Map showing distribution of bottom-water dissolved oxygen from July 28 to August 3, 2015, west of the Mississippi River delta. Black lined areas — areas in red to deep red — have very little dissolved oxygen. | National Oceanic and Atmospheric Administration

The Gulf of Mexico’s ‘Dead Zone’ Could Nearly Double in Size This Year

Agricultural runoff and biological waste from the Mississippi River fuel algae blooms each year in the Gulf of Mexico, depleting marine oxygen and sometimes triggering massive die-offs.