The area near the mouth of the Mississippi River becomes a “dead zone” when phytoplankton die as the result of pollution and strip the water of oxygen. An area of low to no oxygen can kill fish and other marine life. U.S. National Oceanic and Atmospheric Administration scientists forecast the Gulf of Mexico “dead zone” in summer 2019 to be approximately 20,277 square kilometers (7,829 square miles), or roughly the size of the landmass of Massachusetts. The annual prediction is based on U.S. Geological Survey river flow and nutrient data. June 12, 2019.