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Ocean oxygen content (2016)


  • Dissolved oxygen in sea water affects the metabolism of species. Therefore, reductions in oxygen content (i.e. hypoxic or anoxic areas) can lead to changes in the distribution of species, including so called ‘dead zones’.
  • Globally, oxygen-depleted areas have expanded very rapidly in recent decades. The number of ‘dead zones’ has roughly doubled every decade since the 1960s and has increased from about 20 in the 1950s to about 400 in the 2000s.
  • Oxygen-depleted zones in the Baltic Sea have increased more than 10-fold, from 5 000 to 60 000 km2, since 1900, with most of the increase happening after 1950. The Baltic Sea now has the largest dead zone in the world. Oxygen depletion has also been observed in other European seas in recent decades.
  • The primary cause of oxygen depletion is nutrient input from agricultural fertilisers, causing eutrophication. The effects of eutrophication are exacerbated by climate change, in particular increases in sea temperature and in water-column stratification.


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