If global warming continues, the world's blue lakes are in danger of turning green-brown, writes Phys.org, referring to a publication in the journal "Geophysical Research Letters".

The current study presents the first global inventory of the color of lakes., adds BTA.

According to its authors, changes in the colors of their water can mean a deterioration in the health of ecosystems.

Although substances such as algae and sediments can affect the color of lakes, the present study shows that the air temperature, precipitation, depth and altitude of these bodies of water also play an important role in determining the most common water color.

Blue lakes, which make up less than a third of the world's lakes, tend to be deeper and are in cool, high-altitude regions with high rainfall and ice cover in winter.

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Green-brown lakes, making up 69 percent of all lakes, are more widespread and are in drier regions, inland and along coastlines, the study found.

Researchers used more than 5 million satellite images of 85,360 lakes and water reservoirs around the world from 2013 to 2020 to determine the most common color of their water.

"No one has studied the color of lakes on a global scale before," said Xiao Yan, a hydrologist at Southern Methodist University and an author of the study. 

The color of a lake can change seasonally, in part due to changes in algae growth.

For this reason, the authors characterized the color of the lake by evaluating its most common shade over seven years.

The new study also examines how different degrees of warming could affect water color if climate change continues.

The study shows that climate change could reduce the percentage of blue lakes, many of which are found in the Rocky Mountains, northeastern Canada, northern Europe and New Zealand.

"Warmer water, which causes more algal blooms, is likely to shift the lakes toward green colors," said Kathryn O'Reilly, an ecologist at the University of Illinois.

North America's Great Lakes, which are among the fastest warming, are seeing increased algal blooms, O'Reilly notes.

Previous research has found that even in remote Arctic regions there are lakes that are turning greener.

O'Hara Lake, Yoho National Park pic.twitter.com/4ZhkLFtAYz

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While previous studies have used more complex and precise indicators to understand the overall health of lake ecosystems, water color is a simple but reliable indicator of water quality that can be monitored by satellites on a global scale.

This approach makes it possible to study how remote lakes change with climate.

"If you use the lakes for fishing, food or drinking water, the changes in lake quality that are likely to occur with greening will mean that it will be more expensive to treat," O'Reilly adds.

"There may be periods where the water is unusable and the fish species disappear, so we won't get the same ecosystem services from these lakes when they turn from blue to green."

Additionally, changes in water color can have recreational and cultural implications in places like Sweden and Finland.

As warming continues, lakes in northern Europe are likely to lose their winter ice cover, which may affect related activities.

"No one wants to swim in a green lake, so from an aesthetic standpoint, some of these bodies of water that we've always thought of as places of recreation or spiritual refuges can disappear with the color change," says O'Reilly. 

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