Sailfish
The diving patterns of Atlantic sailfish helped scientists figure out that many fish are spending more time in shallower water as low-oxygen zones push closer to the surface. PHOTOGRAPH BY CLAUDIO CONTRERAS, NATURE PICTURE LIBRARY

Oceans are losing oxygen—and becoming more hostile to life

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.

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