Critical coverage thresholds of submerged macrophytes for phosphorus pulse mitigation and resilience enhancement in...

Nutrient pulses caused by heavy rainfall or agricultural runoff can increase the loss of phosphorus from land to adjacent waters, threatening aquatic ecosystems. Using in-situ enclosure experiments that simulated a single moderate PO₄-P pulse (0.05 mg P/L), we tested how the macrophytes at different coverage levels (0-100%) regulate water quality, ecosystem resilience, and eutrophication processes. The system's buffering capacity and resilience were significantly positively correlated with submerged macrophytes coverage (SMC). High SMC enclosures exhibited lower nutrient levels, curbed algal growth, and sustained clear-water conditions, whereas systems with sparse macrophytes retained relatively higher nutrients and algal biomass, along with a greater eutrophication risk. Suspended solids, turbidity, and Chl-a were more sensitive to increases in SMC, meaning SMC enhancement improved these parameters more effectively and visibly than nutrients. Structural equation modeling suggested that submerged macrophytes offered more pathways to improve water quality at higher coverage levels, yet the actual mechanisms remain to be tested by direct measurements of pH, ORP, Ca-P and Fe-P interactions. Furthermore, a critical threshold of 39% SMC (95% CI: 20%-54%) was inferred to inhibit eutrophication, and maintaining coverage above this level is crucial to buffer single moderate P pulses, but the threshold may vary with plant species, lake type, and pulse magnitude and frequency.