text:  The dead zone in the Gulf of Mexico is caused by excess nutrient pollution flowing downriver from the Mississippi River watershed, which drains more than 40 percent of the continental U.S. Efforts to reduce runoff from farms and cities have not made much progress toward a federal goal to reduce the dead zone to 1,900 square miles by 2035. The dead zone can impact key fisheries, for instance, the Gulf shrimp harvest.