First map of B.C.’s dissolved carbon yields surprise findings

Ocean acidification may be worse in B.C.’s ‘protected’ waters than along the outer coast, and is not always tied to low oxygen levels.

A new study

Ocean acidification may be worse in B.C.’s ‘protected’ waters than along the outer coast.

A new study shows water in the Strait of Georgia is far more acidic than the water washing into it from the outer shelf—a finding that is likely true for B.C.’s many other fjords and sheltered river mouths.

It’s an unwelcome surprise, says Debby Ianson, an oceanographer with Fisheries and Oceans Canada who led the study.

“For shellfish farmers and others who may have been looking at the work published on the outer coast and saying, ‘Wow, these conditions get bad,’ conditions are much worse in the Strait of Georgia.”

Ianson’s team did gather data closer to Haida Gwaii, and will publish those results soon.

Because the Hecate Strait is more open than the Strait of Georgia, Ianson said carbon levels in Haida Gwaii waters are more similar to the outer coast.

The Strait of Georgia study is the first attempt to precisely map the dissolved carbon anywhere in B.C. coastal waters.

Besides raising acidity, adding CO2 to the ocean robs shellfish and other marine calcifiers of aragonite and calcite—the stuff they need to make shells, coral, exoskeletons, or the internal ‘scaffolding’ inside many seaweeds.

“If you’re an organism that makes your shell out of aragonite, it means your shell is going to start to dissolve and you’re going to have to put a lot of energy out to rebuild it,” said Ianson, noting that other studies already show a decline of some key calcifiers in B.C. waters, including the tiny pteropod, or ‘sea butterfly,’ that makes up much of the diet for juvenile Pink salmon.

Ianson said one reason the Strait of Georgia study is so surprising is that it shows the strait may be protected from low-oxygen, but not from high CO2.

“Mostly people think about oxygen depletion—hypoxia—and ocean acidification really going hand in hand,” she explained.

Just like in a garden compost, when living things break down in the ocean, they absorb oxygen and release C02.

All of the water washing up on the West Coast is relatively ‘old’—it picks up a lot of decaying organic matter on its 1,200-year journey from the north Atlantic down to Antarctica and across the north Pacific.

So B.C. coastal waters have always had less oxygen and more CO2 than most coastal waters around the world.

But thanks in part to its tidal narrows, the Strait of Georgia breaks that trend by having high oxygen and high CO2.

Narrows like the Haro and Juan de Fuca Straits have quick currents and intense tidal mixing, said Ianson—mixing that helps the ocean “breathe.”

Oxygen and carbon dioxide gases pass between the air and the ocean surface all the time.

By bringing deep water to the surface quickly, tidal narrows allow the ocean to absorb more oxygen.

However, the ocean can exchange oxygen a lot faster than CO2.

“These narrows are allowing the deep waters in the Strait of Georgia and other fjords to refresh their oxygen,” said Ianson.

“But because the the C02 exchange is a lot slower, they’re not letting the ocean get rid of its C02, at least not at the same rate.”

Other factors that affect the Strait of Georgia in particular are the Fraser River, which has high carbon levels, and, to a lesser degree, the carbon added the strait by pulp mills, city sewage, and fish farms.

It varies with the strength of the Fraser, but generally speaking, shellfish and other marine calcifiers already have a very small window of opportunity in the Strait of Georgia.

Even in a good year and in the right season, Ianson only the top 20 metres of water are really biologically favourable for them.

“Most of the water has poor growing conditions for shelled organisms, all the time,” she said.

“The tidal narrows may be protecting us from hypoxia,” she added.

“But they will not be protecting us against ocean acidification.”

The study is available in the journal Geographical Research Letters.

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