What is a Dead Zone?



Dead zones are hypoxic areas, which are low-oxygen areas in the world’s oceans. This is usually caused by excessive nutrient pollution from human activities. The wastes that go into the water are primarily nitrogen and phosphorous from agricultural runoff, sewage and even industrial emissions (McLamb 2008). Thus, oxygen needed for marine life is depleted in these areas of high concentrated discharge, generally along coastlines. Dead zones started to suddenly increase in the 1970s and oceanographers started to notice a decrease in marine life. According to scientists, the number of dead zones has doubled every 10 years since 1960 (Murray 2006). This is attributed to the tremendous increasing conversion of forests and wetlands to agricultural and urban developments that took place in 1950, resulting in more waste and nutrients released into the water.

The major cause of dead zones is an increase in chemical nutrients in water called eutrophication (Steer 1982). These nutrients act as fundamental building block for single-celled organisms named phytoplankton, which lead to a rapid increase in density in water. Increase of nitrogen and phosphorous in water cause cyanobacteria, a type of algae, to increase. As the algae die, bacteria decompose the dead algae and consume huge amounts oxygen. Since the bacteria are consuming oxygen in such large numbers, this eventually leads to extremely low levels of oxygen in the bodies of water. Also, natural phenomena can cause lack of oxygen in water like enclosed bodies of water. Dead zones occur in many areas of the country, especially along the East Coast, the Gulf of Mexico and the Great Lakes. UN Environment program published Global Environment Outlook Year Book, which reported that 146 dead zones exist in the word with various sizes (UNEP 2004).

The Gulf of Mexico dead zone is an area of hypoxic water at the mouth of the Mississippi River. It usually covers up to 6000 to 7000 square miles. The Gulf of Mexico is a major source for the seafood industry, but because dead zones are killing marine life, these industries are having trouble making business. 

This graph shows that the size of dead zone in the Gulf of Mexico has doubled in size over the past 25 years. For example, from 1986 to 2006, the size of dead zone in the Gulf of Mexico more than doubled.

Works Cited

Steer, A. (1982) About out Eutrophication. Eutrophication and Hypoxia. World Resources Institute. (Date Accessed: February 21, 2013.) http://www.wri.org/project/eutrophication/about

Murray, L. (2006) Dead Zones - Investigate current research. Teach Ocean Science. Teach Ocean Science. (Date Accessed: February 21, 2013.) http://www.teachoceanscience.net/teaching_resources/education_modules/dead_zones/investigate_current_research/

McLamb, E. (2008) Ocean Dead Zones Expanding. Ecology.  Ecology Global Network. (Date Accessed: February 21, 2013.) http://www.ecology.com/2008/08/19/ocean-dead-zones-growing/

Steer, A. (1982) About out Eutrophication. Eutrophication and Hypoxia. World Resources Institute. (Date Accessed: February 21, 2013.) http://www.wri.org/project/eutrophication/about

Steer, A. (1982) About out Eutrophication. Eutrophication and Hypoxia. World Resources Institute. (Date Accessed: February 21, 2013.) http://www.wri.org/project/eutrophication/about

United Nations Environment Programme. (2004) Further Rise in Number of Marine ‘Dead Zones.’  United Nations Environment Programme. United Nations Environment Programme. (Date Accessed: February 21, 2013) http://www.unep.org/Documents.Multilingual/Default.aspDocumentID=486&ArticleID=5393&l=en