The lakes of yesteryear have changed a lot. Clear waters succeeded an overabundance of algae. The observation, underlined in a new scientific article, is unequivocal: the production of organic matter thanks to photosynthesis has multiplied by seven in Canadian lakes after the 1960s. And climate change would be the main cause.
“It has been known for a long time that the new anthropogenic nutrients could increase the production of algae in the lakes. Now, we have proof that global warming also contributes to it, ”explains Irene Gregory-Eaves, biology professor at McGill University and author of the new study.
The project, an effort of several years led by scientists from the main universities of Quebec, is based on samples of sediment taken at the bottom of 80 lakes located from one end of Canada to the other. In these muddy “carrots”, researchers manage to detect the traces of the marine life of yesteryear.
In particular, they measure the presence of chlorophyll pigments, these molecules which allow algae to capture sunlight. The new study, published on July 24 in the journal Communications Earth&Environmentis the first to perform this exercise in such a systematic way and for such a large number of lakes, underlines Mme Gregory-Eaves.
The proliferation of algae has many consequences, such as the relaxation of toxins by cyanobacteria (blue-green algae) and the narrowing of fish habitat. That said, the most serious impact is probably the deoxygenation of the lakes – a phenomenon caused by bacteria, which “breathe” more oxygen by decomposing the whole new organic matter available.
From one lake to another, the increase in “primary production” (the expression used by scientists to designate the biomass generated by photosynthesis) varies a lot. By assembling the thousands of chlorophyll measures, made in sediment layers accumulated between 1850 and 2014, the upward trend became obvious. This increase begins in the middle of the XXe century and continues to accelerate today.
A “robust correlation”
Scientists – starting with Hamid Ghanbari, a postdoctoral researcher at Laval University and the first author of the article – tested different variables to check if they were going hand in hand with the multiplication of algae: air temperature, persistence of ice cover, energetic lighting of the sun and the use of land in the watershed.
Result: a “robust correlation” between the presence of algae and the climatic variables, that is to say temperature and ice. The more hot it is, the more the algae abound. This suggests “an alteration on a continental scale of the primary production of lakes in response to warming which accelerates”, write the authors. The energetic lighting of the sun, which varies according to the pollution of the air and the clouds, also plays a small role.
As for the use of the territory, the analysis detects a certain influence on primary production, but less strong than that of the climate. The lakes subject to a strong human imprint (urbanization, agriculture, deforestation, mines, etc.) are more likely to be invaded by algae. But the entire portrait makes the authors say that “even the lakes located in relatively intact watersheds are vulnerable”.
In some lakes “oligotrophes” (limpid and poor in nutrients), a slight increase in vegetable biomass can be beneficial, in particular to supply fish, admits mme Gregory-Eaves. “But in a Mésotrophic or Eutrophe lake [qui contient assez ou trop de nutriments]it will be harmful, ”says the professor, who is interested in upheavals that occurred in the lakes after the industrial revolution.
Researchers are now investigating the consequences of the multiplication of algae on biodiversity of lakes. They scrutinize the invertebrate sub-fossils in sediment carrots. They analyze the environmental DNA contained therein to determine the species involved. With more than 900,000 lakes on its territory, Canada is the country with the largest number of freshwater tablecloths in the world.