Not to be confused with the Stephen King novel of the same name, or the movie starring Christopher Walken, the Gulf of Mexico hypoxic zone or, as it's popularly called, the dead zone, is a patch of ocean off the coast of Louisiana in which marine life cannot be sustained from Spring until Autumn, due to extremely low oxygen content in the water. In the summer of 2001 it covered an area of over 7000 square miles, roughly the size of New jersey, and within that space every living thing either has to leave or die. Divers who have gone down in the dead zone report a totally empty sea with nothing moving, and a sea floor covered with dead crabs, worms and fish.

The dead zone is caused by the enormous amount of dissolved phosphates and nitrates pouring into the sea from the mouth of the Mississippi river. These chemicals are heavily used in fertilizers in the United States, and their use has been increasing steadily since the 1960's. Fertilizers help plant life to grow, and it follows that when you pour them into the sea in huge volumes, something is going to grow. In this case, it's phytoplankton, or algae. Algal blooms have been observed forming on all the world's oceans in larger and larger numbers since the 1980s, and are one of the 'warning signs' for the environmental equivalent of the Doomsday Clock. When summer comes and the temperature rises, these algae swarm on the surface in huge numbers in a process known as eutrophication. Since algae produce oxygen, this might not seem like such a problem, but the oxygen stays near the surface, and the real problem is what happens when the algae die. They float down to the bottom of the sea, where they are consumed by anaerobic bacteria. The more algae die, the more bacteria can feed on them, and the more oxygen those bacteria remove from the bottom waters of the ocean. In the dead zone, this process continues until the oxygen content of the water falls below the critical point for sustaining life, two milligrams per liter. When this hypoxic zone is created at the bottom of the sea, the entire food chain is broken. Bottom-dwelling fish and crustaceans die, and the fish that feed on them die, and the whole process continues until the sea is emptied of life and the sea floor is a graveyard, covered in patches with a dark brown-black bacterial sludge.

The Mississippi and its tributaries and streams drain an enormous area of the United states, covering 31 states and most importantly covering farmland in Minnesota, Iowa, Illinois, Indiana and Ohio, states which are responsible for most of the nitrogen and phosphorous in the river. Nitrate concentrations in farmed soil have tripled in the last 30 years as farming becomes more and more intensive, and farms are also responsible for 1.3 billion tons of livestock manure, most of which eventually finds its way into the sea. In addition to this, the Mississippi contains untreated sewage and other types of urban pollution. For a long time it was thought that the sea was so big that humans could pour their waste into it without end, and without disrupting its ecosystems, but that myth is starting to be dispelled as it becomes more and more difficult to ignore the evidence. As the National Ocean Service puts it:

"Nutrient over-enrichment from anthropogenic sources is one of the major stresses impacting coastal ecosystems."

There is also some evidence that the same hypoxic phenomena that produce the dead zone are also responsible for other strange water conditions in the Gulf of Mexico, such as the red tides, huge, toxic algal blooms that poison fish and shellfish and make them in turn poisonous to humans. The algal blooms in the dead zone are non-toxic, but the hypoxic conditions may indirectly lead to the production of these toxic blooms, which have so far rendered 50% of the shellfish-producing areas of the Gulf coast unusable or 'permanently off-limits'.

The dead zone first reached close to its current size after the Mississippi flooded in 1993, and has remained steady since then, probably aided by the advance of global warming, providing higher water temperatures for the algae to bloom. If it stayed confined to the Gulf of Mexico it might be easy to ignore, but similar, less extreme hypoxic conditions are being observed in coastal waters all around the United States, leading some researchers to believe that large swathes of the ocean are in danger of being rendered permanently uninhabitable. Leaving aside the ugliness and danger of being entirely surrounded by dead, bacteria-infested water, the impact of this on the US fishing industry would of course be catastrophic, and, as usual, this might be the stick that makes the government take notice and introduce some hefty restrictions on the use of chemical fertilizers before it's too late. The problem is that the industry which would be required to make sacrifices (the farming industry, which would be required to cut nitrate use by up to 50%) is unconnected to the fishing business, which would be the main beneficary in financial terms. It is not enough, therefore, simply to introduce restrictions and regulations - a complex and expensive rescue package will have to be put together by the US government. That the money can be raised for such a venture is clear from the quick commitment of tens of billions of dollars recently to the War on Terrorism and the Son of Star Wars plan, and the eventual financial return, in terms of increased marine life available for fishing, should be enough to persuade a far-sighted administration to do something before it's too late.

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