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.
If the prediction stands, it would be the largest dead zone measured since mapping began in 1985, the report says. An assessment is set for the end of the summer.