Warming climate reshapes base of Great Slave Lake’s food web

Climate change has altered algae that make up the base of Great Slave Lake’s food web, according to a new study. The change signals a fundamental shift in the ecosystem of North America’s deepest lake.

It’s still unclear what the change might mean for fish, but the shift is bound to have repercussions, said John Smol, a biology professor at Queen’s University in Kingston and one of the study’s authors.

“To me, it’s almost inconceivable that we’re not going to have some sort of cascading effects going up the food chain,” he said.

The North is warming up to four times faster than the rest of the globe. The region and its residents are feeling the effects of rapid warming.

Lakes are bearing the brunt of climate change, too. Smol and his colleagues have previously studied small to mid-sized Arctic lakes, finding dramatic changes in their algae and invertebrates.

Until now, large northern lakes were generally thought to be buffered from these climate impacts, said study co-author Kathleen Rühland, a research scientist in the biology department at Queen’s University.

“We’ve always thought of these big lakes as being somewhat protected,” she said.

Seeing changes in algae in a lake as big as Great Slave suggests that we’ve “shifted into a new climate regime,” said Jennifer Korosi, an associate professor and limnologist (researcher of inland bodies of water) at York University in Toronto, who said the study was well done. Korosi was not involved in the recent study but was supervised by Smol during her PhD.

Not only could the change in algae have cascading effects on the food chain, but Korosi said the findings highlight that other changes in the lake may be going unnoticed.  

Changes in the lake environment, such as increases in temperature and growing season, that are affecting algae are “undoubtedly impacting other organisms as well,” she said.

“It tells us we need to be taking a look at them, and we need to be monitoring the system closely,” she said.

A ‘poster child’ of climate warming

Compared to big lakes in southern regions, little research has been conducted on large, high-latitude lakes.

“Great Slave Lake is the deepest lake in North America. I mean, it’s the size of Belgium, and it’s deeper than the CN Tower,” Smol said. “Yet we know so little about the biology or how it’s changed.”

To understand how warming has affected Great Slave Lake, the researchers examined sediment cores taken from the bottom of the lake, allowing them to peer back roughly 200 years in time.  

Specifically, they looked at diatoms, single-celled algae that form the base of the lake’s food web and settle at the bottom of the lake when they die.

“They’re very good indicators of what’s happening in the lake,” Smol said.

Unlike most big northern lakes, Great Slave Lake has actually been the subject of some research. Studies of the lake’s characteristics, including diatoms, were conducted in the 1940s and ’50s as well as in the ‘80s and ‘90s. Scientists also took sediment cores from the lake in the ‘80s and ‘90s.

In the new work, Smol and Rühland drew on this existing work and cores from the 1990s. They teamed up with Marlene Evans, of Environment and Climate Change Canada, to collect another set of sediment cores in 2014.

The cores revealed that, until recently, a large, heavy species of diatoms dominated the lake, the researchers reported on Tuesday in the journal Proceedings of the Royal Society B. These heavy diatoms thrived in the lake as they bloom under ice and need wind to mix the water to stay near the surface, where they can photosynthesize, the researchers said.

Conditions have changed, however, with increasing ice-free days, lower wind speeds and higher temperatures recorded at a weather station in Hay River. In the late 1990s to early 2000s, the heavy diatoms were largely replaced with tiny, pancake-shaped species.

“The other ones are still there, but these ones are really dominating,” Rühland said. “We see them kind-of explode after 2000 or so.”

Rühland said the shift is a “poster child signal of climate warming” that has been seen in other lakes.

“We were quite surprised how striking the changes were in Great Slave Lake,” Smol said, adding that the lake was protected from climate impacts for a long time. “But boy, it’s changing over the last two decades. It’s been changing and changing fast.”

Climate ‘winners and losers’

The speed of the shift stands in contrast to another, much slower shift the researchers noticed earlier in the sediment cores.

In the oldest layers of sediment, diatoms were few and far between, but they began to accumulate starting in the 1800s. Up until 2000, Rühland said, the diatom population had been stable.

Smol said the abruptness of the recent change is surprising.

“We live in a world where we think everything happens gradually. With climate, we have tipping points and thresholds. And here’s a classic one,” he said.

The researchers say they still don’t know how these changes will affect the rest of the food web but, with algae serving such an important role in the lake, they imagine there must be an effect on other parts of the ecosystem.

“There are winners and losers in climate change,” Smol said. “With less ice cover, you’re getting more algae, and so you may even have more food, but we don’t know if that’s going to be good or bad.”

For instance, that might increase the numbers of certain fish, increase the number of their competitors, or displace native species, the researchers said.

Smol and Rühland said more research is needed to learn more about how the lake is changing and what these changes mean for higher levels of the food web, including fish populations that feed so many of the NWT’s residents.

With more warming locked in over the coming decades due to emissions to date, Rühland said the findings should serve as a call to attention.  

“I don’t want to alarm people, but I think we need to wake them up a little bit,” she said. “This is really changing quickly.”