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Every spring, while meadows and forests are clearly awakening from winter lethargy, lakes and reservoirs undergo an invisible transformation with major consequences for all water uses: their surface waters warm and in a few weeks will be as hot and will have lost so much density that they will no longer be able to mix with the cold water at the bottom.
“Every year the water in the lakes and reservoirs is stratified during the spring and summer”, explains Daniel Mercado of the Catalan Water Research Institute (ICRA) . “This may seem like an anecdotal fact, but in reality the entire functioning of the ecosystem and all the uses we make of water are adapted to the periodicity of this phenomenon. If the pulse of stratification and mixing changes, the ecosystem and the way we manage and enjoy water in lakes and reservoirs will also have to change.
A study led by Ireland’s Dundalk Institute of Technology and published this week in the journal Nature Communications shows that the duration of the stratification period in Northern Hemisphere lakes has lengthened during the 20th century, in some cases by whole month “This has already had consequences on the water quality of many lakes”, says Rafa Marcé , ICRA researcher and coordinator of the international modeling network (ISIMIP Lake Impact Sector) that has made this work possible . “During stratification, the deep layers are isolated from the atmosphere, and they are depleting their oxygen content, which can become depleted. The oxygen deficit has very negative consequences for the ecosystem and for the quality of the water, and if the stratification lasts longer these problems will only increase.’
The study also makes projections of how the lengthening of the stratification period may affect the lakes in the future if no action is taken to mitigate climate change. Daniel Mercado has contributed to these simulations by modeling the behavior of 17,000 lakes and reservoirs on Earth on Undarius , ICRA’s supercomputer . “For the first time, scientists from all over the world have coordinated to simulate the behavior of a large number of lakes in various climate change scenarios and using many different models,” explains Daniel. “This is impossible to do alone, so we created the ISIMIP Lake Impact Sector to promote an international collaboration that was indispensable.” Models predict that in a future where climate change is not curbed, annual lake stratification would lengthen by 20 to 40 days on average, depending on how much climate change mitigation we apply from now on.
The consequences of this would approach disaster. “We are not aware of the importance of this because, unlike other changes such as the flowering of crops or animal migrations, we do not have a direct sensory experience of what happens in the water of the lakes”, explains Rafa Marcé . “But the consequences for diversity will in many cases be irreversible. Many lakes that are now well oxygenated will develop oxygen deficits, driving out fish species that cannot tolerate it. The quality of the water we use for drinking and other uses will also be affected by this oxygen deficit, which will mean spending more resources to be able to enjoy the water with guarantees.’
But this is not all. Recent studies point out that the lengthening of the stratification period in lakes favors the emission of methane from the sediments, a gas with a powerful greenhouse effect. “It’s a clear example of feedback: more global warming, more stratification and more methane produced in the lakes, which will lead to more warming,” explains Daniel Mercado . “This is just another of the many pieces of evidence we have on the table that should make us act to stop climate change immediately, without delay.”
Publication :
R. Iestyn Woolway, Sapna Sharma, Gesa A. Weyhenmeyer, Andrey Debolskiy, Malgorzata Golub, Daniel Mercado-Bettín, Marjorie Perroud, Victor Stepanenko, Zeli Tan, Luke Grant, Robert Ladwig, Jorrit Mesman, Tadhg N. Moore, Tom Shatwell, Inne Vanderkelen, Jay A. Austin, Curtis L. DeGasperi, Martin Dokulil, Sofia La Fuente, Eleanor B. Mackay, S. Geoffrey Schladow, Shohei Watanabe, Rafael Marcé, Don C. Pierson, Wim Thiery, and Eleanor Jennings (2020) : Phenological shifts in lake stratification under climate change. Nature Communications https://doi.org/10.1038/s41467-021-22657-4
Additional information : WATExR project: https://watexr.eu/
