Canada’s only remaining epishelf lake — made up of freshwater floating on saltwater — is gone, along with the unique ecosystem it supported, scientists confirmed in a new study.
Jérémie Bonneau, the lead author and an assistant professor at Laval University in Quebec City, said its loss is sad, as the Milne Fiord epishelf lake on Ellesmere Island in Nunavut was “the last thing like that in Canada.”
“Suddenly, it’s … drained away and then everything that was living in there is gone.”
Bonneau noted that the complete disappearance of a once common phenomenon shows how a warming climate is transforming the Arctic.
While scientists had previously reported the lake’s demise, the new study, recently published in the journal Scientific Reports, details how it happened and says the irreversible loss “marks a regime shift and the end of a 4000-year era.”
Major ice shelf lake in Canada has disappeared
Glaciers are rivers of ice. Typically, they flow into the sea, and so does any meltwater on their surface. Normally, that freshwater is quickly mixed into the seawater by wind and waves.
But the mouth of Milne Fiord has been dammed for thousands of years by the Milne Ice Shelf — a huge, flat piece of ice sitting on the ocean and attached to the north shore of Ellsemere Island.
The dam stopped freshwater from flowing into the ocean, allowing it to accumulate on top of the saltwater of the Arctic Ocean. It formed a floating layer because freshwater is less dense than saltwater.
Meanwhile, a layer of ice about 50 to 100 centimetres thick covered the lake year-round, protecting the freshwater and saltwater layers from winds that could have mixed them together — and making the lake invisible from the surface.
Bonneau said there’s evidence the lake in Milne Fiord had existed and been accumulating freshwater for more than 4,000 years.
When researchers first measured it in 1983, its freshwater layer was 18 metres deep, said Andrew Hamilton, a University of Alberta researcher who co-authored the new study. That’s similar to the height of a six-storey building.
While some epishelf lakes sit completely on top of an ice shelf — that’s what epishelf means — the Milne Fiord epishelf lake was actually connected to the ocean by an under-ice channel in the ice shelf, which has been thinning for about a century.
By looking at the bacteria, viruses and phytoplankton in the two layers of the lake, researchers discovered practically no overlap between the freshwater and saltwater communities living just 20 centimetres apart.
Bonneau said, “It’s really two unique ecosystems in the same water column, and you don’t get that anywhere else.”
Mary Thaler, who previously analyzed water from the lake for DNA but was not involved in the new study, said that while humans or fish could easily swim across the invisible border between the fresh and saltwater, for single-celled organisms, it’s “an unbreakable barrier.”
Analysis by Thaler and her colleagues at Laval University found bacteria and more complex eukaryotic microbes, whose cells are similar to those of plants and animals, as well as viruses. They included species that usually live in sediments on the bottom rather than in the water itself. Some of the microbes found in the lake had never previously been reported in the Arctic.
But the lake also had an influence beyond its own borders. Hamilton sent a remotely operated vehicle (ROV) into the channel in the ice shelf, where freshwater flowed out of the lake, mixed with saltwater along the way and carried nutrients to a lower depth than they would otherwise be found.
It recorded video footage of ghostly sea anemones, scallops and sea stars thriving in complete darkness below 10 metres of ice, fed by the nutrients draining from the epishelf lake.
“We were absolutely astounded to find a full, flourishing ecosystem existing within kind of this ice channel,” he said.
Then, in 2020, the Milne Ice Shelf broke into pieces off the coast of Ellesmere Island, and its wall no longer held the lake in.
Reverberations of the Milne Ice Shelf collapse
The new study looks at what happened to the ice and the lake itself before and after the collapse.
The researchers had probes called moorings that ran from the floor of the fiord to the surface, measuring salinity at different depths.
They showed that within two months of the ice shelf breaking up, the freshwater had drained and none was left in the fiord.
Thaler said freshwater organisms living there would have died more or less instantly when mixed with seawater.
While the freshwater lake was ice-covered year-round, the saltwater fiord is ice-free for part of the summer, allowing it to warm and be mixed by wind and waves — a typical marine environment suitable for organisms different from those supported by the former lake.
“And so you kind of start to see this huge change,” Bonneau said.
While epishelf lakes are now completely gone from Canada, a few still exist in Greenland and Antarctica.
“Unfortunately,” Hamilton said, “I think their days are numbered…. This kind of highlights that we are on a one-way trip for some of these really astounding ecosystems that make up the beauty of our planet.”
