Dy­ing oceans

Oxy­gen-de­prived zones in oceans across the world have emerged as a huge threat to marine ecosys­tems

Down to Earth - - SCIENCE - DIYA DAS

Iwith­out oxy­gen.That is MAGINESURVIVING what aquatic crea­tures in huge dead ar­eas in oceans are go­ing through the world over. Cur­rently, there are more than 400 dead zones, or ar­eas with ex­tremely low lev­els of oxy­gen, ap­pear­ing at least once a year (see ‘Suf­fo­cated spots’). They harm fish­eries, de­stroy bio­di­ver­sity of the re­gions they oc­cur in and can dam­age ma­jor ecosys­tems of the world.

Such ab­nor­mal de­ple­tion in dis­solved oxy­gen lev­els has in­ten­si­fied ex­po­nen­tially over the past 40 years. One such zone is present off the west coast of In­dia,in the Ara­bian Sea. Here, the colour of the wa­ter be­comes bright green ev­ery win­ter. The colour is so in­tense that it can be seen from space. The cause is a mi­cro­scopic plank­ton, Noc­tiluca scin­til­lan, that “blooms”and spreads over the sea sur­face.

Re­cent stud­ies have shown how marine com­mu­ni­ties are af­fected by de­plet­ing oxy­gen lev­els.A study pub­lished in in April 2015, showed that a dip in oxy­gen lev­els dras­ti­cally changed the struc­ture of the com­mu­nity of seafloor or­gan­isms, re­plac­ing the orig­i­nal bio­di­ver­sity with fewer,hardier species.Th­ese changes occurred over a rel­a­tively short pe­riod (about 150 years), but the re­cov­ery from this col­lapse took more than 1,000 years.

In­dus­trial scale use of fer­tilis­ers is one of the main rea­sons be­hind the for­ma­tion

Ex­ces­sive sewage from Mum­bai and Karachi has cre­ated huge oxy­gen-de­pleted zones in the Ara­bian Sea

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