Louisiana's shrimpers expected 2010 to be a good year. Instead, they got the oil spill. Although many found temporary jobs working cleanup for BP PLC, hopes for recovery turned to 2011.

Now the swollen Mississippi River is expected to deliver another heavy blow to a seafood industry already on the ropes: a massive flush of fertilizer, animal manure, treated sewage, pesticide and urban runoff.

Scientists predict this polluted wash will give rise to the Gulf of Mexico's largest-ever "dead zone," a large swath of ocean devoid of fish, shellfish and other marine life.

"It's a disaster in the making," said Clint Guidry, a third-generation Louisiana fisherman and president of the Louisiana Shrimp Association. "Everybody paid their taxes and fixed their boat up, and they were ready to go back to work this year. It's not looking good."

The same dirty water is expected to wreak havoc on Lake Pontchartrain, just north of New Orleans, by setting off a toxic algae bloom this summer predicted to fill the lake with slime, kill fish, and cause respiratory problems to those who go near it.

"We expect them to have to close the lake, and we expect it to last for months," said Gene Turner, a professor in the oceanography department at Louisiana State University. "It'll really be a soup out there."

Hurricane to the rescue?

The dead zone in the Gulf of Mexico is anticipated to be the largest measured since the early 1970s, when scientists began studying the annual phenomenon that generally appears in March and lasts through September.

The Gulf is essentially fertilized when phosphorus and nitrogen from farm runoff throughout the Corn Belt and treated sewage and wastewater from cities as far north as Chicago wash down the Mississippi River. The result is an algae bloom in the warm, shallow waters of the Gulf that create a lifeless zone 60 miles out from the coastline. The dead zone runs from the Mississippi Delta as far as west as Galveston, Texas.

Both the algae and the bacteria that feed on algae after they die consume the Gulf's dissolved oxygen, creating a state of hypoxia that forces the fish and shellfish that can to flee and leaves the rest to suffocate.

This year's dead zone will likely be at least 5 to 10 percent larger than the largest to date, which appeared in 2002 and spanned 8,500 square miles, said Nancy Rabalais of the Louisiana Universities Marine Consortium.

Rabalais, who takes to the water every year to study the dead zone and is considered among the foremost authorities on the subject, said only a major storm could neutralize the threat by stirring the Gulf and re-oxygenating the water.

"We've been doing these cruises since 1985, and there's no doubt, unless we get a hurricane or tropical storm, this should be the largest one ever," Rabalais said.

The size of the dead zone has more than doubled over the years since scientists began measuring it, with the biggest jump in size coming after floods in 1993. Like today's, that flood shunted huge volumes of polluted runoff from throughout the sprawling Mississippi River Basin, which drains 41 percent of the lower 48 states directly into the Gulf through the funnel of the delta.

"There's a huge area the size of Massachusetts where you just can't catch anything for a long period of time," Rabalais said.

This puts a major strain on the fishing industry, forcing bigger boats to spend the extra time and fuel to fish around the zone and potentially leaving smaller operators high and dry.

"The bigger the zone, the farther they have to look for fish," Turner said.

Turner and Rabalais, husband and wife experts in this area of Gulf chemistry, have not yet officially made their prediction as to the dead zone's size. They are waiting to receive final numbers for May from the U.S. Geological Survey on the nitrate concentrations coming down the river.

"I haven't quite decided what the size is going to be, except for sure it's going to be 5 or 10 percent larger than anything we've seen before," Turner said.

Corn, continental shelf at play

The area of the continental shelf could be the limiting factor, since the dead zone can only form in roughly 200 meters or less of water, where water can separate into the necessary layers, Turner said.

A heavy dose of nitrates was expected to flow down the Mississippi River this year as a result of high corn prices prompting a big planting that would, in turn, result in more fertilizer runoff.

If a record-breaking dead zone does appear and then recedes in the fall, that does not mean a return to normal. The dead zone exhibits a legacy effect, in which the effects of the nitrates pollution tend to accumulate from year to year and help explain why the dead zone gets larger and larger.

"At some point we're going to get twice the size of the zone for the same amount of nitrogen coming in," Turner said.

That is bad news for shrimpers like Guidry, who say this year's catches have already been thin.

"It's off to a real slow start. We're not seeing a lot of shrimp. And that's prior to the high river event," Guidry said. "It really has all the ingredients to be terrible."

But Dean Blanchard, owner of Dean Blanchard Seafood Inc. in Grand Isle, La., says the dead zone is "the least of my worries."

Blanchard, the largest shrimp buyer in the United States prior to the Deepwater Horizon spill, said his daily business remains down 60 percent, or $25,000 per day, since before the spill.

"Normally, we'd be very concerned," Blanchard said of the dead zone. "But you can't kill something twice. It can't make it no worse."

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