Researchers predict the recurring “dead zone” off the Louisiana coast will grow this summer to its largest size in at least 22 years, 8,543 square miles.

The forecast, released Monday by the Louisiana Universities Marine Consortium, is based on a federal estimate of nitrogen from the Mississippi River watershed to the Gulf of Mexico. It discounts the effect large storms or hurricanes might have.

The “dead zone” in the northern Gulf, at the end of the Mississippi River system, is the second-largest area of oxygen-depleted coastal waters in the world. Low oxygen, or hypoxia, can be caused by pollution from sources including farm fertilizer, soil erosion and discharge from sewage treatment plants, according to the U.S. Geological Survey.

Excess nutrients can spur growth of algae, and when the algae die, their decaying takes up oxygen faster than its brought down from the surface. As a result, fish, shrimp and crabs can die or otherwise be adversely affected, the consortium Web site says.

Eugene Turner, a professor of oceanography and coastal sciences at Louisiana State University who is involved with the report, said it’s tough to determine whether fish are dying because of hypoxia or other factors, such as climatic effects. However, “we really don’t want to mess with this, to make it worse,” he said.

The dead zone usually begins forming in the spring and exists through summer and into the fall. Though the size of the dead zone has shrunk some years, on average, it’s steadily grown larger, Turner said.

If the prediction for July stands, it would be the largest dead zone measured since mapping began in 1985, the report says. An assessment by the consortium is set to be made at summer’s end.

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