Oceans Are Losing Oxygen—and Becoming More Hostile to Life

Posted on National Geographic: 13 Mar 2015 — By Craig Welch — Low-oxygen areas are expanding in deep waters, killing some creatures outright and changing how and where others live. It may get much worse.

 The diving patterns of Atlantic sailfish, like this one going after sardines in the Caribbean, and blue marlin helped scientists figure out that many fish are spending more time in shallower water as low-oxygen zones push closer to the surface. 


Marlin and sailfish are the oceans’ perfect athletes. A marlin can outweigh a polar bear, leap through the air, and traverse the sea from Delaware to Madagascar. Sailfish can outrace nearly every fish in the sea. Marlin can hunt in waters a half mile down, and sailfish often head to deep waters too.

Yet in more and more places around the world, these predators are sticking near the surface, rarely using their formidable power to plunge into the depths to chase prey.

The discovery of this behavioral quirk in fish built for diving offers some of the most tangible evidence of a disturbing trend: Warming temperatures are sucking oxygen out of waters even far out at sea, making enormous stretches of deep ocean hostile to marine life.

“Two hundred meters down, there is a freight train of low-oxygen water barreling toward the surface,” says William Gilly, a marine biologist with Stanford University’s Hopkins Marine Station, in Pacific Grove, California. Yet, “with all the ballyhoo about ocean issues, this one hasn’t gotten much attention.”

These are not coastal dead zones, like the one that sprawls across the Gulf of Mexico, but great swaths of deep water that can reach thousands of miles offshore. Already naturally low in oxygen,these regions keep growing, spreading horizontally and vertically. Included are vast portions of the eastern Pacific, almost all of the Bay of Bengal, and an area of the Atlantic off West Africa as broad as the United States.

Globally, these low-oxygen areas have expanded by more than 1.7 million square miles  (4.5 million square kilometers) in the past 50 years.

This phenomenon could transform the seas as much as global warming or ocean acidification will, rearranging where and what creatures eat and altering which species live or die. It already is starting to scramble ocean food chains and threatens to compound almost every other problem in the sea.

Scientists are debating how much oxygen loss is spurred by global warming, and how much is driven by natural cycles. But they agree that climate change will make the losses spread and perhaps even accelerate.

“I don’t think people realize this is happening right now,” says Lisa Levin, an oxygen expert with the Scripps Institution of Oceanography, in San Diego.

Bad Water Rising

Few understand marlin and sailfish better than biologist Eric Prince. He has studied them in Jamaica, Brazil, the Ivory Coast, and Ghana. He has examined their ear bones in Bermuda, taken tissue samples in Panama, and gathered their heads—with bayonet-like bills still attached—during fishing contests in Puerto Rico.

One day a decade ago, while tracking satellite tags attached to these fish, Prince saw something bizarre: Marlin off North Carolina fed in waters as deep as 2,600 feet (800 meters). But marlin off Guatemala and Costa Rica hovered high in the water, almost never descending beyond a few hundred feet. Sailfish followed a similar pattern.

These billfish have special tissues in their heads that keep their brains warm in deep water. So why were they bunching up at the ocean’s surface?

 The culprit, it turned out, was a gigantic pool of low-oxygen water deep off Central America. These fish were staying up high, trying to avoid suffocating below.

Prince’s discovery came just as other scientists were figuring out that rising temperatures were expanding natural low-oxygen zones in the deep ocean, pushing them skyward by as much as a meter (three feet) per year.

Over the next decade, researchers figured out that this change already was driving marine creatures—sailfish, sharks, tuna, swordfish, and Pacific cod, as well as the smaller sardines, herring, shad, and mackerel they eat—into ever narrower bands of oxygen-rich water near the surface.

“It leaves just a very thin lens on the top of the ocean where most organisms can live,” says Sarah Moffitt, of the Bodega Marine Laboratory at the University of California, Davis.

 Congregating alongside their prey appears to be making some bigger fish fatter, as they burn less energy hunting. But living in such a compressed area also may be speeding the decline of top predators such as tuna, sailfish, and marlin by making them more accessible to fishing fleets.

Dungeness crabs can suffocate when low-oxygen waters from the deep ocean are swept near coastal Oregon. 


“It makes the predators much more likely to be caught by the longline fleet,” says Prince, of the National Oceanic and Atmospheric Administration’s Southeast Fisheries Science Center in Florida. “Very slightly, every year, they become more and more susceptible to overfishing.”

Oxygen is so central to life, even in the marine world, that its loss is harming animals in countless other ways, too.

Warming Waters Deplete Oxygen

 Fish, squid, octopus, and crab all draw dissolved oxygen from the water. And just as oxygen levels shift with elevation, oxygen at sea varies with depth. But in the ocean, oxygen is also dynamic, changing daily and seasonally with weather and tides or over years with cycles of warming and cooling.

Sea stars are often early victims when low-oxygen waters get drawn onto the continental shelf from the deep sea. 


 Oxygen gets into the sea in two ways: through photosynthesis, which takes place only near the top where light penetrates, or through the mixing of air and water at the surface by wind and waves.

Deep ocean waters hold far less oxygen than surface waters because they haven’t been in contact with air for centuries. And in many places, decomposing organic matter raining down from the surface uses up what little oxygen remains. These natural deep-water “oxygen minimum zones” cover great swaths of ocean interior.

They are far different from hypoxic coastal dead zones, which are multiplying, too, with more than400 now reported worldwide. Dead zones are caused by nitrogen and other nutrients as rivers and storms flush pollution from farms and cities into nearshore waters.

The expansion of deep-sea low-oxygen zones, on the other hand, is driven by temperature. Warm water carries less dissolved oxygen. It’s also lighter than cold water. That leaves the ocean segregated in layers, restricting delivery of fresh oxygen to the deep and making these oxygen-poor zones much bigger.

 Breathless seas

 Oxygen is as essential for life in the sea as it is on land. Oxygen levels normally vary with depth. But deep ocean areas already low in oxygen are losing more as seas warm, wreaking havoc on marine life. Here are four elements of that change (see animated graphic).

“The natural thing to expect is that as the ocean gets warmer, circulation will slow down and get more sluggish and the waters going into the deep ocean will hang around longer,” says Curtis Deutsch, a chemical oceanography professor at the University of Washington, in Seattle. “And indeed, oxygen seems to be declining.”

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