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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.”