Abstract: Aims Free-surface flow-constructed wetland is a powerful means for the reduction of contaminants from agricultural runoff. Wetlands dominated by submerged aquatic vegetations (SAVs) may take up nutrients, particularly phosphorus (P), from surface flow with high efficiency. The objective of this study was to assess P removal performance by the SAV community under high and low P concentrations.
Methods Weekly or biweekly inflow and outflow water samples were collected from four small constructed wetlands (test cells) planted with SAV in South Florida, USA, between September 1999 and September 2001. These test cells were divided into two groups, with the north test cells receiving a higher inflow total phosphorus (TP) concentration (average = 75 μg l^-1) than the south test cells receiving a lower TP concentration (average = 23 μg l^-1). Limerock (LR) berms were installed in two of these test cells to allow an evaluation of the efficiency of this physical barrier to enhance wetland performance.
Important findings North test cells displayed high TP removal of ~60% while the removal efficiency of the south test cells was only ~20%. Soluble reactive phosphorus concentrations in both north and south test cells were sequestered down to near-detection limit. High removal efficiencies for particulate phosphorus were also observed in the north test cells. The LR berms at the two test cells were found to be associated with decreases of an average TP removal of 2 μg l^-1. Outflow TP concentration did not increase with inflow TP concentration, but increased with nominal hydraulic loading rates. Findings from this study demonstrated high P removal from inflow water containing high TP concentration by the SAV wetland and the importance of hydraulic regime to wetland performance.

Key words: constructed wetlands, hydraulic loading rates, limerock berm, removal efficiency, submerged aquatic vegetation (SAV), total phosphorus (TP)