Floating treatment wetland influences on the fate and removal performance of phosphorus in stormwater retention ponds

•A retention pond retrofitted with a Floating Treatment Wetland was monitored.•A similar conventional retention pond with same inflow was monitored as a control.•Phosphorus plant, sediment, inlet and outlet concentrations were quantified.•The FTW pond showed higher total phosphorus treatment efficiency.•FTW increased phosphorus sedimentation while plant uptake seemed limited. A field trial comparing the fate and removal performance of phosphorus (P) in two parallel stormwater retention ponds, one retrofitted with a Floating Treatment Wetland (FTW) and one without any vegetation (a control), was carried out near Auckland, New Zealand. Results suggest that inclusion of a FTW would significantly improve P removal efficiency exhibiting 27% lower TP outlet event mean concentrations (EMCs) than a conventional retention pond. The low SRP inlet EMC did not allow the performance of either pond to be differentiated. Inlet particulate bound P (PP) is thought to have been associated with particulate copper on fine particles like colloidal organic matter and/or clay and trapped into the sticky biofilm of the roots to subsequently settle on the bottom of the pond. The FTW pond induced a more neutral water column pH and higher organic release into the water column, likely promoting dissolved phosphorus sorption onto particles. Surprisingly, the reduced (low redox potential) sediment observed below the FTW did not induce P release probably due to the more neutral pH conditions allowing re-adsorption onto organics and/or clay minerals (e.g. Al-OH). This resulted in higher P sediment accumulation in the FTW pond. P uptake by plants is not thought to be a significant removal pathway. Sorption of dissolved P, physical entrapment of PP in roots and settlement are thought to be the main P removal pathways for ponds equipped with FTWs.