Reducing Nitrogen Inputs Can Rapidly Reverse Eutrophication in Shallow Phosphorus-Rich Lakes
J. Thad Scott, Jason M. Taylor, Isabelle M. Andersen, Alexa K. Hoke, Patrick T. Kelly
Abstract
Although phosphorus (P) has been the primary focus of lake eutrophication management efforts globally, we show that reducing nitrogen (N) inputs to shallow P-rich lakes can rapidly reverse accelerated eutrophication. We conducted a long-term field mesocosm manipulation experiment testing the effects of variable N and P stoichiometry on the eutrophication of lakes, particularly the comparative productivity of lakes with low N:P inputs versus high N:P inputs and the subsequent effects of reducing N inputs while maintaining P inputs. Nitrogen enrichment in excess of balanced N:P ratios (20-50 molar) resulted in phytoplankton biomass well above thresholds indicative of healthy lake ecosystems. After 4 years of both N and P enrichment, we eliminated N fertilization while maintaining P fertilization and observed a 57 ± 3.1% reduction in phytoplankton biomass across all fertilization treatments. Comparative analysis of our experimental results with the 2022 U.S. National Lakes Assessment suggested that excess N is contributing to eutrophication for approximately 40% of U.S. lakes. Despite the capacity of lake ecosystems to import N from N fixation, reducing N inputs to shallow P-rich lakes has the potential to decrease phytoplankton biomass and the harmful effects of accelerated eutrophication.