Marine dead zones are hypoxic (or low-oxygen) areas in the world's oceans and lakes caused in large part by massive algal blooms. Huge blooms form near the mouths of major rivers as nutrients from agricultural runoff, such as nitrogen and phosphorus, pump into the sea. As the microorganisms feast on the influx of nutrients, the murky bloom can grow to a mammoth size, essentially snuffing out all the oxygen in the water.
The largest dead zone in the U.S., and second largest in the world, forms annually at the mouth of the Mississippi River in the Gulf of Mexico (it is shown here in red). The algal bloom can cover an area in excess of the size of New Jersey, and has become a major threat to the local fishing industry.