Climate change modulates the impact of nitrogen and enhances phosphorus-driven top-down control by zooplankton on harmful algae

Eutrophication and climate change are the main drivers of the increase in Microcystis biomass. These environmental factors also regulate the dynamics between planktonic predators and their prey. Here, we present the results of a controlled laboratory study investigating the top-down control efficiency of Paramecium on Microcystis under different nutrient (nitrogen and phosphorus) levels and climate conditions (current climate: 25( degrees )C, 400 ppm; predicted future climate: 30 degrees C, 750 ppm). The results showed that Paramecium completely eliminated Microcystis populations under all combinations of nutrient levels and climate conditions. Furthermore, the ability of Paramecium to control Microcystis was modulated by nutrient enrichment and climate change. Specifically, key performance parameters-including the Microcystis population decline index, time for Microcystis elimination, ingestion rate and specific growth rate of Paramecium, , and time to reach maximum Paramecium abundance-were inhibited by increasing nitrogen levels but improved by increasing phosphorus levels. This indicates that nitrogen enrichment weakened the ability of Paramecium to control Microcystis, , whereas phosphorus enrichment enhanced it. Further analysis revealed that, regardless of nutrient availability, these parameters were optimized under the high-temperature-CO2 scenario, suggesting that climate change has consistently enhanced the top-down control of Microcystis by Paramecium. . Notably, an in-depth examination of the overall change patterns of performance parameters and principal component analysis (PCA) revealed that climate change altered the response patterns of these parameters (or their comprehensive scores) to increased nitrogen and enhanced their response magnitude to increased phosphorus. Essentially, climate change compensated for the negative effects of increased nitrogen levels on the performance of Paramecium in controlling Microcystis and amplified the positive effects of increased phosphorus levels. These findings reveal that protozoan predators might enhance their top-down control of harmful algae under the interaction of eutrophication and climate change. This study, using Paramecium and Microcystis as models, provides valuable insights for managing freshwater ecosystems and applying protozoa to mitigate harmful algal blooms.