How Fertilizer Creates Dead Zones in the Ocean

Below is a clear definition of how dead zones are formed in the ocean. They're called a hypoxic zones. They are regions in the ocean that have such a low level of oxygen that animal life suffocates and dies. A dead zone forms in the Gulf of Mexico every summer. The following is excerpted from the book, The Man Who Planted Trees: A Story of Lost Groves, the Science of Trees, and a Plan to Save the Planet by Jim Robbins:

In the 1960s there were 49 hypoxic zones in the world’s oceans, and that number has doubled every decade since. Dead zones are caused by nitrogen and phosphorus, largely from agricultural fertilizers. As the nutrients run off farm fields and make their way into the ocean, they fertilize the blooms of naturally occurring algae. When the algae naturally die and sink to the bottom of the sea, bacteria feed on them and consume the dissolved oxygen that is needed to sustain fish and other life. As the bacteria continue to eat, the level of dissolved oxygen declines to the point where the ocean in that region is no longer able to support life. The most infamous of dead zones, extending over 8,500 square miles—bigger than the state of New Jersey—is in the Gulf of Mexico, fed by nutrient runoff from large-scale agriculture along the length of the Mississippi River.