Canada’s deepest lake feeling immediate effects of climate change, new research finds

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Climate change has reached into Canada’s deepest lake and rewired it at the microscopic level.

That’s the conclusion of scientists investigating the ecological history of Great Slave Lake, one the world’s largest bodies of fresh water and an economic and cultural focal point for the Northwest Territories. In the first such study conducted since the 1990s, the researchers have discovered a wholesale replacement of the plankton species that inhabit the lake and form the base of its food chain. And though the transformation is invisible to the eye, its implications are profound.

“The lake is strikingly different than it was just 25 years ago‚” said John Smol, a professor at Queen’s University in Kingston and a co-author on the study. “The long-term consequences of this are still unknown, but we can be sure it’s going to cascade throughout the whole ecosystem.”

Dr. Smol specializes in limnology – the science of lakes – and is known for his studies of small bodies of water across Canada and elsewhere. But Great Slave Lake study, done in collaboration with Environment and Climate Change Canada, shows evidence of rapid transition in a far larger aquatic system.

Environmental change is already a common theme across the Canadian Arctic, a part of the country that is warming three to four times faster than the global average. In the Great Slave region, the effects of rising temperatures are apparent on land, from melting permafrost to the multiple wildfires raging around the perimeter of the lake that forced the evacuation of Yellowknife earlier this summer.

The new findings, published Tuesday in the research journal Proceedings of the Royal Society B, show that major changes are also afoot beneath the waterline. They have been hidden until now only because Great Slave Lake is not nearly as well studied as large bodies of water that lie farther south, including the Great Lakes. Yet it is bigger than both Lake Ontario and Lake Erie and, with a maximum depth of 614 metres, is half again as deep as Lake Superior – enough to submerge Toronto’s CN Tower with room to spare.

The results have surprised other researchers who expected the sheer size of Great Slave Lake would help buffer it from the more immediate effects of global warming.

“It is startling to see the magnitude of change in such a large ecosystem,” said Warwick Vincent, an expert in Arctic lake ecology at Laval University who was not involved in the study. “It’s providing yet another wake-up call that the North is changing very rapidly, along with the terrible fires that we’ve seen this year.”

The study suggests that warmer weather in the vicinity of the lake has reduced ice cover in the winter months and altered wind patterns that would otherwise stir the lake’s upper layers during the summer.

Kathleen Ruhland, a research scientist at Queen’s and co-author on the study, said the change in conditions has become pronounced enough to favour different species of diatoms – single-celled algae that are a primary food source in the lake. The study is based on an examination of sediment cores extracted from the bottom of the lake in 2014 – the most recent available. By comparing them with data from 20 and 70 years before, Dr. Ruhland consistently found “almost complete shifts” diatom species.

The data show that the lake was formerly dominated by diatoms that measure about 20 microns or two-100ths of a millimetre across. These are large by microscopic standards and so they require a degree of vertical turbulence in the water to stay suspended near the lake’s surface, where they can gather sunlight, but those conditions have changed.

“A longer growing season and reduced wind speed gives us a much more stable water column,” she said.

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The result, Dr. Ruhland said, is that the heavier diatoms sink to the bottom and species that measure about one-10th their size have proliferated in their place – “a potential early warning sign for future changes to other parts of the lake’s food chain.”

There are certain to be winners and losers in the transformed ecosystem, Dr. Smol said. Smaller diatoms make a better meal for different kinds of microscopic animals, which in turn may shift the food supply for larger species. A longer term question is how this may affect commercial and recreational fishing on Great Slave Lake, and the lake’s role as a food source for people in the area.

Indeed, it was the need to understand the potential for food productivity during the Second World War that prompted the first research efforts at Great Slave Lake. Those were conducted in the 1940s by Donald Rawson, a pioneering limnologist from the University of Saskatchewan. His research provided a key anchor point for the Queen’s study, as did core samples taken in the mid 1990s. Between those two time periods, changes in the lake were gradual, though it is clear that more diatoms were growing there than in the 19th century, when conditions were somewhat colder. The species shift occurred more quickly and more recently, between 1995 and 2014, matching an accelerated period of warming in the North.

Dr. Smol said more consistent monitoring is needed to track changes in the years since 2014. One worrying sign noted in the study is the presence of algal blooms, which have started to appear in portions of Great Slave Lake in the past 10 years and could pose risks to various species and to human health in the future.

“This portends widespread ecological consequences with a potential for increased likelihoods of algal blooms in the future for even large, cold lakes,” said Sapna Sharma, a researcher at York University in Toronto who studies the effects of environmental stress on lakes.

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