I have found something different that I thought might be interesting to share:
Dead zones are hypoxic (low-oxygen) areas in the world’s oceans, the observed incidences of which have been increasing since oceanographers began noting them in the 1970s. These occur near inhabited coastlines, where aquatic life is most concentrated. (The vast middle portions of the oceans, which naturally have little life, are not considered “dead zones”.) The term can also be applied to the identical phenomenon in large lakes.
In March 2004, when the recently established UN Environment Programme published its first Global Environment Outlook Year Book (GEO Year Book 2003), it reported 146 dead zones in the world’s oceans where marine life could not be supported due to depleted oxygen levels. Some of these were as small as a square kilometre (0.4 mi²), but the largest dead zone covered 70,000 square kilometres (27,000 mi²). A 2008 study counted 405 dead zones worldwide.
Another Article related to the matter:
World’s Largest Dead Zone Suffocating Sea:
Numbers of this once-threatened predator have soared from 1,000 to more than 23,000 in the Baltic Sea (map) since pollutants including DDT, an eggshell-thinning pesticide, and PCBs, chemical compounds used in electrical equipment, were banned in the 1970s, Westman said.
But there is a new danger to the eagle and many other marine species: An explosion of microscopic algae called phytoplankton has inundated the Baltic’s sensitive waters, sucking up oxygen and choking aquatic life.
Though a natural phenomenon at a smaller scale, these blooms have recently mushroomed at an alarming rate, fed by nutrients such as phosphorous and nitrogen from agricultural fertilizers and sewage. When it rains, farm fertilizers are washed into the sea. Sewage-treatment facilities also discharge waste into the Baltic ecosystem.
As a result, the Baltic is now home to seven of the of the world’s ten largest marine “dead zones”—areas where the sea’s oxygen has been used up by seabed bacteria that decompose the raining mass of dead algae.
Source with the picture : http://news.nationalgeographic.com/news/2010/02/100305-baltic-sea-algae-dead-zones-water/
More about Baltic Sea:
More about Dead Zones: