Combining Ludwigia leptocarpa with the dried biomass of Pontederia crassipes for mitigating cyanobacteria: a sustainable way to use macrophyte waste

Constructed Floating Wetlands (CFWs) with emergent macrophytes offer a low-cost, sustainable strategy to mitigate eutrophication. We evaluated the combined use of <i>Ludwigia leptocarpa</i>, a native macrophyte from the Americas, and dried biomass of <i>Pontederia crassipes</i> (DB Pc) as substrate in small-scale CFWs to suppress a natural cyanobacterial bloom. <i>Ex-situ</i> mesocosm experiments (40 L) were conducted over 16 days with treatments: <i>L. leptocarpa</i> with DB Pc (S+), <i>L. leptocarpa</i> alone (S-), DB Pc alone, and living <i>P. crassipes</i> as positive control. <i>L. leptocarpa</i> (S-) and <i>P. crassipes</i> reduced Soluble Reactive Phosphorus by >85%, but <i>L. leptocarpa</i> produced 10 times less biomass. Treatments with DB Pc increased nutrient and phenol concentrations. All treatments led to reduced chlorophyll-a and phytoplankton density, especially cyanobacteria, along with pH reduction. 16S rRNA sequencing revealed higher bacterial diversity in the <i>L. leptocarpa</i> rhizosphere than in the water, suggesting a role in phytoremediation. The combination of <i>L. leptocarpa</i> and DB Pc in CFWs shows potential for cyanobacterial control through nutrient uptake, allelopathy, and pH modulation. This method supports sustainable water management by utilizing a native, slow-growing macrophyte and repurposing waste biomass that would otherwise harm aquatic ecosystems.