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When the atmosphere heats up, lake beds cool down

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septembre 26, 2019 // par Evelyne Dufresne
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“We shouldn’t just assume that lakes are getting warmer due to global warming. It’s actually much more complex than that,” says Isabelle Laurion, professor and limnologist at Institut national de la recherche scientifique (INRS). The most recent publication co authored by Professor Laurion sheds light on the situation and challenges notions previously accepted by the scientific community. 
 
© INRS
 
The results of the study, published in Limnology and Oceanography Letters  in August 2019, show that in response to climate change, bottom waters in lakes show an inverse tendency to cool down. This challenges the idea, generally accepted in the scientific community, that lakes in their entirety are warming up.  In addition, greater thermal stratification increases the rate of carbon burial in sediments as well as the production of methane, a greenhouse gas (GHG) more powerful than carbon dioxide (CO2), and its release into the atmosphere. The authors of the study hope their findings will encourage researchers to approach the issue from a new angle.
 
 
A problem rooted in transparency loss
Eutrophication is an ecosystem imbalance caused by the excessive growth of algae.

Browning is the result of an overabundance of dissolved organic matter in lake water.
Eutrophication and browning in fresh water leads to a reduction in lake-water transparency and plays a key role in the thermal stratification of lake systems. More solar energy gets trapped in the surface layers, resulting in bottom-water cooling and oxygen depletion. This disrupts the microbial activity of the carbon cycle, particularly with regard to methane production. 
 
The study also revealed that smaller, shallower lakes are more acutely and quickly affected by climate change than larger lakes. 
 
 
Lakes as GHG emitters
“Lakes play a significant role in the global carbon cycle,” explains Professor Laurion. “They receive carbon from the terrestrial environment and from algae in the form of carbon dioxide (CO2) or methane and then release some of that carbon into the atmosphere.” 
 
According to the study, bottom-water cooling slows down microbial activity, which promotes carbon burial. While it is known that global warming causes lakes to emit a significant amount of CO2, the model results from the study indicate that GHG emission potential increases when methane production increases. 
 
More intense and stable stratification combined with longer summers leads to oxygen depletion in lakes. “When there’s less oxygen, microbial activity produces methane rather than CO2,” explained Professor Laurion. The decrease in oxygen offsets the effect of the lower water temperature, which adversely affects methane production. Overall, the amount of methane produced is increasing.
 
Effects of climate change combine with lower temperatures and lower oxygen levels in bottom waters due to transparency loss.
 
Effects of climate change combine with lower temperatures and lower oxygen levels in bottom waters due to transparency loss.
 
The two-part study was conducted in collaboration with the University of Basel in Switzerland and Université de Montréal. The research team started by analyzing the observational data collected in scientific literature, then ran simulations using the FLake modeling tool. FLake allows researchers to predict the vertical temperature variation and mixing conditions of lakes at various depths over several years. The team used the tool to simulate the synergistic effects of climate change and transparency loss. 
 
Isabelle Laurion has been a professor at INRS since 2002 and is a member of Groupe de recherche interuniversitaire en limnologie (GRIL) and the Centre for Northern Studies (CEN). She studies the effects of climate change on lake dynamics, with a focus on Arctic landscapes. Professor Laurion has a master’s degree in oceanography from Université du Québec à Rimouski and a PhD in biology from Université Laval. ♦

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