The world's oceans have lost more than two per cent of their oxygen since 1960, with potentially devastating consequences for sea plants and animals, marine scientists said Wednesday.
In those five and a half decades, parts of the oceans devoid of oxygen, called anoxic waters, have quadrupled, said a study in the science journal Nature.
And the production and flow of nitrous oxide, a powerful greenhouse gas, "will probably have increased", it said.
Oceans cover nearly three-quarters of the Earth's surface, provide about half of the oxygen we breathe and feed billions of people every year.
In a comment on the study that Nature also published, research scientist Denis Gilbert of Fisheries and Oceans Canada wrote that a "twoper cent decrease of ocean oxygen content may not sound like much".
But, he warned, "the implications of this for marine ecosystems couldbe severe in parts of the ocean where oxygen is already low".
The report found that the largest decrease happened near areas where oxygen was already low, in so-called "dead zones", where oxygen levels declined by 4 per cent every decade.
Most oxygen was lost in the Equatorial and North Pacific Ocean, the Southern Ocean, and the South Atlantic Ocean.