The Ocean Is Running Out of Breath, Scientists Warn

https://www.scientificamerican.com/article/the-ocean-is-running-out-of-breath-scientists-warn/?fbclid=IwAR3edpsO7Yh45goSzNQ00ZGbu4Wp-ubkwlXnDb0AvuK3yuFbuuq16BL8f5A

By Laura Poppick on February 25, 2019

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.

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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.

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As oxygen-rich regions become scarcer, current fish habitats will also shrink and force economically important species—such as tuna, which globally generate an estimated $42 billion annually—into new ranges. In the northeastern tropical Atlantic researchers have found habitat for tuna as well as billfish fisheries shrank by 15 percent from 1960 to 2010 (pdf) due to oxygen loss.

Coastline fisheries can also face the added pressure of agricultural runoff fertilizing algal blooms that consume copious oxygen as they decay—as has long been the case in the Gulf of Mexico near the mouth of the Mississippi River. These “dead zones” force some fishes to seek higher oxygen areas on the edges of their typical ranges. This can help fishermen find them because the fishes congregate in these condensed areas, but it also provides a false sense of abundance and will not be sustainable in the long-term, Seibel notes.

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