A dead zone, or as scientists prefer, a hypoxic zone, is an area in estuaries and coastal waters around the mouth of larger rivers entering the sea where the oxygen content is too low to support most forms of marine life. Seminal research conducted by two marine ecologists in 2008, Dr. Bob Diaz (Virginia Institute of Marine Science) and Dr. Rutger Rosenberg (University of Gothenburg in Sweden) found that the number of such zones had increased by one third between 1995 and 2007.
The United Nations currently estimates the number of dead zones at more than 500 globally, covering 250,000 square miles, with no significant coastal waters left untouched by this phenomenon. Some of the world’s larger lakes and bays, such as Lake Erie (U.S. Upper Midwest), Lake Taganyika (East Africa), and Lake Titicaca (South America), as well as the Caspian Sea (Central Asia) and the Chesapeake Bay (U.S. East Coast) also have sizable hypoxic zones.
The first references to such low oxygen zones were found in scientific journals in the early 20th Century according to research by Dr. Diaz. The phrase ‘dead zone’ was first used in a 1971 article about the environmental impacts of industrial development in the Soviet Union. Such zones have likely existed throughout history, but it has only been in recent decades that their number and magnitude have become a matter of public policy concern.
The low oxygen content of these waters is primarily the result of nitrogen and phosphorus fertilizer seeping from fields and golf courses as agricultural and urban runoff (, making its way from local streams and rivers into the main river of major watersheds, such as the Mississippi River watershed which empties into the Gulf of Mexico (known officially at the federal level as the Gulf of America) about 100 miles southeast of the city of New Orleans, LA. The Mississippi watershed covers parts or all of 31 U.S. states and two Canadian provinces, including some of the most productive (and heavily fertilized) farmland in the world in states like Iowa, Illinois, and Minnesota. The total watershed (also known as river basin or drainage area) of the Mississippi system encompasses 1.2 million square miles.
While the hypoxic zone at the mouth of the Mississippi River draining into the Gulf of Mexico is not the largest one in the world–that dubious honor belongs to the Gulf of Oman in the Arabian Sea, which has multiple rivers from Iran and wadis from Oman discharging into it–the Gulf of Mexico’s hypoxic zone is probably the most heavily studied in the world.
U.S. shrimpers first reported a dead zone affecting the size of their catches in the Gulf of Mexico in 1950, but serious research into the causes of that phenomenon did not begin until 1970. Extensive research has been conducted into the causes of the zone by U.S. marine scientists, especially a team from Louisiana State University in Baton Rouge headed by Dr. Nancy Rabalais starting in 1985. As with nearly every hypoxic zone, the Gulf of Mexico zone varies in size over the course of a year, with the peak coming in late summer as the applied fertilizer in Midwest fields that is not taken up by the corn and other grains being grown seeps into the subsidiary streams and rivers that feed the Mississippi River.
The Gulf’s hypoxic zone also varies in size between years, depending in large part on the amount of rainfall received upriver in the Mississippi River basin and the amount of nutrients captured in those waters. As of July 2025, the five-year average size was 4,755 square miles in area. The National Oceanic and Atmospheric Administration (NOAA) has estimated the area of the hypoxic zone for the last 34 years, the work always conducted in late July to maintain data consistency.
In the fall of 1997, the Environmental Protection Agency (EPA) established a Hypoxia Task Force to better understand the causes and effects of eutrophication in the Gulf of Mexico and coordinate activities to reduce the size of the dead zone. Experts and officials were appointed to this Task Force by both the federal government and from state government agencies within the Mississippi River basin. In 2001, the Task Force issued an Action Plan describing a national strategy to reduce the frequency, duration, size and degree of oxygen depletion of the Gulf’s hypoxic zone. This plan was updated in light of new scientific findings in 2008.
In response to the 2008 Action Plan, states within the Mississippi River basin began work in 2012 to develop nutrient management plans to reduce the nitrogen runoff levels identified as coming from their lands. That effort was completed by 2015, and each state issues regular reports on progress that has been made to address the causes of the Gulf hypoxic zone. The Task Force set a long-term goal to reduce the size of the hypoxic zone to less than 1,900 square miles by 2035 in terms of a five-year average. The most recent estimate described above is about 250 percent greater than the target level.
Each state’s nutrient management strategy is different, although for the most part they focus on ways to encourage farmers to reduce fertilizer use and/or runoff by greater adoption of conservation practices known to reduce runoff, such as reduced tillage and use of cover crops. One such piece was the decision by the state of Iowa to offer a $5 per acre incentive under the federal crop insurance program to farmers adopting cover crops in 2017, a program copied by Illinois, Indiana, and Wisconsin in subsequent years. About 15 other states provide other forms of financial assistance to farmers to encourage cover cropping, most although not all of them within the Mississippi River basin. Some states have documented reductions in runoff from their strategies, such as Indiana’s data indicating that 1.7 million tons of sediment, 4.3 million pounds of nitrogen, and 2.2 million pounds of phosphorus were prevented from entering Indiana waterways through use of cover crops in 2022. However, the overall state of the Gulf’s waters has not improved in recent years, likely a result of more frequent heavy rainfall events across the Midwest, believed to be caused to some degree by climate change.
