As Earth is warming up, the ocean is losing its breath and can cause serious damage to marine life, affect livelihoods of millions and trigger release of dangerous greenhouse gases, says study.
Over the past half century, the open ocean has lost around 2 per cent of its dissolved oxygen, vital for sustaining fish and other marine life.
There has also been a ten-fold increase in low oxygen sites, known as “dead zones”, in coastal regions during this period.
Oxygen saturation is a major limiting factor that affects ocean productivity, as well as the diversity of creatures living in it and its natural geochemical cycling.
The new study, published in the journal Science, represents the most comprehensive view yet of ocean oxygen depletion.
Pollution and climate change both play significant roles in depleting the ocean’s oxygen levels and the authors emphasise the role humans must play in addressing these issues.
“Oxygen is fundamental to life in the oceans,” said lead author Dr Denise Breitburg, a marine ecologist with the Smithsonian Environmental Research Centre. “The decline in ocean oxygen ranks among the most serious effects of human activities on the Earth’s environment.”
The work was published by scientists from GO2NE – Global Ocean Oxygen Network – a United Nations working group set up to investigate the impact of oxygen loss from the oceans.
“Combined effects of nutrient loading and climate change are greatly increasing the number and size of ‘dead zones’ in the open ocean and coastal waters, where oxygen is too low to support most marine life,” said Dr Vladimir Ryabinin, executive secretary of the International Oceanographic Commission, which formed GO2NE.
Nutrient loading refers to pollution from sewage and fertiliser run-off that contains nutrients that stimulate the growth of algae in the water. Blooms of algae form and when they die the bacteria that degrades them consumes the oxygen present in the water.
Warming surface waters resulting from climate change also make it more difficult for oxygen to penetrate into the ocean’s depths.
As the entire ocean gets warmer, this effect means less oxygen can be held inside it.
In dead zones oxygen levels tend to be so low that any animals living there suffocate and die. As a result, marine creatures avoid these areas, resulting in their habitats shrinking.
Even in areas where oxygen depletion is less severe, smaller decreases in oxygen levels can impact animals in various non-lethal ways such as stunting their growth and hindering reproduction.
The researchers warned that the effects of oxygen depletion in the oceans are extensive and ecological impacts go hand in hand with direct effects on the humans that rely on the sea for their livelihoods.
“It’s a tremendous loss to all the support services that rely on recreation and tourism, hotels and restaurants and taxi drivers and everything else,” said Dr Lisa Levin, a biological oceanographer at Scripps Institution of Oceanography, one of the study’s co-authors.
“The reverberations of unhealthy ecosystems in the ocean can be extensive.”
Lyndsey Dodds, head of UK marine policy at WWF, added: “This just shows even more pressures on our oceans. We hear a lot about plastics now, and unsustainable fishing, but this is certainly an issue that seems to have less attention, despite the potentially catastrophic impact.”
However, the authors of the new study state that despite the dramatic figures, the problem of oxygen depletion can be dealt with.
They highlight efforts to provide better sewage treatment in Chesapeake Bay in the eastern US, which have resulted in substantial increases in water oxygen levels.
“This is a problem we can solve,” said Dr Breitburg. “Halting climate change requires a global effort, but even local actions can help with nutrient-driven oxygen decline.”