FLATHEAD LAKE – Large-scale, rapid advances in wastewater treatment systems are on the rise all over the world. However, the impact on the balance of key nutrients in the waterbodies that these wastewater treatment systems feed into rarely has been examined on a broad scale over a long period of time.

In a recent study published in the Proceedings of the National Academy of Sciences, an international team of researchers discovered a link between the dramatic improvement in wastewater treatment and potentially harmful changes in nutrient ratios that could threaten the biodiversity and ecosystems of freshwater lakes.

The team included Jim Elser, director of the University of Montana’s Flathead Lake Biological Station.

“We know from recent studies in lakes that not only the absolute concentrations of nutrients like nitrogen and phosphorus matter for the health of a lake,” Elser said. “It is also important to pay attention to their relative concentrations. Strongly imbalanced ratios of N and P can have unappreciated and undesired outcomes.”

Analyzing continuous lake monitoring data from 46 lakes in China over a 10-year span, scientists found that the ratio between nitrogen and phosphorus concentrations increased in 24 of the 46 lakes. The ratio between nitrogen and phosphorus decreased in only three of the 46 lakes.

This discovery is important because food webs in lakes work best when there is a balanced supply of nitrogen and phosphorus. If that balance is altered – if, for example, a local wastewater treatment plant is extremely effective at removing phosphorus but not nitrogen from the water it returns back to the lake – then the microorganisms at the bottom of the food web have a change in their nutrient diet. This, in turn, can impair the performance of organisms higher in the food web.

Researchers involved in the study emphasize that improving municipal wastewater collection and treatment worldwide remains an important and worthy target within the 17 sustainable development goals presented by the United Nations.

However, given the potential ecological impacts on biodiversity and ecosystem function of altered nutrient ratios in wastewater discharge, researchers do suggest that long-term strategies for domestic wastewater treatment not only focus on the total reduction of nutrient discharges but also should consider maintaining the appropriate nutrient balance for the respective body of water.

For this study, researchers primarily relied on data from highly productive lakes in China’s more populated regions. But Elser believes the study’s findings may also apply to Flathead Lake here in lightly populated western Montana. He currently is working on a new manuscript that examines the concentrations and ratios of nitrogen and phosphorus in Flathead Lake over the past few decades to determine the impact of local wastewater treatment on Flathead Lake’s food web.

“Flathead Lake has low nutrient levels due to its relatively pristine watershed, as well as the implementation of wastewater treatment, which is great,” said Elser. “However, Flathead Lake also has imbalanced concentrations of nitrogen and phosphorus, which may possibly be a result of stringent removal of P but not N in those treatment plants. Our emerging paper explores this and describes some of the potential consequences.”

Other authors on the study include Yindong Tong, Mengzhu Wang and Xueyan Liu from Tianjin University in China; Josep Peñuelas from CREAF in Spain; and Hans Paerl from the University of North Carolina-Chapel Hill. The full study is online at https://www.pnas.org/content/117/21/11566.