Ocean dead zones are ocean regions where the amount of oxygen dissolved in the water has fallen so low that most or all marine life is incapable of surviving and has either died or fled to nearby, more oxygen-rich areas. There are currently several hundred known dead zones around the world, ranging from less than a square mile to tens of thousands of square miles in surface area.
– About Dead Zones –
Dead zones can result when the number of basic chemical nutrients in the water increases to the level that eutrophication occurs, in which enormous blooms of single-celled and plant-like organisms such as algae occurs, using up available oxygen. (Algae do produce oxygen through photosynthesis to an extent during the day, but consume more through respiration at night, and through decomposition after they die.) The result is that oxygen in the area is drawn out of the water, leaving less available for other marine life. If oxygen levels fall far enough, then marine life is unable to survive on what remains. The process would be exactly analogous to what surface life, including humans, would suffer if oxygen were somehow drawn out of the atmosphere.
Although algae blooms are a well-understood natural cause of oxygen depletion, it is believed that an increase in dead zones in recent times is also the consequence of human alteration and pollution of the environment. In particular, chemicals released into the water via fertilizer, sewage, and city waste can cause dead zones or eutrophication (the process described above).
There is, of course, a considerable range of increasingly severe effects resulting from lowering oxygen levels, rather than a simple “termination” point. As oxygen levels fall, the evidence suggests that fish reproductive rates decline. Fish and fast-moving animals probably move toward oxygen-rich areas where possible, but small bottom-dwellers, like clams and lobsters, as well as animals naturally found in large and more-or-less immobile colonies, would not be able to relocate in the same way and, if oxygen levels fall far enough, must perish.
Several hundred dead zones have been identified in oceans around the world. These include dead zones in the northern Gulf of Mexico, the Pacific Northwest, and parts of the Black Sea, the latter of which is a natural phenomenon resulting from stagnant water. Those which attracted the most early concern were in the Chesapeake Bay, U.S., the Kattegat strait in Scandinavia, and elsewhere. Where the Mississippi deposits the detritus of Midwest agriculture into the northern Gulf of Mexico, a gargantuan dead zone has developed which spans over 8500 square miles. The Gulf of Mexico will likely develop another massive dead zone as a result of the BP oil spill at Deepwater Horizon, oil plumes from which will have a devastating effect on marine life.
Where dead zones are a product of human pollution, in theory they can be mitigated or even eliminated through conservation efforts. The Black Sea itself became largely a dead zone as a result of fertilizer pollution during the Soviet era, but has recovered to a considerable extent since the Soviet collapse took with it much of the agricultural activity which affected the sea. European and North American pollution control efforts have also had some success reducing dead zones resulting from pollutants running out to sea via major rivers.