💧 Northern lakes transforming rapidly with warming

In 25 years of climate change, how have the lakes of the North changed? This question motivated a group from the Faculty of Forestry, Geography and Geomatics affiliated with the Centre for Northern Studies to travel to James Bay and Hudson Bay in 2021, after a first expedition in 1995.


At the time, Professor Reinhard Pienitz had sampled nearly 70 lakes located in the boreal forest to the south and in the tundra to the north. The researcher had studied diatoms, single-celled algae that reproduce very quickly and form the base of the food chain. "Aquatic microorganisms are good indicators of conditions in lakes," says Professor Pienitz. This scientific mission allowed for sketching a picture of the physical, chemical, and biological conditions of the lakes before warming.

In 1995, the state of the lakes aligned with the transition in terrestrial vegetation. "Lakes in the boreal forest are supplied with a lot of organic matter, which gives them a brownish color, like tea," the researcher recounts. "To the north, the lakes are almost transparent because they contain few nutrients," reports Professor Pienitz.

A quarter century of transformation

Professor Dermot Antoniades and PhD student Marie Alibert returned 25 years later to replicate the sampling campaign. The bare rocks and shrubs of the tundra had given way to dense vegetation. Lakes once easily accessible were now hidden. "We had to use machetes to carve a path through the brush," says the PhD student, first author of the study.

This northward shift of the boreal forest had the effect of modifying lake conditions and, consequently, the population of microorganisms living there. "When conditions are more favorable to certain species that are better adapted to heat or nutrients, for example, they will take advantage to reproduce more quickly. The dominant species will change," explains Professor Antoniades.

By comparing data from 1995 and 2021, the scientists found that the typical conditions of southern lakes—more brownish and nutrient-rich—were now found much further north. In just 25 years, this ecological transition zone had shifted by at least 68 km (about 42 miles) and up to 148 km (about 92 miles) in places, a considerable distance according to Marie Alibert.

Another striking difference: the transition zones associated with the boreal forest and tundra were no longer always aligned with the vegetation, unlike what was observed in 1995. "Lakes are responding more quickly to climate change," explains Professor Antoniades. Even though vegetation plays a big role, other factors such as permafrost thaw or lake warming can transform water properties.

To better see whether these upheavals are entirely new or have occurred in the past, the team also collected sediment cores from the bottom of the lakes. These columns make it possible to reconstruct the history of the lakes and verify whether the ecosystems have experienced transitions as rapid as these before.

Effects still poorly understood

This shift of conditions from south to north could have effects on biodiversity. "There is a risk of homogenization of species if all aquatic environments become similar. Species that were more adapted to cold and low organic matter will tend to fade away," adds Marie Alibert.

According to Reinhard Pienitz, the study helps highlight almost invisible manifestations of climate change. "Even if we don't notice them, they have already begun and we don't know the long-term consequences, especially for northern communities that depend on these lakes for drinking water and fishing."

The research team would like to return more often to this northern region to document the changes. For Professor Pienitz, this continuity is precious. "It's quite rare to have a long data series, spanning a quarter of a century. We have a golden opportunity to assess the consequences of warming."

The study was published in the journal Scientific Reports.