Global warming has significantly altered Earth’s atmosphere. While oceans in general have long suffered from low oxygen levels, the Gulf of St. Lawrence has been warming and losing oxygen faster than almost any other place of global oceans. In a latest study, researchers have investigated causes of this rapid deoxygenation.

The Gulf of St. Lawrence is a large part of ocean that drains North America’s Great Lakes and also harbors an incredibly diverse marine ecosystem. Researchers suggest that the unprecedented drop in oxygen levels of the Gulf of St. Lawrence is attributed to two of the ocean’s most powerful currents: the Gulf Stream and the Labrador Current. The changes in these currents have caused lower-oxygen water to invade Canada’s Gulf of St. Lawrence.

“The oxygen decline in this region was already reported, but what was not explored before was the underlying cause,” said lead researcher Mariona Claret from University of Washington, who did the work while at Canada’s McGill University. “Observations in the very inner Gulf of St. Lawrence show a dramatic oxygen decline, which is reaching hypoxic conditions, meaning it can’t fully support marine life.”

Canada’s fisheries agency has been tracking temperature and oxygen levels of St. Lawrence region for decades, but the connection between the two has never been considered.

To find out what caused oxygen loss in the region and what role climate played, researchers used output from NOAA’s Geophysical Fluid Dynamics Laboratory model. Model output combined with the historical observations showed that the Gulf Stream has shifted northward and the Labrador Current has weakened due to high carbon dioxide levels over the past century. As the carbon dioxide level increases, it causes Gulf Stream’s warm, salty and oxygen-poor water to enter deeper parts of the St. Lawrence gulf. Since warm water is less effective at holding oxygen, it results in more oxygen loss. The shift in the large-scale ocean circulation causes warming and deoxygenation in the Gulf of Saint Lawrence.

“We relate a change in oxygen on the coast to a change in large-scale currents in the open ocean,” said Claret. “Being able to potentially link the coastal changes with the Atlantic Meridional Overturning Current is pretty exciting.