Sea•thospedia: What are Dead Zones?

Oct 25 2011

Dead zones are a very unfortunate oceanic occurrence.  They form in hypoxic (very low-oxygen) areas in the ocean near coast lines where aquatic life is abundant. Dead zones are caused by an increase of chemical nutrients such as phosphorous and nitrogen in the water, a process known as eutrophication. These chemicals enter into the ocean through runoff from human activities such as agriculture and urban development.  One of the main chemical sources is fertilizer used in agriculture. 

These chemicals are the nutrients that help form phytoplankton- plant like organisms that swim freely near the surface of the ocean. An increase of these chemicals in the water lead to excessive growth of phytoplankton, particularly algae, a process known as algal bloom. Algae depletes oxygen from the water at night when it under goes a process known as cellular respiration.

When algae dies, it sinks to the ocean bottom and is decomposed by bacteria which uses up oxygen in the water and further depletes oxygen levels.  These two processes, excessive algae growth and algae decomposition, result in areas with very low oxygen (hypoxic areas). Fish and other marine organisms need oxygen to survive. Starved of oxygen, marine life either leave to other areas or die, and the hypoxic area becomes a dead zone.  

 As you can see in the pictures, dead zones can be seen from the surface of the ocean and they are not a pretty sight. There is no fishing in these areas because there is literally no marine life to fish! Sadly, the incidences of dead zones have become more numerous since they were first discovered in the 1970′s.

A major dead zone near the U.S. is in the Gulf of Mexico (pictured to the right) which formed from chemical runoff that enters the gulf from the Mississippi river. Check out this website about the Gulf of Mexico dead zone, it has a lot of cool interactive information including maps and videos! Let’s stop dead zones from forming by reducing the amount of chemicals released into the ocean!


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