Between 2007 and 2012, lakes tinted with a greenish-brown hue—officially classified as “murky”—stealthily overtook blue lakes to become the country’s dominant lake type, Cara Giaimo reports for Atlas Obscura.

At the start of this five-year period, blue lakes represented 46 percent of the freshwater bodies included in the Environmental Protection Agency’s National Lakes Assessment (NLA). By 2012, this figure had dropped to 28 percent; at the other end of the spectrum, the percentage of murky lakes skyrocketed from 24 percent to 35.4 percent.

Researchers from the EPA, Virginia’s Longwood University and Washington State University relied on NLA data to evaluate the current state of America’s lakes and, according to a press release, assess encroaching murkiness’ “potential negative consequences for water quality and aquatic life.” The team’s findings are newly published in Limnology and Oceanography.

Color can reveal information about a lake’s nutrient load, algal growth, water quality and surrounding landscape, explains Kevin Rose, an environmental biologist at New York's Rensselaer Polytechnic Institute, in a separate article published in online journal Lake Scientist.

There are three main categories of lake color: blue, green and brown. Although each type carries its own connotations, none is inherently worrisome.

Blue lakes contain low concentrations of algae and organic matter, meaning they can only support certain fish populations. According to Rose, such lakes speak to both “the pristine character of the water and low human impact in the surrounding watershed.”

Green lakes carry more nutrients than blue lakes and therefore encourage the growth of chlorophyll-filled algae. This algae produces green lakes’ hue and enables them to host thriving fish families, but as the warmer summer months approach, poor water quality conditions increasingly deprive many fish of oxygen, eventually killing them.

Brown water lakes, which are often found near forests or wetlands, are filled with organic matter such as dirt and dead plants. These lakes offer little light, curbing the quantity of algae and organisms capable of surviving in their dark conditions.

Murky lakes contain high quantities of both algae and organic matter, Atlas Obscura’s Giaimo notes. Lakes bearing such greenish-brown or brownish-green tints tend to be of low water quality.

Still, lead author Dina Leech, a biologist at Longwood University, tells Giaimo that “lakes could naturally exist in any one of these states.” The largest cause for concern, Leech concludes, is “over five years—this short period of time—we see lakes shifting to murky.”

The scientists’ findings draw on data procured by the EPA’s quinquennial National Lakes Assessment, a statistical survey designed to “provide information on the extent of lakes that support healthy biological condition and recreation, estimate how widespread major stressors are that impact lake quality, and provide insight into whether lakes nationwide are getting cleaner.”

According to Giaimo, volunteers visit more than 1,000 lakes across the country to collect information on their size, depth and water temperature. For the purposes of the new study, researchers narrowed down the data to the amount of phosphorus found in a lake and its “true color,” which is determined by comparing filtered water samples to a color wheel.

Laboratory Equipment’s Seth Augenstein reports that Northern Appalachia, the Southern Plains and other arid regions experienced the largest increase in murky waters. It’s unclear what factors caused this shift, but a press release notes the scientists suspect three main culprits: land cover, climate change and land use patterns, which means murky lakes tended to host more agricultural endeavors in their watersheds, or the regions from which they receive runoff.

“Blue lakes typically are those that do not show evidence of nutrient pollution or elevated organic matter while murky lakes have high levels of both,” Leech said in a statement. “A shift toward murkiness is a management concern because murky lakes tend to have more algae, including potentially harmful cyanobacteria. And with poor food quality at the base of the food web, over time murky lakes may not be able to support a healthy fishery.”