The document discusses the "dead zone" in the Gulf of Mexico caused by hypoxia from nutrient pollution from the Mississippi River watershed. It notes that the dead zone can be up to 22,000 square kilometers and is widespread and severe from June to September each year. The hypoxia is caused by excessive nutrients, particularly nitrogen and phosphorus from agricultural runoff, flowing down the Mississippi River which leads to an overgrowth of algae that sinks and decomposes, depleting oxygen levels in bottom waters.