The spectre of low-oxygen “dead zones” is surfacing along BC’s Central Coast, threatening the region’s rich marine ecosystems and fisheries. 

Widespread hypoxia — when oxygen levels in the ocean fall below levels required by marine life — is being detected in the deep waters of Queen Charlotte Sound for the first time, said Sam Stevens, an oceanographer at the Hakai Institute and lead author of a new study. 

Aggravated worldwide by climate change since warmer oceans generally hold less oxygen, marine hypoxia is now beginning to impact the sound, sandwiched between northern Vancouver Island and south of Haida Gwaii. Marine life, such as crabs, fish and a host of sea bed creatures like sea stars and anemones, begin to suffer from hypoxia when dissolved oxygen levels fall below two milligrams per litre, Stevens said. 

“In 2022 and 2023, we saw large areas below this threshold,” he said, adding projections from the research data suggest marine oxygen levels will get significantly worse by mid-century. Preliminary data gathered during 2024 and to date in 2025 suggest that the region’s deep waters continue to experience oxygen starvation. 

If that trajectory continues, by 2050, about half of the sea floor of the Queen Charlotte Sound could suffer hypoxia during summer, along with 40 per cent of the volume of deep water below 100 metres, he said. 

“That’s a large fraction of the system,” Stevens said. “It suggests big changes for ecosystems and the species that depend on them.”

Marine die-offs and fisheries impacts

The research raises alarms about the future of some of Canada’s most productive waters.

The deep waters of the sound shelter celebrated conservation areas, like ancient glass sponge reefs and important commercial fisheries for groundfish like halibut, rock cod and hake. The region also falls within the Great Bear Sea, or BC Northern Shelf bioregion, where a string of new marine conservation areas are being developed. 

The hypoxia levels on the Central Coast raises the threat of serious marine die-offs like those experienced in southern waters off the coast of Washington and Oregon, where in several instances in the past two decades, huge numbers of Dungeness crabs perished, said Stevens. 

Persistent low oxygen levels along the Central Coast could also cause a shift in the distribution of fish, which unlike slower bottom dwelling creatures, will migrate away from hypoxic areas, he said. 

Data about changing oxygen levels can better help inform monitoring of fish stocks and conservation decisions, he noted. 

“That might look like managing the fisheries for certain species that might be more or less impacted by these conditions,” Stevens said. Examining hypoxia data could help predict what areas in the region might be least affected — possibly acting as refuges for species needing protection.  

 Global drivers, local fallout

The new study also suggests that the local oxygen levels are being driven by global warming halfway across the globe in the subarctic waters near Japan and Russia, Stevens said. 

“It was particularly interesting to make the connection between the Queen Charlotte Sound and climate change processes happening thousands of kilometres away on the other side of the Pacific,” he said. 

Oxygen levels of deep waters arriving on the coast of BC appear to be affected by climate impacts on the massive ocean circulation system called the North Pacific Gyre that rotates ocean water clockwise from North America west across the Pacific Ocean and back again. 

However, winters in the distant subarctic waters may be changing with global warming, lessening the mix of atmospheric oxygen into ocean waters as a result of storms and winds. Freshwater layers on the surface of the ocean, caused by melting ice, also act like a cap that makes it more difficult for oxygen to mix into deeper seawater for eventual transport back to North America. 

The combined effects mean there is less oxygen in deep ocean water when it returns to flow over the continental shelf along the BC coast eight to 10 years later, he said. 

Additionally, the nutrient-rich but low-oxygen water becomes even more hypoxic once it wells upward in coastal waters and triggers plankton growth. When organic matter and dead microscopic creatures from these plankton blooms sink as “marine snow” into deeper water, bacteria breaking down the materials consume oxygen, aggravating hypoxia, Stevens said. 

Ocean gliders an invaluable tool 

The research team drew on nearly 20 years of historical ocean data, much derived from shipboard studies. However, the new findings were discovered using robotic ocean gliders, Stevens said. 

The autonomous, torpedo-shaped gliders collect detailed measurements on things like oxygen, temperature and salinity. The fleet of gliders with names like “Eve” or “Hal” dive from the ocean’s surface to the sea floor, and give a comprehensive, expansive picture of how ocean conditions change over time.

“They can be out there year-round through any ocean conditions, and they take data at very high resolution,” he said. 

“Whereas, us humans don’t really like to be at sea all year round — especially not in the winter. Boats can be challenging and expensive. So, these gliders offer a really, really great platform to study things like this in high detail.”

The fallout of low oxygen in deep water could ripple into coastal inlets which are part of the sound’s ecosystem, Stevens said. Fjords such as Knight Inlet, Bute Inlet and Rivers Inlet all draw on oxygen-rich waters from the sound and if dead zones develop, these ecosystems may also decline.

“The events we’re seeing now are unique in the historical record,” Steven said. 

“If these trends continue, hypoxia will continue to worsen in the coming decades.”

Rochelle Baker / Local Journalism Initiative / Canada’s National Observer