This article discusses why scientists feel that the ocean is running out of breath. Widespread and sometimes drastic marine oxygen declines are stressing sensitive species—a trend that will continue with climate change.
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.
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.
The effects of even very subtle dips in oxygen on where zooplankton—animals at the base of the food web—congregate in the water column were documented in a December 2018 Science Advances report.
Aside from food web disruptions, animals face various other physiological challenges as their bodies adjust to lower oxygen levels. Chinese shrimp flip their tails less vigorously to conserve energy in lower oxygen environments, becoming less agile as a result, a Marine and Freshwater Behavior and Physiology study reported last month. Also, some male fishes produce fewer and less motile sperm as oxygen levels decline—and the trend does not seem to bounce back in future generations when oxygen levels improve, researchers reported in Nature Communicationsin 2016.