The dead zone in the Gulf of Mexico has doubled in size since last year to 7,728 square miles, larger than it’s ever been before — and larger than New Jersey. The dead zone, a layer of water at the bottom of the Gulf so low in oxygen that it can’t support life, forms annually, in the spring and early summer. Scientists attribute much of the blame for the dead zone to the use of nitrogen-based fertilizers by farmers along the Mississippi and its tributaries, which run into the Gulf.