What are dead sea zones?
Slabs of ocean, as large as 100,000 sq km, that are so depleted of oxygen nothing can survive in them. These zones are having a devastating effect on marine ecosystems and, according to a 2006 report by the United Nations Environment Program (UNEP), they’ve swelled in number by more than a third since 2004.
Where are they?
Shrimp trawlers in the 1960s noticed areas in the Gulf of Mexico where nothing was ever caught. In the 1970s, oceanographers began to document this area, then found a similar zone near Oregon. Later, zones were identified off the coasts of China, Japan, South America, Britain, Portugal, and the Scandinavian fjords. There are two dead zones off the coast of south-east Australia.The largest zone is in the Baltic Sea.
How did this happen?
Pollution and climate change. Some dead zones are caused by agriculture: phosphorus- and nitrogen-rich fertilisers find their way to the sea and cause massive blooms of phytoplankton that die, sink and are eaten by bacteria which use up all the oxygen in the water. Dumped sewage, animal waste and fossil fuel burning are culprits too.
Also, rising land temperatures are causing changes in the wind patterns that affect currents and allow deep, oxygen-poor water to rise (‘upwelling’).
Should I be worried?
Yes. There are now more than 200 dead sea zones. Last year, the zone near Oregon grew to an unprecedented size, and went from low-oxygen to no-oxygen. Not only do these zones kill off marine life, they threaten the livelihood of people who depend on sea industries.
Can anything be done?
Yes. Dead sea zones are reversible. The Black Sea zone was once the largest in the world, but between 1991 and 2001 it almost disappeared when fertilisers became too costly for farmers to use.