Ocean Oxygen Content Down Up To 40% In Past 50 Years In Some Tropical Areas; Global Average Down 2%
Escaping predators, digestion and other animal activities—including those of humans—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. Some coastal regions around the equator naturally are low-oxygen hotspots because they contain nutrient-rich waters where bacterial blooms consume oxygen as they break down dead marine life. But shifts in ecosystems elsewhere—including in the open ocean and around the poles—especially surprises and concerns Oschlies and others because these regions were not considered as vulnerable. Climate models projecting future change have also routinely underestimated the oxygen losses already observed around the world’s oceans, he and his colleagues reported in Nature last year—another reason why this trend calls for more attention, he says.
EDIT
https://www.scientificamerican.com/article/the-ocean-is-running-out-of-breath-scientists-warn/
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