Natural processes used to occasionally create a “dead zone” of oxygen-poor water grows in the waters of the Gulf of Mexico. It would happen when nutrients from upstream got washed out to sea, feeding phytoplankton in the area. As that phytoplankton population exploded, and then decomposed, bacteria would absorb the water’s oxygen, making it deadly to animals, from crustaceans to fish.
But these days farming fertilizers provide a regular source of nutrients to the phytoplankton, and the dead zone is a regular thing. NASA’s Earth Observatory explains:
Once infrequent, the Gulf of Mexico dead zone is now an annual event, triggered by phosphorus and nitrogen in fertilizers used on farms throughout the central United States and as far away as Saskatchewan, Canada. In the fall, strong winds from seasonal storms stir the water, mixing the oxygen-poor deep water with oxygen-rich surface water, bringing a reprieve until the next spring.