Unusually robust spring floods in the U.S. Midwest are flushing agricultural runoff—namely, nitrogen and phosphorus—into the Gulf and spurring giant algal blooms, which lead to dead zones, or areas devoid of oxygen that occur in the summer.

The forecast, developed by the University of Michigan and Louisiana State University with support from the U.S. National Oceanic and Atmospheric Administration, estimates a Gulf dead zone of between 7,286 and 8,561 square miles (18,870 and 22,172 square kilometers). The largest ever reported in the Gulf, 8,481 square miles (21,965 square kilometers), occurred in 2002.

On the flip side, the Chesapeake Bay—the country's biggest estuary—will likely experience a smaller-than-average dead zone this summer.

The forecasts are made using computer models, which are based