Trouble on the bayou.

Each April, the people of Terrebonne Parish convene in Chauvin, some 80 miles south of New Orleans, for the Blessing of the Fleet, heralding the ar rival of shrimp season. As spring gathers, though, a threat to this community's traditional means of livelihood will begin to arrive. Hypoxia, or severe oxygen depletion, is forcing aquatic animals to flee their homes. Those unable to do so are dying, choking on an overload of nutrients (primarily nitrogen and phosphorus), delivered to the Gulf of Mexico via 2,552 miles of Mississippi River, creating what is commonly known as the "Dead Zone"

Hypoxia arrives in the spring, when the sun has begun to heat the Mississippi. The water layers--warmer fresh water on top; cooler, denser gulf water beneath--restrict the re-supply of oxygen to the bottom. The newly arrived nutrients stimulate the growth of phytoplankton, the decomposition of which exacerbates oxygen depletion. In autumn, the fresh water begins to cool, top and bottom waters once again mix, and oxygen levels rise.

"We talk a lot about how human activities such as paving shorelines, agricultural practices and increasing run-off of pollutants negatively affect coastal waters and resources," says Andy Shepard of the NOAA Undersea Research Center at the University of North Carolina-Wilmington. "The northern gulf is the poster child for this global issue."

Nancy Rabalais of the Louisiana Universities Marine Consortium knows these Waters well. An assessment she helped compile points to a general decline in brown shrimp catch, the most commercially valuable along the Gulf Coast, mirroring the rise in hypoxia.

As a result of two tropical storms last summer, the hypoxia-affected area Rabalais mapped in July was somewhat smaller than the previous 10-year average. Rabalais predicted the improvement would be short lived. More hypoxia was found in the following weeks. Former Louisiana state representative and local marina owner Johnny Glover recalls phoning Rabalais several years back to say, "Nancy, you're ruining my business. My fishermen aren't coming because they think there's no fish down here." Rabalais showed him her data; today, he's a staunch ally.

The Blessing of the Fleet in Chauvin is a daylong affair, the centerpiece of which is a parade that begins at St. Joseph's Catholic Church and proceeds down the bayou. Often now, shrimpers are forced to trawl farther out, past the hypoxic zone. It can make earning a living more difficult. But, at least for now, they're still bringing them in. CONTACT: Louisiana Universities Marine Consortium, [phone], www.lumcon.edu; NOAA Undersea Research Center, UNC-Wilmington, [phone], www.uncw.edu/nurc.