19/04/2021

Climate change alters the heartbeat of lakes and reservoirs

  • Collaborative study detects for the first time global changes in lake water mixing patterns caused by global warming


Every spring, as meadows and forests awaken noticeably from winter dormancy, lakes and reservoirs undergo an invisible transformation with fundamental consequences for all water uses: their surface waters warm up and within a few weeks they will be so warm and have lost so much density that they can no longer mix with the cold water at the bottom.

"Every year the water in lakes and reservoirs stratifies during the spring and summer," explains Daniel Mercado of the Catalan Institute for Water Research (ICRA). "This may seem anecdotal, but in reality the whole functioning of the ecosystem and all the uses we make of water are adapted to the periodicity of this phenomenon. If the heartbeat of stratification and mixing changes, the ecosystem and our way of managing and enjoying water in lakes and reservoirs will also have to change".

A study led by the Dundalk Institute of Technology in Ireland and published this week in the journal Nature Communications shows that the length of the stratification period in Northern Hemisphere lakes has lengthened during the 20th century, in some cases by up to a full month. "This has already had consequences for the water quality of many lakes," says Rafa Marcé, ICRA researcher and coordinator of the international modelling network (ISIMIP Lake Impact Sector) that made this work possible. "During stratification, the deep layers are isolated from the atmosphere and gradually deplete their oxygen content, which can become depleted. The oxygen deficit has very negative consequences for the ecosystem and for water quality, and if stratification lasts longer, these problems will only increase.

The study also makes projections of how the lengthening of the stratification period may affect lakes in the future if no action is taken to mitigate climate change. Daniel Mercado contributed to these simulations by modelling the behaviour 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 behaviour of a large number of lakes under 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 the international collaboration that was indispensable. The models predict that in a future where climate change is unchecked, annual lake stratification would lengthen by 20 to 40 days on average, depending on the degree of climate change mitigation we implement from now on.

The consequences of such a phenomenon would be close to disaster. "We are not aware of the importance of this because, unlike other changes such as crop flowering or animal migrations, we have no direct sensory experience of what happens in lake water," 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, which will drive out fish species that cannot tolerate it. The quality of the water we use for drinking and other purposes will also be affected by this oxygen deficit, which will mean spending more resources to enjoy the water safely.

But that is not all. Recent studies show that the lengthening of the stratification period in lakes favours the emission of methane from sediments, a gas with a potent greenhouse effect. "It's a clear example of feedback: the more global warming, the more stratification and the more methane produced in 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/

Platform for automatic measurement of water quality in the Sau reservoir. Among other variables, the platform monitors the stratification and oxygen deficit in the water. ©Rafael Marcé
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