Particulate phosphorus transformations in south Florida stormwater treatment areas used for Everglades protection

Six large treatment wetlands (352–6698 ha), designated Stormwater Treatment Areas (STAs), have been constructed to sequester phosphorus (P) in drainage waters before entering the Everglades. We performed oxic and anoxic laboratory incubations to assess the stability of P in suspended particles from inflow and outflow waters for selected flow-paths of STA-2, a “well-performing” wetland, and STA-1 West (W), an “under-performing” wetland. Particles were concentrated by filtration prior to incubation. STA-2 flow-paths exhibited a reduction in the particulate P (PP) concentration (e.g., 28 to 3–8 μg/L), as well as an increase in particle stability (i.e., reduced conversion of PP to soluble reactive P [SRP]) with passage through the wetland. By contrast, while the STA-1W flow-path exhibited a PP reduction of 68–46 μg/L under anoxic incubations, the particles exiting the wetland were more labile than those entering. For PP collected from both STAs, anoxic incubations produced higher conversion rates to SRP than did oxic conditions. This redox influence on SRP release was more pronounced for outflow PP from STA-1W than for PP exiting the STA-2 flow-paths.STA-2 is comprised of flow-paths dominated either by emergent vegetation (Typha domingensis and Cladium jamaicense) or submerged (Najas guadalupensis, Potamogeton illinoensis, periphyton) macrophytes. Internal sampling of water column P species in the submerged macrophyte wetland revealed a sharp decline in SRP levels near the wetland inflow, and more gradual declines in PP and dissolved organic P (DOP) concentrations, primarily within the first half of the wetland. Mean outflow TP concentrations (15 μg/L) during these monitoring events were comprised of 13% SRP, 27% PP and 60% DOP. Particles exiting the emergent macrophyte wetlands were higher in organic matter content, but lower in total suspended solids, algal cell, chlorophyll a, and carbonate concentrations than particles in the inflow waters. By contrast, particles exiting the submerged macrophyte wetland were more similar to inflow waters with respect to all of the above characteristics except total suspended solids.This study demonstrates that the behavior and transformation of PP constituents will have a marked impact on the utility and cost-effectiveness of watershed and STA phosphorus control efforts. Observed differences between STA-2 and STA-1W suggest that the STA P loading rate history is a prominent factor in dictating both the total mass of PP, as well as bioavailable fraction of PP, discharged from the STAs.