An unknown process is dumping phosphorus into streams and lakes across the U.S., according to a new study (Environ. Sci. Technol. 2016, DOI: 10.1021/acs.est.5b05950). The resulting increase in phosphorus 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.
EPA stumbled on these results while analyzing data collected as the agency tracked the health of the nation’s waters, says EPA biogeochemist John L. Stoddard. Every five years, the agency measures the concentration of important ions and nutrients in a selection of lakes and streams.
Phosphorus was the only measured nutrient that changed, Stoddard says. Across the country, median total phosphorus in streams more than doubled from 26 to 56 µg/L over the past 10 years. In lakes, levels rose from 20 µg/L in 2007 to 37 µg/L in 2012. “The change was most dramatic in the more pristine parts of the country,” he says.
To avoid significant changes to ecosystems, phosphorus concentrations must stay below 10 µg/L. By 2014, only 1.6% of stretches of streams tested fell below that level.
“This is a surprising and provocative result that we wouldn’t have seen if EPA hadn’t been doing routine monitoring,” says Emily H. Stanley, an aquatic biogeochemist at the University of Wisconsin, Madison.
Stanley and the EPA scientists have some ideas about the sources of the extra phosphorus. Big storms, which have increased in the past 20 years, can wash soil particles rich in phosphorus into streams and lakes.
But perhaps the most likely cause is atmospheric deposition from sources such as dust. “Phosphorus sticks to soil and clay particles in dust and gets blown around by wind,” Stanley says.
The increases in big storms and dust are linked to climate change, Stoddard says, so it will be difficult to stop the trend.