Photograph by Brian Skerry, Nat Geo Image Collection
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New research shows that oxygen loss in the ocean could shift and shrink habitat for Atlantic cod and many other marine species.

Photograph by Brian Skerry, Nat Geo Image Collection

Fish, Crabs Are Losing Homes as Oceans Lose Oxygen

About 20 percent of ocean habitat for Atlantic cod and crabs could be gone by end of century. Other marine life is at risk, too.

Atlantic cod, the storied catch of New England’s fishing industry, have little in common with bottom-dwelling rock crab, which are perhaps best known for swiping bait from lobster traps. But a largely unheralded byproduct of climate change–loss of oxygen in the ocean–will hit both dramatically by limiting where they can live, according to a new study published Thursday.

The oxygen losses accompanying global warming could reduce by 20 percent the amount of ocean suitable for cod and crab by the end of the century, according to the study in the journal Science.

The new research suggests this oxygen loss may shift and shrink marine habitats for a multitude of species globally, potentially upending marine food webs far more substantially than previously thought.

"You're talking about shuffling the deck and reorganizing who interacts with who and that introduces all kinds of new dynamics into the system," says lead author Curtis Deutsch, a University of Washington oceanographer.

Deutsch's work is the latest in a spate of research highlighting the ways oxygen loss may alter ocean life. (See also: "Oceans Are Losing Oxygen – And Becoming More Hostile to Marine Life.")

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.
Ocean mixing
Euphotic zone
Oxygen can get into the sea as wind and waves stir the surface or through photosynthesis, which takes place in surface regions where light penetrates.
Warm water is lighter. As the upper ocean heats up, it gets harder for that water to penetrate cold layers below through ocean circulation. Such mixing is how oxygen gets down from the surface.
Warm water holds less oxygen. As temperatures rise at the surface, that water loses its ability to carry oxygen.
Oxygen minimum zones (OMZs)
OMZs are not dead zones, but are vast midwater areas far out at sea that hold less oxygen than surface waters do. Organic matter decomposes as it rains down from the surface, robbing OMZs of O2.
In some regions, such as the U.S. West Coast, wind draws oxygen-poor water up from the deep and blows it into shallow areas over continental shelves.
Low-oxygen zones expanding out and up kill some animals and drive others to thinner bands of oxygen-rich water near the surface. That alters the food chain and makes large predator fish easier to catch.
The bottom
Waters below OMZs are less depleted of oxygen. Most organic matter sinking in the ocean decomposes before it reaches the bottom.
Zones' depths vary with location.
SOURCES: Francis Chan, Oregon State University; William Gilly, Stanford University's Hopkins Marine Station

"The specific impacts are going to differ from species to species, but the way this will affect the distribution of marine life is universal," says Joanie Kleypas, a marine ecologist at the National Center for Atmospheric Research in Boulder, who wrote an opinion piece published alongside Deutsch's work. "Some species that have never been together before are going to get pushed together, and what happens after that is just a big burning question."

As temperatures rise in the ocean, the amount of oxygen in seawater decreases. But, at the same time, the amount that creatures require goes up, so just when marine life needs more oxygen, there will be less of it.

Because oxygen already plays a huge role in dictating where ocean species live, Deutsch and his colleagues wanted to see how some typical animals might respond when oxygen dropped.

They examined rock crab and three Atlantic fish species – cod, warm-water sea bream, and cold-water eelpout–whose physiology and distribution already had been thoroughly studied. They calculated and mapped their metabolic index–the ratio of oxygen they need to the oxygen available. Then they used climate models to project where future oxygen and temperature changes will allow these animals to live.

"The habitat shrinks," Deutsch says. "And the loss is significant."

Even though these four species live in different environments, the volume of sea that could accommodate the needs of each shrank roughly 20 percent.

Cod fishing in the United States, while a fraction of what it once was, is still a $32-million annual industry, and there are small markets for rock crab. But the significance of the research goes far beyond these species.

“We don’t think those examples are exceptions, but rather indicate that habitat restrictions … are likely to be common and many species worldwide will likely show similar effects,” says co-author Brad Seibel, a University of Rhode Island marine biologist.

Effects will vary by animal and location. Dramatic oxygen loss in Antarctica, for example, won't harm marine life as much because the region is oxygen-rich. The Pacific, on the other hand, is naturally low in oxygen and could see far greater oxygen losses with warming.

“It’s likely that impacts will be more substantial in the Pacific. But impacts will likely be most substantial in tropical waters of both oceans relative to temperate or polar latitudes,” Seibel says.

Oxygen also isn’t the only factor shuffling where marine creatures live. Changes in marine chemistry and shifts in predator-and-prey relationships also will alter where fish and other sea creatures live.

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