Reducing Nitrogen Inputs Can Rapidly Reverse Eutrophication in Shallow Phosphorus-Rich Lakes.

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.