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 Ludwigia leptocarpa, a native macrophyte from the Americas, and dried biomass of Pontederia crassipes (DB Pc) as substrate in small-scale CFWs to suppress a natural cyanobacterial bloom. Ex-situ mesocosm experiments (40 L) were conducted over 16 days with treatments: L. leptocarpa with DB Pc (S+), L. leptocarpa alone (S-), DB Pc alone, and living P. crassipes as positive control. L. leptocarpa (S-) and P. crassipes reduced Soluble Reactive Phosphorus by >85%, but L. leptocarpa 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 L. leptocarpa rhizosphere than in the water, suggesting a role in phytoremediation. The combination of L. leptocarpa 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.