A large, goose-shaped lake stretches across Canada’s Northwest Territories. Two red circles and two yellow triangles mark sites where samples were taken from the lake, and the Slave River and the Hay River are both labeled.
Great Slave Lake in Canada’s Northwest Territories is North America’s deepest lake. Recent research shows that its enormous mass of cold water is no longer shielding it from climate impacts. Credit: European Union, Copernicus Sentinel-2A imagery

From the air, Great Slave Lake looks like a giant goose winging across Canada’s Northwest Territories. Spanning an area the size of Belgium and reaching depths of up to 614 meters, it’s the 10th largest freshwater lake in the world and North America’s deepest. Its huge mass of cold water helped shield Great Slave Lake from the climate impacts that have upended the ecosystems of shallower lakes in high northern latitudes.

But no longer, according to a new study in Proceedings of the Royal Society B.

Spurred by accelerating Arctic warming, the microscopic algae, or phytoplankton, at the foundation of this massive lake’s food web have made a radical regime shift since the turn of the century. These single-celled organisms, called diatoms, leave behind silica shells that are preserved in lake sediment records. By analyzing the sediment, scientists found that the hefty, chain-forming diatoms that had long ruled Great Slave Lake have now been supplanted by tiny, pancake-shaped counterparts. This upheaval in the system’s primary energy source could affect the lake’s productivity and carbon dynamics, alter its food web, and disturb a significant food and cultural resource for nearby First Nations and Métis communities.

Profound Change, Cascading Consequences

“In recent history, which we can link to climate warming and less ice cover, it really is a profound change,” said John Smol, a paleolimnologist at Queen’s University in Kingston, Ont., Canada, and coauthor on the study. Although the consequences of the regime shift are not yet known, Smol said, the impacts are almost certain to cascade throughout the ecosystem.

Smol and colleagues compared lake bottom cores extracted in 2014 and the mid-1990s. The two sets of cores provided sediment records stretching back roughly 200 years, from which researchers analyzed diatom remains for changes in species abundance that signal ecosystem transition. Sediments show that for much of the past century, the lake’s ecosystem was dominated by Aulacoseira islandica—a heavy, 20-micrometer diatom shaped like a tin can. But beginning in the mid-1990s, a heterotrophic array of small, buoyant plankton about a tenth that size began to muscle in. The shift sharply ramped up around 2000. By the mid-2010s, the analysis finds, the tiny interlopers had completely taken over.

Two blue-tinted photomicrographs of diatoms. The left diatom is shaped like a can with a bumpy texture; the right one is shaped shaped like a pancake with ridges along the outer edge
Photomicrograph of Aulacoseira islandica, a diatom that dominated Great Slave Lake prior to 2000 (left). In the 21st century, diatoms in Great Slave Lake diversified and included more small-celled organisms like Discotella (right). Credit: K. Rühland, Queen’s University

Lake scientists are surprised and concerned by this radical transition, which is unprecedented in the sediment record. Although the warming climate has been triggering rapid shifts in smaller and medium-sized northern lakes since the mid-1900s, the findings show that very large bodies like Great Slave Lake, which had until recently been protected by their extensive ice cover and thermal inertia, are no longer able to fend off change.

“The nature and magnitude of these effects is startling.”

“The nature and magnitude of these effects is startling,” said Warwick Vincent, an ecologist specializing in Arctic lakes at Laval University in Quebec who was not involved in the research. “We expected North America’s deepest lake to have a large buffer capacity against global change, and it seems that this has now been exceeded.”

Rising Temperatures, Declining Ice Cover, Slowing Winds

Great Slave Lake’s abrupt transformation corresponds to accelerating Arctic climate change, said the study’s lead author, Queen’s University paleolimnologist Kathleen Rühland. The region is now warming several times faster than the global average. Since 2010, average air temperatures in the Arctic have climbed by 1°C. Lake ice cover has also declined significantly, and wind speeds have slowed in recent decades. As a result, the lake is growing calmer, said Rühland, which is bad news for large phytoplankton like A. islandica that require turbulence to stay afloat and in reach of light for photosynthesis.

“They quickly sink out of the water column,” Rühland said. “But these conditions are ideal for these small, pancake-shaped diatoms that are more buoyant.”

The regime shift is “an early warning sign that things are really changing,” Rühland said. Further research and monitoring are needed to determine the broader implications. One big question is how the shift will affect the algal primary production—or the creation of organic matter from inorganic carbon compounds—driving this vast ecosystem’s food system. Between 2003 and 2018, primary production in Great Slave Lake rose 27%, according to a remote sensing study in the journal Water. But the smaller algae now fueling the food web could provide less nutrition for the organisms that eat them, ultimately affecting the food supply for fish and other aquatic life—and the communities that rely on them. In addition, the amount of carbon the new regime sequesters through primary production could rise or fall.

The researchers are now turning their attention to the Northwest Territories’ Great Bear Lake, which is farther north, colder, and even bigger than Great Slave Lake. Preliminary data on this lake, the eighth largest in the world, suggest a similar upheaval is underway there too.

“We think of the Arctic as the miners’ canaries of the planet, and the lakes are recording it,” said Smol. “And within the lake, the canaries are probably the algae.”

—Cheryl Katz (@ckatz99), Science Writer

Citation: Katz, C. (2023), Arctic warming triggers abrupt ecosystem shift in North America’s deepest lake, Eos, 104, https://doi.org/10.1029/2023EO230406. Published on 24 October 2023.
Text © 2023. The authors. CC BY-NC-ND 3.0
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