Influence of hydrologic regime and vegetation on phosphorus retention in Everglades stormwater treatment area wetlands

The Florida (USA) Everglades ecosystem has been impacted due to increased loading of nutrients, in particular phosphorus (P), primarily from adjacent agricultural areas. Consequently, restoration measures involve the establishment of stormwater treatment areas (STAs) comprising a series of constructed wetlands. A series of mesocosms were established at the inflow of the Everglades Nutrient Removal Project wetland, the first such STA constructed. These mesocosms were designed to mimic STAs, as they operated as flow-through systems and were packed with native soil. The objective of the study was to determine the effects of vegetation and hydrologic fluctuations on P retention/release by the wetland soil and on effluent water quality. Four treatment combinations consisted of continuously flooded with emergents (Typha), intermittently flooded with emergents, continuously flooded with no emergents, and intermittently flooded with no emergents. Intermittently flooded treatments underwent two 1 month drawdown events during the year. Soils were collected to determine the various pools of P and surface water samples were collected twice weekly to determine mass P flux in and out of the mesocosms. Results showed that the majority of the P was stored in the calciumand magnesium-bound fraction, as well as the refractory pool in the soil. Approximately 91% of the inflow soluble reactive P (SRP) mass was retained within the mesocosms for the continuously flooded treatment, and 80% was retained in the treatments subjected to periodic drawdown events, regardless of vegetation type. There was a net annual flux of dissolved organic P (DOP) out of the mesocosms for the drawdown treatments, whereas the net reduction in the DOP concentrations for the continuously flooded treatments was just 17%. These results demonstrate that, although these wetland systems perform well in reducing surface water SRP, additional research may need to focus on improving the reduction of DOP in order to reduce further the P loads to the nutrient-sensitive Everglades system. Copyright  2004 John Wiley & Sons, Ltd.