Stretches of ocean off British Columbia’s coast are increasingly being starved of oxygen — an emerging threat that, if left unchecked, could have devastating impacts on the region’s marine life and fisheries, a new study says.
The research, recently published in the Journal Global Biochemical Cycles, focused on Queen Charlotte Sound, Pacific Canada’s largest sea sitting over a continental shelf.
Samuel Stevens, a researcher at the Hakai Institute who led the study, said the more frequent, more widespread low-oxygen events, known as hypoxia, tended to occur in the summer months.
In some cases, extreme hypoxia — the point where falling oxygen levels damage and stress marine organisms — was found to be more severe than anything previously recorded.
“For a number of reasons, climate change is causing the ocean to lose oxygen,” said Stevens. “Oxygen is clearly an important thing for life in the ocean.”
“The fact that it's now appearing — and it's occurring over large portions of the of the Central Coast — is quite alarming.”
Stevens carried out the research with colleagues from the University of British Columbia, University of Victoria, the Hakai Institute, and Fisheries and Oceans Canada.
The research team used underwater moorings, ship surveys, and torpedo-shaped autonomous drones called ocean gliders to collect data along the Sea Otter Trough, a 120-kilometre-long ocean-floor feature located off the northern tip of Vancouver Island. They then compared the recent data to records going back to 2003.
The result: shelf-wide hypoxic events spiked to more than 13 per cent of the time in 2022 and 2023 from 2.5 per cent recorded in the previous two decades.
The numbers represent the first documented case of persistent and widespread hypoxia in the Canadian Pacific north of Vancouver Island, according to the study.
Changing ocean conditions in Japan, Russia reach B.C.
Based on their findings, Stevens and his colleagues identified two main reasons for the drop in seawater oxygen levels.
The first is a long-term decline in oxygen across the Pacific Ocean, likely linked to large-scale changes in how much oxygen is absorbed near Japan and Russia. While the exact causes are not fully understood, research suggests that changes in wind and tide patterns, as well as an influx of fresh water from the melting Arctic, are hindering the mixing of atmospheric oxygen at the ocean's surface.
“These processes that occur all the way on the other side of the ocean can be measured in Queen Charlotte Sound,” said Stevens.
The second cause was found to be more frequent changes driven by local factors, like the strength and timing that deep water wells up onto the continental shelf.
As the water travels out of the deep ocean and across the sound's relatively wide continental shelf, naturally occurring bacteria have more time to consume what little oxygen remains through a process called respiration, said Stevens. This further depletes oxygen levels before the water reaches the mainland.
Submarine canyons, which act as important conduits for deep, low-oxygen water, were also flagged in the study as likely contributing to hypoxic conditions in the region.
Trend points to coast-wide shift in oxygen levels
Using their data, the scientists then predicted future conditions. If the trends continue, they found oxygen-starved ocean conditions in the Queen Charlotte Sound would occur more frequently over the coming decades.
By 2050, large portions of the region’s deep water layer could become “largely hypoxic” at certain times of year, the study said.
The scientists’ findings off B.C.’s Central Coast may serve as a warning for other parts of the province’s coast, including nearby areas on the Canadian Pacific shelf, such as Hecate Strait and Dixon Entrance.
It also comes after past research has shown similar declines in oxygen in B.C.’s Strait of Georgia, off southern Vancouver Island and other parts of the Central Coast.
“The declines you see over long time scales are roughly the same no matter where you are,” said Stevens. “We’re seeing [the] whole coast is shifting slowly towards a low-oxygen state.”
A lack of oxygen has already led to mass die-offs of crab species off the U.S. Pacific coast.
The latest study notes that hypoxia was a likely factor contributing to the absence of Pacific hake — a migratory groundfish normally found from Southern California to Alaska — north of Vancouver Island in 2023.
The scientists also point to research suggesting low oxygen levels in ocean water could fuel ocean acidification, which in turn, stymies the formation of seashells and can put stress on the entire food chain.
What sets the Queen Charlotte Sound apart is its potential for outlier months, when local conditions could drive already paltry oxygen levels dangerously lower than the rest of the B.C. coast, said Stevens.
Pinpointing the precise danger will require more research — including fish and seafloor surveys — to link low-oxygen events to their impact on local marine ecosystems, he said.
