Researchers found that efforts to reduce phosphorous in Minnesota's lakes could actually cause more pollution in the water.
Techniques that reduce phosphorous could also kill off microbial processes that help cut down nitrogen, a University of Minnesota press release reported.
When too much nitrogen builds up in large lakes it can cause pollution to flow downstream and even into coastal areas.
Man-made acceleration of phosphorous and nitrogen cycles have helped out nitrogen removal processes, but this effect is counteracted in some of the larger lakes that undergo successful phosphorous removal.
"Freshwater ecosystems, including lakes, streams and wetlands, are a large global sink for reactive nitrogen," lead author Jacques Finlay, an associate professor in the College of Biological Sciences (CBS), said. "By reducing one aquatic pollutant - phosphorus - we are in some cases reducing the ability of lakes to remove nitrogen."
The researchers looked at how much nitrogen enters the lake and how much leaves downstream to make their conclusion. The team also employed "time-series analyses" to analyze the concentrations of nitrogen and phosphorous in the lakes.
"The work was motivated by our thinking about the case of a single lake - Lake Superior. This lake is one that we would expect to efficiently remove nitrogen, but it doesn't, and it has extremely low phosphorus, so this work arose from efforts to generalize beyond a single system," Finlay said.
The excess nitrogen stem from a number of sources including: "sewage, lawn fertilizer, vehicle exhaust and pets." Farming is also a huge contributor to nutrient runoff, as well as airborne pollution.
The researchers stressed that the study was not suggesting efforts to remove phosphorous from the lakes should be reduced.
We need to pay attention to the way that nutrients interact in ecosystems and maintain our focus on reducing phosphorus and nitrogen pollution," Finlay, said. "If we do that, we'll be taking steps toward improving water quality locally as well as downstream."
Gaston Small, an assistant professor at the University of St. Thomas, and Robert Sterner, a fellow CBS ecology professor, co-authored the study.
