Warmer Ocean Waters Will Shift Marine Habitats

Warmer ocean waters are creating such a challenge for marine species worldwide, that new research shows they will consequently shift entire marine habitats.

University of Washington (UW) researchers report in the journal Science that these higher temperatures will speed up the animals' metabolic need for oxygen, but the warmer water will hold less of the oxygen needed to fuel their bodies - similar to what happens at high altitudes.

These changes will act together to push marine animals away from the equator. About two thirds of the respiratory stress due to climate change is caused by warmer temperatures, while the rest is because warmer water holds less dissolved gases.

"If your metabolism goes up, you need more food and you need more oxygen," lead author Curtis Deutsch, a UW associate professor of oceanography, explained in a news release. "This means that aquatic animals could become oxygen-starved in the warmer future, even if oxygen doesn't change. We know that oxygen levels in the ocean are going down now and will decrease more with climate warming."

During the study, Deutsch and his colleagues focused on four Atlantic Ocean species - Atlantic cod, Atlantic rock crab, sharp snout seabream, and common eelpout, a bottom-dwelling fish. Using climate models, they sought to determine how these four species would be able to meet their future energy needs in a presumably low-oxygen environment.

If emissions continue at their current rate, they found, then near-surface ocean water will warm several degrees Celsius by the end of this century. That's bad news for these oxygen-breathing animals, as seawater at that temperature would hold five to 10 percent less oxygen than it does now.

Habitats of Atlantic rock crab, for example, which live in coastal waters, would thereby be restricted to the shallow surface. And for all four species, living near the equator will no longer be an option, given that peak oxygen demand would become greater than the supply. This would in effect reduce their current habitable ranges by 14 to 26 percent.

"This simple metabolic index seems to correlate with the current distributions of marine organisms," noted co-author Raymond Huey, "and that means that it gives you the power to predict how range limits are going to shift with warming."

And because all of the studied marine species live in such diverse environments - from the open ocean to the subtropical Atlantic and Mediterranean - researchers believe that these findings apply to just about all marine species that rely on oxygen for survival.

"The Atlantic Ocean is relatively well oxygenated," Deutsch said. "If there's oxygen restriction in the Atlantic Ocean marine habitat, then it should be everywhere."

The Pacific Ocean, in particular, will be especially vulnerable by the year 2100 because its oxygen levels are already relatively low.

Previously, marine scientists thought about oxygen more in terms of extreme events that could cause regional die-offs of marine animals, also known as dead zones.

"We found that oxygen is also a day-to-day restriction on where species will live, outside of those extreme events. Ranges will shift for other reasons, too, but I think the effect we're describing will be part of the mix of what's pushing species around in the future," Deutsch concluded.

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