Rapid phosphorus removal promotes a sustained decrease in nitrogen in eutrophic lakes

Abstract Excess phosphorus (P) leads to lake eutrophication, prompting interventions to reduce P availability. However, we have a limited understanding of how P availability impacts nitrogen (N), which also affects lake water quality. The in‐lake application of alum rapidly decreases P availability, presenting an opportunity to explore how P influences N availability and cycling. Analyzing 46 alum treatments, we found that total N (TN) decreased and TN:TP increased after treatment. This result is supported by case studies in other regions. The decrease in TN was positively related to the decrease in TP and the increase in clarity in shallow lakes. Decreased TN persisted despite TP increasing over 8 yr following treatment. Detailed analysis of two lakes indicated that the expansion of submerged macrophytes promoted TN removal from the water column. Our findings reveal important connections between N and P cycling and indicate that controlling P may provide additional benefits through dual‐nutrient control.