Warmer, Lower-Oxygen Oceans Drive Marine Species toward the Poles

Living in a warming ocean won’t just be uncomfortably hot for marine animals, it’s also likely to suffocate them.

According to a newly published  in the journal , the combined stresses of rising ocean temperatures and the resulting drop in oxygen levels will put too much physiological strain on marine animals living closer to the equator.

As water temperatures rise, the animal’s metabolism speeds up, increasing the demand for more oxygen. At the same time, the rising temperatures reduce the amount of oxygen that the upper ocean can hold, so the concentration of the gas will go down, said Curtis Deutsch, the study’s lead author and an associate professor in the School of Oceanography at the University of Washington, Seattle.

“Put these two things together and it’s kind of a double whammy,” he said.

As a result, many species will see their ranges become smaller, especially close to the equator. In order to survive, a number of species may move closer to the poles, though there is no guarantee these new habitats will be hospitable, the researchers said.

Other factors like cold intolerance, ocean acidification and pollution will also play a role in species survival, according to the study.

Deutsch and his colleagues based their conclusions on an integrated analysis of global ocean temperature and oxygen data, combined with metabolic research from over a dozen different species.

In order to compare how species ranges corresponded with current oxygen levels and the species oxygen demands, the researchers created a global metabolic index (the ratio of oxygen supply to the resting metabolic demand). They focused on four species in the Atlantic Ocean—cod, rock crabs, sea bream and eelpout—because they had the most available data, according to Deutsch.

The researchers discovered that like most terrestrial animals, marine species such as fish can only live in areas where they have two to five times more oxygen than what they need when they are inactive. This ensures that when the fish’s activity increases, like when swimming away from a predator, there will be enough oxygen available to allow it to get away.

“What we found was that at the edge of their range, on the southern side toward the equator, these areas coincided with the areas where oxygen was insufficient to enable their energy needs,” said Deutsch. “What this means is oxygen limitation is a real constraint on where [species] can live.”

A ‘universal’ breathing requirement

“It means there’s something universal about the amount of energy organisms need,” Deutsch said.

Based on existing estimates of ocean temperature increases and oxygen loss over the course of the century, the researchers predicted that the amount of available oxygen relative to metabolic demand is likely to go down by 20 percent globally and by approximately 50 percent in high-latitude regions.

However, that doesn’t necessarily mean habitats will be smaller, according to Brad Seibel, the study’s co-author and an associate professor in the Biological Sciences Department at the University of Rhode Island.

“We expected the greatest habitat losses near the tropics, where species are already living near their critical metabolic index. In the tropics, oxygen demand is high due to high temperature and supply is low due to low solubility among other factors. Tropical species don’t have much metabolic index to lose,” Seibel wrote in an email.

He cautioned that more research is needed on other species and in other habitats to back up the findings. The researchers plan to extend their research to include gelatinous organisms and very small species that form a critical part of the food web over the next several years.

David Johnson, an assistant professor at the Virginia Institute of Marine Science, who was not involved with the study, described the research as compelling.

“We often are concerned with direct temperature effects on aquatic animals, but this study shows us that we should also be concerned with loss of oxygen in the water. And that as the waters warm, species may move towards the cooler, oxygen rich waters of the poles,” he wrote in an email.

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