Quirks & Quarks

The Gulf of St. Lawrence is running out of oxygen thanks to climate change

Shifting currents is bringing oxygen poor water to the deepest parts of the gulf

Shifting currents is bringing oxygen poor water to the deepest parts of the gulf

A drop in oxygen levels has significant implications for bottom-dwelling fishes and crustaceans living in the Gulf of St.Lawrence — including the Atlantic cod, which is of economic importance to the region. (The Associated Press)

For more than a decade, scientists have been trying to understand why deep water oxygen levels in the Gulf of St. Lawrence have been declining.

Now new research explains why. Climate change has shifted Atlantic ocean currents, and brought warmer, but oxygen-poor water into the gulf.

The gulf is one of Canada's most important and historic places. It's home to all sorts of marine life, and it's a central cultural and tourist hub for five provinces.

A drop in oxygen levels has significant implications for bottom-dwelling fishes and crustaceans living there, including cod and wolffish, whose survival in the area could be threatened if oxygen levels continue to decline.

Dr. Denis Gilbert, a climate research scientist with the Department of Fisheries and Oceans, first discovered the drop back in 2005. But he didn't have enough evidence then to link it to climate change.

In the new study, Gilbert and his colleagues used computer models to simulated two earth systems, with different levels of carbon dioxide in the atmosphere. One had pre-industrial carbon dioxide levels held constant at 200 parts per million; the second model increased carbon dioxide levels by one percent each year until it doubled - to roughly the level it's at today.

"What we see very clearly is that when you leave carbon dioxide constant, the water temperature and oxygen in the deeper parts of Gulf of St.Lawrence essentially remains constant," said Gilbert. "When you increase carbon dioxide, there's a shift in the dynamics of the two major currents that we find in the Northwest Atlantic that causes rapid warming and lower oxygen."

An increase in carbon dioxide leads to more heat on Earth.This changes the distribution of heat in the air and oceans, and thereby the Earth's atmospheric and ocean processes.

Climate change has shifted the dynamics of the Gulf Stream and the Labrador Current at the Tail of the Grand Banks, where they meet. As a result, more oxygen-poor Gulf Stream water is brought into the Gulf of St.Lawrence. (Figure taken from the study report)

As a result, the oxygen-poor Gulf Stream from Florida has shifted northward and the oxygen-rich Labrador current from the Arctic has slowed down. The two currents meet at the tail of the Grand Banks, east of the Gulf of St.Lawrence, but because the Labrador Current is pushed up, the oxygen-rich water struggles to get into the gulf and is displaced by oxygen-poor waters from the Gulf Stream.

In recent years, Gilbert noticed oxygen levels have started to drop again in the gulf after a period of stability. He says this downward trend is likely to continue if we don't limit our carbon emissions.



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