A new analysis of water body color confirms that lakes around the U.S. are reacting to their environments and changing their processes across entire landscapes, generating a useful tool for water managers and scientists for future study.

Researchers have understood the seasonal cycles of algal growth in lakes for years, though only across a relatively small number of bodies of water, maxing out at hundreds at a time. The new study, published May 11 in Water Resources Research, accounts for more than 26,000 lakes, making it the largest of its kind. It shows that lakes in the northeastern U.S. may be trending toward higher algae content, while those in the Pacific Northwest may be getting bluer.

"In the U.S. and globally, we spend billions of dollars on policy focused on protecting our freshwater resources, and if we're basing that policy off of a handful of lakes that we have good data for, it's very possible that that handful of lakes isn't really representative of what's happening across the landscape as a whole," said lead author Simon Topp, a recent geological sciences Ph.D. graduate from the University of North Carolina at Chapel Hill.

Lake color can reflect a range of water-quality attributes, such as chlorophyll content, sediments and colored dissolved organic matter, which, according to Topp, one can think of like the dark tannins in a cup of black tea. While color is highly correlated with these qualities, Topp says it cannot definitely indicate them without further analysis. 

Over the 36 years of data that their new model accounted for, Topp and his colleagues found that the number of green eutrophic lakes appears to have been increasing across the entire U.S. since the mid-1990s. These lakes, according to Topp, have high algae concentrations year-round, where other lakes tend to bloom seasonally, such as in summer or spring. 

Eutrophic lakes, teeming with algae, can starve lakes of oxygen and crater biodiversity. Not only are they the result of higher temperatures at higher latitudes, they emerge thanks to added nutrients in water, which can come from a number of sources. Agriculture, according to Topp, is a primary source of these nutrients because of fertilizer runoff, which contains phosphorus and nitrogen. 

While these lake processes are commonly known, they have never been studied at such large scales. Over time, lakes tended to change based on varying climate conditions and populations. 

"Lakes are not these constant, stable systems; lakes are constantly changing; they're constantly integrating the things that are going on around them," Topp said in an interview with The Academic Times. "If we treat them kind of like stable systems and take a snapshot of a lake and say, 'OK, this is how this lake is and how this lake has always been,' that's just not an accurate representation."

For instance, another study predicts that lake heat waves, periods of accentuated surface temperatures that can foster toxic algal blooms, will intensify by the end of the century.

While color signals indicating eutrophic lakes seemed to increase across much of the U.S., those suggesting clearer blue lakes became more common in the Pacific Northwest. These lakes, also known as aseasonal blue lakes, may have multiplied due to 1970s-era environmental protections such as the Clean Water Act, amended in 1986 and 1996, and the Endangered Species Act. Though he cannot definitely say that these laws improved the Pacific Northwest's water quality, a prior study of his confirmed that water clarity has been improving across the U.S. since 1984, even as old industrial pollution continues to put mercury into the Great Lakes. 

To create their model, Topp and his colleagues drew on the HydroLAKES dataset that contains the polygonal shape, among other features, of 1.4 million lakes around the world that are larger than 10 hectares (24.7 acres). They then examined Landsat observations from 1984 to 2020 of over 56,000 U.S. lakes in the HydroLAKES dataset. Landsat is NASA's satellite observation program that has been providing images of Earth's surface since the early 1970s, allowing scientists to study landscape changes over time. 

Where the HydroLAKES data provided the shapes of lakes, the Landsat images gave the color from reflectance signals, from which the researchers could determine the color of each lake. After controlling for lakes with the clearest seasonal color signals, the researchers incorporated data from over 26,000 lakes into their color analysis.

Because researchers cannot discern water body quality through color alone, Topp intends for his model to be useful for those who already understand what color signifies in their local lakes. For example, a water manager in Colorado, who ostensibly knows what's going on in the state's lakes, could access Topp's dataset to analyze and better understand how Colorado's water bodies have been changing over time. 

In future research, Topp wants to improve his model to account for the variables that account for water body color, such as chlorophyll, sediments and colored dissolved organic matter, rather than color alone. 

"Water inherently is a very connected system, so by taking what we know in individual lakes and trying to extrapolate that out, we learn more about those connections by looking at the landscape," Topp said. "And through long time periods, we can start to have a better grasp of how these things interact with each other."

The study, "Shifting patterns of summer lake color phenology in over 26,000 US lakes," published May 11 in Water Resources Research, was authored by Simon N. Topp, Tamlin M. Pavelsky and Xiao Yang, University of North Carolina at Chapel Hill; Hilary A. Dugan, University of Wisconsin-Madison; John Gardner, University of North Carolina at Chapel Hill and University of Pittsburgh; and Matthew R.V. Ross, Colorado State University.