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Oxygen Levels in Oceans are Dropping

Posted: 03.10.2019 no comments


Widespread and sometimes drastic marine oxygen declines are stressing sensitive species — a trend that will continue with climate change.


Read the entire article at Scientific American


Escaping predators, digestion and other animal activities require oxygen. But that essential ingredient is no longer so easy for marine life to obtain, several new studies reveal. In the past decade ocean oxygen levels have taken a dive—an alarming trend that is linked to climate change, says Andreas Oschlies, an oceanographer at the Helmholtz Center for Ocean Research Kiel in Germany, whose team tracks ocean oxygen levels worldwide. “We were surprised by the intensity of the changes we saw, how rapidly oxygen is going down in the ocean and how large the effects on marine ecosystems are,” he says.



It is no surprise to scientists that warming oceans are losing oxygen, but the scale of the dip calls for urgent attention, Oschlies says. Oxygen levels in some tropical regions have dropped by a startling 40 percent in the last 50 years, some recent studies reveal. Levels have dropped more subtly elsewhere, with an average loss of 2 percent globally.


Ocean animals large and small, however, respond to even slight changes in oxygen by seeking refuge in higher oxygen zones or by adjusting behavior, Oschlies and others in his field have found. These adjustments can expose animals to new predators or force them into food-scarce regions. Climate change already poses serious problems for marine life, such as ocean acidification, but deoxygenation is the most pressing issue facing sea animals today, Oschlies says. After all, he says, “they all have to breathe.”


A warming ocean loses oxygen for two reasons: First, the warmer a liquid becomes, the less gas it can hold. That is why carbonated beverages go flat faster when left in the sun, Oschlies says. Second, as polar sea ice melts, it forms a layer of buoyant water at the sea surface above colder, more saline waters. This process creates a sort of lid that can keep currents from mixing surface water down to deeper depths. And because all oxygen enters this habitat at the surface — either directly from the atmosphere or from surface-dwelling phytoplankton producing it during photosynthesis — less mixing means less of it at depth.


To address the overall deoxygenation problem, Oschlies helped organize an international conference on the subject in Kiel last September. Attendees drafted an impromptu declaration called the Kiel Declaration on Ocean Deoxygenation to raise awareness among international governments, the United Nations and the public as well as to call for immediate action. They want governments and international groups to make more serious strides to slow climate change and cut back on the coastal runoff pollution that exacerbates oxygen decline. The researchers modeled the new declaration after the Monaco Declaration (pdf), which Oschlies thinks successfully helped raise international awareness around ocean acidification in 2008.