Water quality impairment by nutrient enrichment from agricultural activities has been a concern worldwide. Phytoremediation technology using aquatic plants in constructed wetlands and stormwater detention ponds is increasingly applied to remediate eutrophic waters. The objectives of this study were to evaluate the effectiveness and potential of water lettuce (Pistia stratiotes L.) in removing nutrients including nitrogen (N) and phosphorus (P) from stormwater in the constructed water detention systems before it is discharged into the St. Lucie Estuary, an important surface water system in Florida, using phytoremediation technologies.

In this study, water lettuce (P. stratiotes) was planted in the treatment plots of two stormwater detention ponds (East and West Ponds) in 2005–2007 and water samples from both treatment and control plots were weekly collected and analyzed for water quality properties including pH, electrical conductivity, turbidity, suspended solids, and nutrients (N and P). Optimum plant density was maintained and plant samples were collected monthly and analyzed for nutrient contents.

Water quality in both ponds was improved, as evidenced by decreases in water turbidity, suspended solids, and nutrient concentrations. Water turbidity was decreased by more than 60%. Inorganic N (NH₄ ⁺ and NO₃ ⁻) concentrations in treatment plots were more than 50% lower than those in control plots (without plant). Reductions in both PO₄ ³⁻ and total P were approximately 14–31%, as compared to the control plots. Water lettuce contained average N and P concentrations of 17 and 3.0 g kg⁻¹, respectively, and removed 190–329 kg N ha⁻¹ and 25–34 kg P ha⁻¹ annually.

Many aquatic plants have been used to remove nutrients from eutrophic waters but water lettuce proved superior to most other plants in nutrient removal efficiency, owing to its rapid growth and high biomass yield potential. However, the growth and nutrient removal potential are affected by many factors such as temperature, water salinity, and physiological limitations of the plant. Low temperature, high concentration of salts, and low concentration of nutrients may reduce the performance of this plant in removing nutrients.

The results from this study indicate that water lettuce has a great potential in removing N and P from eutrophic stormwaters and improving other water quality properties.