Improvement in municipal wastewater treatment alters lake nitrogen to phosphorus ratios in populated regions

Significance Due to different rates of change in total nitrogen (TN) and total phosphorus (TP) concentrations in lakes, increases in TN/TP mass ratios were observed in many China’s freshwater lakes during 2008 to 2017. This growing imbalance has important implications for aquatic ecology that remain poorly considered and understood. Here, we show that changes in municipal wastewater treatment are a major driver for increases in lake TN/TP mass ratios, as phosphorus is more effectively removed than nitrogen from wastewater. Our findings highlight the need for more efficient nitrogen reduction in addition to phosphorus reduction in wastewater treatment to reduce risk for phytoplankton blooms and toxin production and to maintain ecosystem biodiversity in downstream waterbodies.