Reducing Phosphorous In Lakes May Weaken Their Ability To Remove Nitrogen

Attempts to manage algae blooms and improve the water quality of large lakes may cause such bodies of water even more harm, according to new research. Lowering the level of phosphorous in large lakes can in turn hinder the microbial processes that remove nitrogen -- another potentially harmful nutrient present in the water -- University of Minnesota researchers reported in a study published online in the journal Science Friday.

Phosphorus is found in Minnesota’s many lakes after it is washed into them, frequently in the form of agricultural and lawn fertilizers. Excessive amounts of the nutrient can then cause algae blooms and otherwise lower the quality of the water, an issue that many water-cleanup efforts attempt to correct by reducing the phosphorous levels in affected lakes. The University of Minnesota researchers note this focus on cutting levels of phosphorous can result in significant accumulations of nitrogen in the lakes and increased nitrogen pollution in rivers and coastal regions downstream from them.

“Freshwater ecosystems, including lakes, streams and wetlands, are a large global sink for reactive nitrogen,” lead author Jacques C. Finlay, an associate professor in the College of Biological Sciences, said in a statement. “By reducing one aquatic pollutant -- phosphorus -- we are in some cases reducing the ability of lakes to remove nitrogen.”

The researchers reached their conclusion by analyzing differences in the amount of nitrogen that goes into lakes and the amount of nitrogen that later comes out of them downstream. They combined these data with “time-series analyses of nitrogen and phosphorus concentration in large lakes,” allowing them to gain a better understanding of the way removing phosphorus and nitrogen affects the levels of these nutrients and the quality of water.

“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 researcher added that the study highlights the need to “pay attention to the way that nutrients interact in ecosystems and maintain our focus on reducing phosphorus and nitrogen pollution.” And Finlay is convinced that combining these two goals will ultimately lead to better local and downstream water quality.