Quantifying Critical Thresholds of Submerged Macrophyte Coverage to Buffer Climate-Amplified Ammonium Pulses and Stabilize Clear-water States.

Climate change intensifies nutrient pulses through extreme rainfall and agricultural runoff, yet the buffering capacity of submerged macrophytes against such disturbances remains unquantified. Through a large-scale enclosure experiment simulating ammonium pulses (1.24 mg/L NH-N), we tested how submerged macrophytes coverage (SMC, 0-100%) modulates water quality, ecosystem resilience, and regime shifts (from clear to turbid). The system's buffering capacity and resilience stability increased significantly with SMC, whereas its recovery stability decreased. High SMC (>50%) accelerated NH-N removal (96 h vs 168 h in controls), suppressed phytoplankton blooms (Chl- increase: 102.5% vs 237.4%), and sustained clear water. Conversely, low and medium SMC (<50%) did not prevent transitions to algal-dominated states. Furthermore, NH-N stress was inversely correlated with SMC, and persistently high NH-N at low SMC increased macrophyte degradation risk. Structural equation modeling revealed that macrophytes-mediated nutrient competition and light stabilization underpinned these effects. Additionally, we identify a critical SMC threshold (39-51%) to mitigate pulse impacts─a finding urgently needed to guide lake restoration in a changing climate. This work bridges the gap between pulse ecology and adaptive management, offering actionable strategies for SDG 6 (Clean Water) and 13 (Climate Action).