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The recent implementation of agricultural best management practices (BMPs) and treatment wetlands called stormwater treatment areas (STAs) have reduced phosphorus (P) concentrations and loadings to the Everglades Protection Area (EPA) in Florida (USA). There is a concern that despite reductions in external P loadings, internal loading from the legacy P enrichment of the EPA wetland soils will continue to elevate water column P concentrations, and may impede restoration outcomes. In an effort to explore ways to reduce soil P efflux, we retrieved intact, vegetated (cattail, Typha domingensis) soil monoliths from two P-enriched areas of the EPA and deployed them at a location where they received pre-treated (low P) surface water as ex situ flow-through mesocosms for 21 months with a mid-study 7-week dry down to mimic natural hydroperiod conditions. Two treatments were tested for soils from both sites, using triplicate mesocosms for each treatment. After applying a herbicide (glyphosate) to eliminate the cattail vegetation, iron (Fe as liquid FeCl) amendments provided no P retention benefits in the organic soils from the two sites, and did not yield significantly (P > 0.05; n = 43) lower flux rates (6.1 and 3.5 mg Pm d) than the herbicide/no soil amendment control (3.9 and 2.1 mg Pm d), as was expected. A combination of low oxidation-reduction potential, heightened organic matter P mineralization, high pH, and sulfide production acted interactively to enhance Fe and P mobilization in the Fe-amended mesocosms. The herbicide/limerock (CaCO)-amended soils exhibited significantly lower (P ≤ 0.05; n = 43) P flux (1.3 and 1.1 mg Pm d) than the herbicide/no soil amendment control soils, but it remains unknown whether the observed reduction in P efflux (ranging from 48 to 67%) would justify the expense and potential environmental impacts of applying a surficial limerock amendment to large regions of the P-enriched wetlands. [ABSTRACT FROM AUTHOR]

Annual phosphorus (P) removal in the six Everglades stormwater treatment areas (STAs) was examined during the period of record water years 1995–2011. The STAs have received approximately 13.6 billion m 3 of stormwater inflow containing an annual total P (TP) concentration (flow-weighted mean) of 143 ± 62 μg P L −1 , with annual P loading rate (PLR) of 1.56 ± 0.91 g P m −2 year −1 and annual hydraulic loading rate (HLR) of 3.1 ± 1.36 cm day −1 , and retained 1500 metric tons of TP. The STAs produced discharges with an annual TP concentration of 41 ± 31 μg P L −1 and have annual load-based, TP retention of 76 ± 11%. Annual areal removal rate was 1.1 ± 0.5 g P m −2 year −1 , and annual k value (settling rate for TP removal) was 16.1 ± 7.4 m year −1 . Regression analysis indicates that outflow TP concentrations increased with increasing inflow TP concentrations ( R 2 = 0.705, p < 0.05, n = 59) and PLRs ( R 2 = 0.435, p < 0.05, n = 59). The k value increased with increasing HLRs ( R 2 = 0.663, p < 0.05, n = 59) and PLR ( R 2 = 0.289, p < 0.05, n = 59), and decreased with increasing inflow TP concentration ( R 2 = 0.105, p < 0.05, n = 59). STA performance, in terms of outflow TP concentration and/or k value, depended on three primary variables: HLR, inflow TP concentration, and PLR. To improve and sustain P removal in the STAs, it is operationally necessary to maintain these important variables at optimal ranges. Additional data on internal P removal mechanisms, such as P flux and microorganism activities, along the flow-ways should be collected to further understand the controlling variables and processes in the STAs. Challenges for STA management and operation were also discussed. [ABSTRACT FROM AUTHOR]

The Everglades Stormwater Treatment Areas (STAs) have been constructed as a component of the Everglades restoration in southern Florida, U.S.A. These large-scale treatment wetlands function to remove excess phosphorus (P) from agricultural and urban runoff. The STAs can receive variable inflow hydraulic and P loading in response to storm events or regional droughts and are operated under state and federal permits. An evaluation of the long-term P removal performance for two of the best performing STAs, STA-2 and STA-3/4 is presented along with the challenges in managing these large wetlands. The trends in the inflow loadings and P retention over the period of record for STA treatment flow-ways were examined on an annual and wet/dry seasonal basis. Daily water depths and weekly outflow P concentrations were also examined. The period of record average annual inflow hydraulic loading rates were relatively low (2.0–3.8 cm d −1 ) and the average annual P loading rates were low to moderate (0.6–1.6 g m 2 yr −1 ) compared to the other STAs, although variable seasonal and annual loadings were observed. Despite receiving variable hydraulic and P loads, these STAs have reduced surface water TP concentrations from 100 and 87 μg P L −1 at inflow structures to 23 and 18 μg P L −1 at outflow structures in STA-2 and STA-3/4, respectively. The outflow concentrations did not consistently respond to changes in the inflow loading rates, suggesting that the P assimilative capacity of the wetlands may not be exceeded on a long-term basis. The occasional elevated outflow TP concentrations indicate that internal physical processes, such as sediment transport or resuspension, may have occurred or the short-term phosphorus assimilative capacity of the wetland was temporarily exceeded, but it appears as though the long-term mechanisms related to biogeochemical processes were not affected. [ABSTRACT FROM AUTHOR]

The Everglades ecosystem in Florida, USA, is naturally phosphorus (P) limited, and faces threats of ecosystem change and associated losses to habitat, biodiversity, and ecosystem function if subjected to high inflows of P and other nutrients. In addition to changes in historic hydropattern, upstream agriculture (sugar cane, vegetable, citrus) and urbanization has placed the Everglades at risk due to nutrient-rich runoff. In response to this threat, the Stormwater Treatment Areas (STAs) were constructed along the northern boundary of the Everglades as engineered ecological systems designed to retain P from water flowing into the Everglades. This research investigated data collected over a period from 2002 to 2014 from the interior of the STAs using data mining and analysis techniques including (a) exploratory methods such as Principal Component Analysis to test for patterns and groupings in the data, and (b) modelling approaches to test for predictive relationships between environmental variables. The purpose of this research was to reveal and compare spatial trends and relationships between environmental variables across the various treatment cells, flow-ways, and STAs. Common spatial patterns and their drivers indicated that the flow-ways do not function along simple linear gradients; instead forming zonal patterns of P distribution that may increasingly align with the predominant flow path over time. Findings also indicate that the primary drivers of the spatial distribution of P in many of these systems relate to soil characteristics. The results suggest that coupled cycles may be a key component of these systems; i.e. the movement and transformation of P is coupled to that of nitrogen (N). [ABSTRACT FROM AUTHOR]

Low-phosphorus (P) wetland discharges (e.g., < 20 μg/L), such as those now commonly observed in Stormwater Treatment Areas (STAs) for Everglades restoration, are likely influenced by internal P loads. This study estimated for the first time the "aggregated" internal P loading rate (iPLR) (including soil and biotic vectors) in a low-P treatment wetland along with calculated vertical soil diffusive fluxes. In situ mesocosms (1.5-m diameter polyethylene cylinders) equipped with closeable windows were deployed at three locations in an STA, enclosing parcels of local vegetation and soil. The mesocosms isolated the water column P response from longitudinal P transport (flow) so the local soil-water-biota P dynamics and equilibria could be observed during no-flow conditions. The water column inside and outside the mesocosms was sampled at each location during three two-week no-flow batch incubations. A first-order model was calibrated to mesocosm batch reaction time series to estimate the iPLR (with uncertainty) at each study site. Soil diffusive flux rates were estimated from vertical porewater P concentration profiles measured at each site. Results showed that: 1) strong gradients in water column equilibrium P concentrations existed along the direction of flow (92–20 μg/L from inflow to outflow), 2) a corresponding gradient also existed for model-predicted iPLR (~6.6 to 1.9 mg/m2-d), 3) these aggregated internal loads were more than 20× greater than calculated soil diffusive fluxes, and 4) an annual extrapolation suggested that average iPLR could be approximately equivalent to the average annual inflow P loading rate to the wetland (1.3 g/m2-yr). This study demonstrates that aggregated internal P loading can greatly exceed more commonly estimated vertical soil diffusive fluxes in low-P wetlands and can be a substantial contributor to water column P concentrations. Furthermore, measuring and modeling water P equilibria during no-flow intervals was a useful method to estimate internal loading in these wetlands. • Novel methods measured internal P loading rate (iPLR) in a treatment wetland. • iPLR magnitude was comparable to external P load. • Diffusion from soils contributed less than 5% of the iPLR. • Nutrient pumping by macrophytes is probable contributor to iPLR. • Insights offer opportunity to improve Everglades restoration projects. [ABSTRACT FROM AUTHOR]