Worrying drop in ocean oxygen documented off B.C. coast
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Researchers have documented a worrying drop off in oxygen in the waters off British Columbia’s central coast, and say the problem could become widespread and persistent by mid-century, devastating marine life.
The study, conducted by the Hakai Institute, used ocean gliders — remotely-operated vehicles — to measure oxygen levels in Queen Charlotte Sound, a coastal region that stretches from the north of Vancouver Island to the southern tip of Haida Gwaii.
Hakai Institute post-doctoral researcher Sam Stevens, who served as lead author on the study, said the team measured a concerning spike in low-oxygen states in 2022 and 2023, primarily during the summer months.
“Once it gets below a certain threshold, there’s not enough oxygen for life in the ocean to function properly, and that’s what we call hypoxia,” Stevens explained.
“For various reasons related to climate change, we’re seeing declines in oxygen in the ocean globally.”
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The team focused its data collection on a deep water area known as the Sea Otter Trough, where they observed bottom hypoxia from June through October, peaking in August.
Researchers compared their results to 20 years of data collected in the area, and the change with historically documented levels was dramatic.
Prior to 2022, hypoxic bottom waters were rare, showing up in just 2.5 per cent of observations. The data from 2022 and 2023 documented hypoxic conditions in 13.1 per cent of observations.
The consequences for marine life in the area could be serious, particularly for species that dwell on the ocean floor.
“In Queen Charlotte Sound specifically we’re seeing some emerging evidence that these oxygen levels are starting to impact the ecosystem and so there are some fish species which we haven’t found in the region over the last few years that are usually quite abundant there,” Stevens said.
Pacific Hake, a migratory groundfish that lives in coastal waters from California to Alaska was documented as absent in the study area in 2023.
Stevens said invertebrates like crabs, sea stars and sea cucumbers that can’t move out of low oxygen areas are at even greater risk. He pointed to mass die offs on the coast of Oregon and Washington as an example of potential dangers.
“That’s a very drastic but possible outcome of these very low oxygen conditions,” he said.
The source of the problem, Stevens said, is complex and global.
As ocean surface waters warm, they become less receptive to absorbing oxygen; fresh water from melting artic ice can also contribute to the process, he said.
In the case of Queen Charlotte Sound, the researchers believe some of the hypoxic waters actually originated across the Pacific Ocean in subarctic waters off Japan and Russia, up to a decade or longer ago.
“The combination of these things make it harder for oxygen from the atmosphere to be injected into the ocean over on the other side of the Pacific and then subsequently we see less oxygen,” he explained.
“That water travels all across the Pacific to us. They’re in B.C. and we see less oxygen in our waters because of that.”
The study projects that if trends continue, more than half of the seafloor in Queen Charlotte Sound could by hypoxic in the summer.
He said the consequences of the process could have important ripple effects on B.C.’s ocean ecosystem, and call for the careful monitoring of species’ health, not just in Queen Charlotte Sound, but across the coast.
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