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A new study reveals that an unknown continental-scale process is dumping phosphorus into streams and lakes across the U.S. (Environ. Sci. Technol. 2016, DOI: 10.1021/acs.est.5b05950). Rising phosphorus measured in these water bodies could lead to toxic algal blooms and degraded habitat for fish, birds and frogs.

High phosphorus levels in streams and lakes typically result from sewage discharge and agricultural runoff. But the new work finds phosphorus pollution in remote areas far from such sources, leaving researchers scratching their heads about where it came from. What evidence they have suggests the phosphorus inputs are probably linked to climate change, and are unlikely to be tamed anytime soon.

Phosphorus is an essential nutrient. But when levels top 10 µg/L in water bodies, ecosystems start to change. The kinds of algae that feed a healthy ecosystem begin to disappear, and undesirable species take over, says Emily H. Stanley, an aquatic biogeochemist at the University of Wisconsin, Madison, who was not part of the study. One group of undesirables, cyanobacteria, can produce toxic blooms that threaten drinking water sources and cost the U.S. economy over $2.2 billion per year.

To track the health of the nation’s waters, the Environmental Protection Agency monitors a selection of lakes and streams, measuring the concentration of important ions and nutrients every five years.

EPA stumbled on the new result while analyzing these data, says John L. Stoddard, a biogeochemist with the EPA. Phosphorus was the only measured nutrient that changed, he says. Uniformly across the country, median total phosphorus in streams more than doubled from 26 µg/L to 56 µg/L over the last 10 years. In lakes, levels rose from 20 µg/L in 2007 to 37 µg/L in 2012.

The researchers used national data on land cover to determine whether the landscape at each sampling site was urban, farmed, or wild. Stoddard also estimated rainwater runoff at the remote sites based on U.S. Geological Survey records.

“The change was most dramatic in the more pristine parts of the country,” he says. In 2004, nearly 25% of stream kilometers in the U.S. had total phosphorus levels in the healthy range, below 10 µg/L. By 2014, only 1.6% of stream kilometers fell in that range.

Wastewater and farm runoff are not to blame, Stoddard says. These sources carry excess nitrogen along with phosphorus, but nitrogen levels did not rise at the sampled sites. And the largest phosphorus increases were in remote areas far from sewage plants and big agriculture.

“Because the phosphorus increases were continental in scale, there are two major processes that might be responsible,” Stanley says. Big storms, which have increased in the last 20 years, can wash soil particles rich in phosphorus into streams and lakes, she says. Although the researchers did not find evidence of large-scale increases in runoff, Stoddard cautions that their data sources may not be fine-grained enough to capture an increase in episodic high-runoff events.

Atmospheric deposition of phosphorus is the remaining explanation, Stanley says. “Phosphorus sticks to soil and clay particles in dust and gets blown around by wind.” In studies focused on the western U.S. and Spain, researchers have measured increased dust deposition in water bodies and linked it to higher phosphorus concentrations and increased algal biomass. Other research suggests that atmospheric deposition of phosphorus is on the rise largely due to increased dust and emissions from burning biomass, Stoddard says.