret spillways spreading over farmland and other residential areas, the river is collecting tremendous amounts of fertilizer and pesticides. This is contributing to what scientists say may become the largest dead zone ever, and posing a serious threat to already taxed marine life.

During the rainy season, fertilizer, animal waste, sewage and car exhaust wash into the Mississippi and the Atchafalaya rivers, flow south and empty into the mouth of the Gulf.

Nitrogen and phosphorous from farm runoff and animal waste are especially toxic to ocean life. They act as natural fertilizers, feeding harmful algae and causing it to bloom wildly. As bacteria consume these blooms, they suck oxygen from the water, depleting the ocean's oxygen reserves. Scientists call this oxygen depletion hypoxia.

"We're expecting probably the largest-ever amount of hypoxia," said Nancy Rabalais, a marine scientist and executive director of the Louisiana Universities Marine Consortium. "That's the the prediction based on the amount of nitrogen coming down the river."

A surge of fresh water creates a layering effect in the seawater, which compounds the problem. The freshwater sits above the heavier saltwater, acting as a cap that prevents oxygen from reaching the deeper water levels.

"The bottom layer of the ocean gets so low in oxygen that sea life has to swim away and vacate the area, and if they can't get away, they suffocate," said Matt Rota, science and water policy director for the Gulf Restoration Network.

Flooding could cause further injury to fisheries in the northern Gulf of Mexico, already reeling from last year's oil spill, Rabalais said. Dead zones alter the habitat for crab, shrimp, fish and lobster, often forcing them to shallow areas. This includes catchable seafood, like shrimp and snapper, which are vital to the area's fisheries.

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