Some ornamental blue lakes may be so blue in the future due to climate change.
In the first global map of lake color, researchers estimate that nearly a third of Earth’s lakes are blue. But, if average summer air temperatures rise a few degrees, some of those crystalline waters could turn dark green or brown, the team reports on Sept. 28. Geophysical Research Letters.
The different colors could change how people use those waters and offer clues about the stability of the lake’s ecosystem. Lake color depends in part on what is in the water, but factors such as water depth and surrounding land also play a role. Compared with blue lakes, green or brown lakes have more algae, sediment and organic matter, says Xiao Yang, a hydrologist at Southern Methodist University in Dallas.
Yang and colleagues used satellite images from 2013 to 2020 to analyze the color of more than 85,000 lakes around the world. Because weather and seasons can affect the color of a lake over time, the researchers ranked the most common color of any lake observed over a seven-year period. The researchers also created an interactive online map that can be used to explore the colors of these lakes.
The approach is “super cold,” says Dina Leech, an aquatic ecologist at Longwood University in Farmville, Va., who was not involved in the study. These satellite data are “only powerful.”
The scientists then looked at local climates at the time to see how lake colors around the world might be linked. For many small or remote bodies of water, temperature and precipitation records are not available. Instead, the researchers also “suddenly” relied on counting every spot of the globes that were connected to each other among the rare records.
Researchers found that lakes in areas with average summer air temperatures that were below 19° Celsius were bluer than lakes in warmer summers. But up to 14 percent of blue lakers studied near that threshold. If average summer temperatures rise another 3 degrees Celsius — as much as scientists think is plausible by the end of the century — those 3,800 lakes could turn green or brown (SN: 8/9/21). That’s because warmer water helps algae bloom more, which changes the properties of the water, giving it a green-brown color, Yang says.
Extrapolating beyond this sample pool is a bit tricky. “We don’t even know how many lakes there are in the world,” says study coauthor Catherine O’Reilly, an aquatic ecologist at Illinois State University in Normal. Many lakes are too small to be reliably detected by satellite, but by some estimates tens of thousands of larger lakes could lose their blue color.
If some businesses become less blue-collar, people probably lose some resources that they have come to value, says O’Reilly. A lake is often used for water, food or recreation. If the water is more clogged with algae, it can be unforgivable or more expensive to drink.
However, color changes do not necessarily mean that the lake is less healthy. “[Humans] “They don’t appreciate a lot of algae in the lake, but if you’re a certain type of fish, maybe you’re like, ‘This is great,'” says O’Reilly.
The color of a lake can hint at the stability of a lake’s ecosystem, with different shades indicating changing conditions for the critters living in the water. One benefit of the new study is that it gives scientists a basis for assessing how climate change will affect Earth’s freshwater resources. Large, continuous surveys can help scientists detect future changes.
“[The study] It sets the stage for what we can compare to future events,” said Mike Pace, an aquatic ecologist at the University of Virginia in Charlottesville, who was not involved in the study. “That, to me, is the big strength of this study.”
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