The Gulf of Mexico's annual "dead zone" will likely grow to an unprecedented size this summer, a leading expert tells MNN. The lifeless expanse of oxygen-starved seawater is a regular problem for both marine animals and fishermen throughout the region, but due to historic Mississippi River floods this spring — combined with relentless thunderstorms dumping rain across the central U.S. — the Gulf dead zone is expected to grow up to 15 percent larger in 2011 than ever before.

The dead zone is fueled by excess nutrients flowing down the Mississippi River, namely synthetic fertilizers and animal waste from Midwestern farms (although urban pollutants are also to blame). And based on measurements taken so far in 2011, the swollen river is carrying a lot more nutrients than usual, says Nancy Rabalais, executive director of the Louisiana Universities Marine Consortium and a prominent authority on environmental issues in the Gulf of Mexico.

"The best predictor [of the dead zone] is the river's nitrate load in May," Rabalais says. "And the amount that's coming down right now indicates it's going to be the largest one ever. It could be 5, 10, 15 percent larger."

The largest Gulf dead zone measured so far was in 2002, when dangerously low oxygen levels (a condition called "hypoxia") spread across 8,500 square miles. It has averaged about 6,000 square miles in recent years, but based on all the nutrients now flowing south, it may grow to nearly 10,000 square miles this summer, Rabalais warns. Scientists have been measuring hypoxia in the region since 1985, she adds, "minus one year when we didn't have any funding."

The dead zone doesn't necessarily kill all the sea life in its path — plenty of [skipwords]fish and other marine animals simply flee to more breathable waters. But all that physical stress can still take a toll on an ecosystem, especially when animals must travel hundreds or thousands of miles just to breathe. And it's not only animals that suffer, Rabalais points out: The people who depend on them end up in dire straits, too.

"When the water is hypoxic to less than 2 parts per million, any fish, shrimp or crabs in that area have to leave," Rabalais says. "So that will significantly decrease the area where you can conduct fishing. Inshore fisheries in Louisiana have smaller boats, so many of them just won't be able to fish[/skipwords] or trawl. The distance required and the fuel costs right now could keep them in port." And, she adds, the Gulf Coast is hardly in a position to handle another crisis: "This just further complicates their inability to trawl since last year because of the BP spill."

Rabalais and her colleagues are currently testing dissolved oxygen levels off Terrebonne Bay, La., and will then venture out in boats next month to examine how big the dead zone is growing. Full-fledged mapping operations will begin July 24, she says.

For more information about the Gulf dead zone, including how and why it forms, check out this interactive explainer from MNN.