Some picturesque blue lakes may not be so blue in the future, thanks to climate change.
In the first global count of the lake’s color, researchers estimate that about a third of the lakes on Earth are blue. But, if the average summer air temperature rises a few degrees, some of those crystal-clear waters could turn a cloudy green or brown, the team reports in the Sept. 28 issue. Geophysical Investigation Letters.
The changing hues could alter how people use those waters and offer clues to the stability of lake ecosystems. The color of the lake depends in part on what is in the water, but factors such as the depth of the water and the surrounding land use are also important. Compared to 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 his colleagues used satellite photos from 2013 to 2020 to analyze the color of more than 85,000 lakes around the world. Because storms and seasons can temporarily affect a lake’s color, the researchers focused on the most frequent color observed for each lake over the seven-year period. The researchers also created an interactive online map which can be used to explore the colors of these lakes.
The approach is “super cool,” says Dina Leech, an aquatic ecologist at Longwood University in Farmville, Virginia, who was not involved in the study. This satellite data is “so powerful.”
The scientists then looked at local climates during that time to see how they might be related to the color of lakes around the world. For many small or remote water bodies, temperature and precipitation records do not exist. Instead, the researchers also relied on “back forecasts” of climate calculated for each location in the world, pieced together from relatively sparse records.
Lakes in places with average summer air temperatures below 19° Celsius were more likely to be blue than lakes with warmer summers, the researchers found. But up to 14 percent of the blue lakes they studied are close to that threshold. If average summer temperatures rise another 3 degrees Celsius, an amount that scientists believe it is plausible at the end of the century — those 3,800 lakes could turn green or brown (Serial number: 8/9/21). That’s because warmer water helps algae bloom more, which changes the properties of the water, giving it a greenish-brown tint, Yang says.
Extrapolating beyond this sample of lakes is a bit tricky. “We don’t even know how many lakes there are in the world,” says study co-author Catherine O’Reilly, an aquatic ecologist at Illinois State University in Normal. Many lakes are too small to reliably detect by satellite, but by some estimates tens of thousands of larger lakes could lose their blue hue.
If some lakes become less blue, people will likely lose some of the resources they have come to value, says O’Reilly. The lakes are often used for drinking water, food or recreation. If the water is more clogged with algae, it could be unappealing to play in or more expensive to clean for drinking.
But color changes don’t necessarily mean lakes are less healthy. “[Humans] don’t value a lot of algae in a lake, but if you’re a certain type of fish species, you might say ‘this is great,’” O’Reilly says.
Lake color can hint at the stability of a lake’s ecosystem, with changing shades indicating changing conditions for creatures that live in the water. One of the benefits of the new study is that it gives scientists a baseline to assess how climate change is affecting Earth’s freshwater resources. Continuous monitoring of the lakes could help scientists detect future changes.
“[The study] it sets a marker against which we can compare future results,” says Mike Pace, an aquatic ecologist at the University of Virginia in Charlottesville, who was not involved in the study. “That is, to me, the great power of this study.”