The environmental impact of human activity is measured in bodies of water, not just in the atmosphere and climate. New research shows the largest offshore "dead zone" ever recorded in the United States. It affects the Gulf of Mexico at the mouth of the Mississippi River, where a low-oxygen zone covers nearly 8,800 square miles — about the size of New Jersey, the National Oceanic and Atmospheric Administration says. The below-normal oxygen levels underwater can't adequately support fish, crabs, shrimp and other marine life.

The situation is linked to farming along the Mississippi River. Unusually heavy rains over the Midwest last spring flushed fertilizer and animal waste from farms down into the Gulf via the massive waterway, which drains part or all of 31 states. When that nitrogen and phosphorus pollution reaches the sea, it feeds algae blooms and bacteria that suck the oxygen out of the water -- bad news for commercial fishing fleets in Louisiana that produce more than 40 percent of American seafood.

"This is the largest one we've ever measured. And the northern Gulf of Mexico dead zone is the second largest human-caused dead zone in the ocean," says Nancy Rabalais of Louisiana State University, who began measuring Gulf oxygen levels in 1985. Underwater video shows the transition from life to death as green fades to black. It becomes so dark, divers need flashlights to find their way. "When you're a scuba diver, you're used to having fish swimming all around you. From 30 to 60 feet, we won't see any fish – nothing," Rabalais says of the research video. The abyss stretches over an enormous portion of the Gulf.

A federal Environmental Protection Agency task force will try to shrink the size of the dead zone. "The solution lies upstream in the watershed with better agricultural management practices -- a switch to crops that have deeper roots and don't need as much fertilizer and are still just as profitable as corn," Rabalais suggests. Agricultural experts also suggest crop rotation, selective applications of fertilizer, wastewater treatment and storm water controls to keep rain runoff out of the river.

Scientist says: "We predicted it would be large, and it is large." -- Nancy Rabalais, marine ecologist at Louisiana State University

Fisherman says: "A lot of the fish, they not as healthy as they used to be. The shrimping is depleting pretty much every year. The season gets later and you catch smaller shrimp." – Reggie Walker, lifelong commercial fisherman on Mississippi’s coast

Marine life expert says:"We are now seeing more animals [such as sea turtles and dolphins] gasping for air and dying. These animals are top of the food chain and are a good indicator of the environment. And I can tell you that there is something that is not looking very good." -- Moby Solangi, head of the Institute for Marine Mammal Studies in Gulfport, Miss.