by Judith Willson

Created on: September 12, 2010   Last Updated: October 18, 2010

The Gulf of Mexico has the second largest dead zone in the world.  Dead zones are regions of the ocean that have become unusually depleted of oxygen and can no longer support oxygen dependent marine life.  They can be caused naturally and are not necessarily permanent. In the Gulf of Mexico the cause of the dead zone was the same as all the others. 

Ironically, dead zones are caused by too many nutrients.  This can be due to upwelling, when deeper water full of nutrients comes to the surface, or run-off from rivers.  Human use of fertiliser and disposal of sewage provides the nutrients for most of the ocean dead zones we now see.

The nutrient rich waters support an abundance of phytoplankton (algae), which is often good news for life in the region.  However when there is too much algae for other small organisms to eat it will die and decompose.  As bacteria decompose organic matter the oxygen is removed from the water, resulting in hypoxic (low oxygen) environments.  Most other marine life cannot cope with these.  Rising ocean temperatures may be exacerbating the problem as less oxygen can be dissolved in warmer waters.  

Ocean ecosystems develop a balance and when this is drastically changed, problems arise.  Too many nutrients are as bad as too few.  An excess of nitrogen and other nutrients effectively kills off an area of the Gulf of Mexico every summer.

Nitrogen and phosphorus rich fertilisers keep farmland in the United States productive, but plenty runs off into streams and rivers before it is used by the crops.  This ends up in the oceans, supporting only blooms of algae.   Human and animal waste also contributes.  Even polluted rainwater containing nitrous oxides plays a part. 

The Gulf of Mexico dead zone starts from the mouth of the Mississippi river, and this is where most of the nutrients are coming from.  The area the dead zone covers varies, but is around six to seven thousand square miles, rising to over ten thousand on occasion.  The dead zone is primarily an annual phenomenon, beginning in the spring and continuing through until late summer.   It was first noticed by marine scientists during the 1980s but had probably been developing for several decades previously.

Benthic (sea floor) organisms are most affected.  This is where the dead zones manifest and also slow moving animals cannot physically remove themselves

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