Phosphorus Reduction in Lakes May Limit Their Ability to Remove Nitrogen

A study of a Minnesota lake reveals that while cleanup efforts aimed at reducing phosphorus have been highly successful, they could ultimately prove problematic.

The report builds off of previous research regarding nitrogen levels in Lake Superior and could have implications for pollution control efforts throughout the world, according to the study's authors.

The researchers found that as phosphorus levels dropped, so did the microbial processes responsible for removing nitrogen from the water.

Based on their findings, the researchers posit that the anthropogenic acceleration of global nitrogen and phosphorus cycles have in turn led to an increase in the processes that remove nitrogen in small and medium-sized lakes.

Thus, in those lakes where phosphorus is controlled, nitrogen is allowed to accumulate, ultimately making its way downstream into rivers and coastal regions.

"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 in a statement. "By reducing one aquatic pollutant -- phosphorus -- we are in some cases reducing the ability of lakes to remove nitrogen."

Nitrogen and phosphorus pollution comes from a variety of sources. In the case of more developed communities, it often can be traced back to sewage, lawn fertilizer, vehicle exhaust and pets. In agricultural areas and other remote places, both air pollution dispersed through the atmosphere and farming are more frequently the cause.

According to Finlay, the study does not mean that phosphorus reduction efforts in lakes should stop, but that a more holistic approach should be adopted.

"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."