Extreme rainfall and melting permafrost associated with a warming climate are causing more organic matter to wash into lakes, rivers and coastal waters.
This so-called “browning” is decreasing the ability of sunlight to disinfect water by a factor of 10.
This could have serious implications for drinking water supplies and coastal fisheries across the globe as it reduces the ability of the sun’s ultraviolet rays to disinfect water effectively, and could lead to an increase in diseases caused by waterborne germs, the researchers said.
The finding stems from a study that analyzed water samples collected from lakes around the world, from Pennsylvania to New Zealand.
Using a model from the U.S. National Center for Atmospheric Research, the investigators calculated the ability of UV radiation from the sun to destroy pathogens in the water of each lake, known as the solar inactivation potential.
The researchers determined how much UV light hits the surface of the various lakes throughout the year and how deeply it reaches. After measuring how much dissolved organic matter each water sample contained and assessing the wavelengths of light it absorbed, they estimated the impact this dissolved matter had on the sun’s germ-killing power.
“Much of the research emphasis up to this point has been on the browning itself, not the ecological consequences,” the study’s lead author, Craig Williamson, said in a news release from the research center.
“We were able to determine that, in some cases, browning is decreasing the ability of sunlight to disinfect water by a factor of 10. This could have serious implications for drinking water supplies and coastal fisheries across the globe,” he explained.
Williamson is an ecologist at Miami University in Oxford, Ohio.
The study, published online recently in Scientific Reports, found that browning in regional lakes has reduced the potential for UV radiation from the sun to inactivate pathogens in the water.
Areas around the lakes’ edges, which are heavily used by people, have a much higher concentration of dissolved organic matter than their centers, the researchers found.
The sun’s disinfecting power can also wane after a heavy rainfall, the study showed. For instance, using water samples from the region where the Manitowoc River flows into Lake Michigan, scientists found that the pathogen-killing effects of UV radiation were reduced by 22 percent after a strong storm, which washed more organic matter into the water. Lake Michigan supplies drinking water to more than 10 million people.
Water-treatment plants would also have more trouble killing waterborne germs because of an increase in dissolved organic matter, the study authors said.
The new findings highlight the importance of understanding the full effects of climate change, the researchers said.
“What happens in the atmosphere affects what happens in lakes,” Williamson said. “These are not separate compartments of the world. These things are all connected.”