Some scenic blue lakes may not be so blue in the future, thanks to climate change.
In the first global lake color count, researchers estimate that about a third of Earth’s lakes are blue. But, if average summer air temperatures rise a few degrees, some of these crystal-clear waters could turn a murky green or brown, the team reports in September 28. Geophysical Research Letters.
The changing hues could alter how people use these 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 water depth and surrounding land use are also important. Compared to blue lakes, green or brown lakes contain 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 frequently observed color 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, Va., 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 relate to the color of lakes around the world. For many small or remote water bodies there are no records of temperature and precipitation. Instead, the researchers also relied on calculated climate “forecasts” for each location on the globe, which are pieced together from relatively sparse records.
Lakes in places where the average summer air temperature was below 19° Celsius were more likely to be blue than lakes with warmer summers, the researchers found. But up to 14% of the blue lakes they studied are close to that threshold. If average summer temperatures rise another 3 degrees Celsius – an amount scientists think is plausible by the end of the century — these 3,800 lakes could turn green or brown (SN: 08/09/21). This is because warmer water promotes the proliferation of algae, which changes the properties of the water and gives it a green-brown tint, Yang explains.
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 at Normal. Many lakes are too small to be reliably detected by satellite, but according to some estimates, tens of thousands of larger lakes could lose their blue tint.
If some lakes become less blue, people will likely lose some of the resources they’ve come to appreciate, O’Reilly says. 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 the color changes wouldn’t necessarily mean the lakes are less healthy. “[Humans] don’t give much importance to algae in a lake, but if you’re a certain type of fish species, you might think to yourself ‘that’s awesome,’” says O’Reilly.
Lake color can allude to the stability of a lake’s ecosystem, with changing shades indicating changing conditions for creatures living in the water. One of the benefits of the new study is that it gives scientists a basis for assessing how climate change is affecting the Earth’s freshwater resources. Continuous monitoring of lakes could help scientists detect future changes.
“[The study] defines a marker to 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, to me, is the great power of this study.”