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In the northeast Pacific, the upper 3,000 meters of water has lost 15 percent of its oxygen over the past 60 years, and the top 500 meters is simultaneously becoming more acidic at an unprecedented rate, a study by Fisheries and Oceans Canada scientists has found.
The changes, which the scientists say are a consequence of climate change, threaten the survival of the black coral, brittle stars, rockfish, and other species that live around the towering seamounts that lie off the British Columbia coast. The scientists say the seamount ecosystem—regarded as an oasis of life in the deep ocean—will be irreversibly changed, and there will likely be local extinctions.
The research shows the previously studied large oxygen minimum zone between 480 and 1,700 meters has some of the lowest oxygen levels in the global ocean, and it is expanding. The 15 percent loss of oxygen puts the northeast Pacific way above the two percent loss seen in oceans globally over the past 70 years.
The result is that the deep-sea ecosystem in the northeast Pacific is facing a double whammy: from the decrease in oxygen as the warming surface water absorbs less oxygen; and from ocean acidification, which affects the ability of marine organisms to maintain shells and skeletons.
Biogeochemical oceanographer Debby Ianson, one of the study’s three authors, says that because carbon dioxide concentrations in the atmosphere are already high from burning fossil fuels, absorption of carbon dioxide by the ocean will continue for at least a decade even if carbon emissions are somehow stopped. “The train has left the station,” she says.
Many species around the seamounts, such as cold-water corals and rougheye rockfish, can live for centuries. But life for many of these creatures is already changing. For example, the changes in water chemistry mean that the depth at which the water becomes acidic enough to dissolve calcium carbonate skeletons is rising by two meters a year.
The discovery stems from data collected by the Line P oceanographic monitoring program, which has been testing the water from the southern tip of Vancouver Island, British Columbia, far offshore into the Alaska Gyre since 1956. Line P is one of the longest-running deep-sea time series in the world, allowing scientists to see trends in ocean chemistry.
Ianson says she and her colleagues found it shocking that there were clear indications of climate change at such depths.
“We’re talking about deep water—between 500 meters and three kilometers. It is old water that has not been at the surface for a long time and, way down there, we weren’t expecting it to experience any climate change,” Ianson says.
Study coauthor and physical oceanographer Tetjana Ross also says she didn’t expect to see such pronounced chemical changes in the deep water because much of the water around the seamounts was last at the surface 1,000 years ago, before humans started releasing large amounts of carbon dioxide into the atmosphere. This is also the last time the water was able to exchange oxygen with the atmosphere.
But now that organisms such as corals are being affected, says study coauthor and marine biologist Cherisse Du Preez, side effects are inevitable.
“The corals can be meters tall, complex with beautiful branches that create homes for shrimp, octopus, nudibranchs,” she says. “Without those corals, those animals no longer have a home. They no longer have shelters, or nurseries, or places where feeding is easier. Their entire lifestyles are just going to fall apart,” she says.
The study predicts that within 40 years the environment will no longer be hospitable, and within 100 years many animals will undergo local extinction.
“While there might be winners and losers in the short run, if it keeps going at this rate, I think the ultimate ending is going to be losses for all the life down there,” Du Preez says.
Verena Tunnicliffe, a recently retired Canada Research Chair in Deep Ocean Research at the University of Victoria in British Columbia who was not involved in the research, says the study illustrates the value of taking measurements over decades.
Scientists have only recently started paying attention to the unique ecosystems of the seamounts off British Columbia, and the study suggests we are witnessing the demise of that ecosystem, she says.
“One of the key points of that paper is, yes, we have to do something about the emissions. But, we have to live with what we have got, so we have to think about proactive management methods that diminish the other threats,” Tunnicliffe says.
In their paper, the scientists emphasize the need to mitigate human impacts such as fishing and undersea mining on the seamount ecosystem by designating the area as a marine protected area. More than 133,000 square kilometers of the region has already been identified by the Canadian government as an area of interest for a potential protected area.