Stormwater Treatment Areas Research Articles

Evaluating water quality change with hydrologic restoration in the Western Everglades (Florida, USA), an application of WAM

Hydrologic restoration at the watershed scale is a complex process that can influence other processes and conditions within the watershed and downstream systems. Given these interrelated watershed scale processes, tools like models are essential to assess the potential effectiveness of sub-watershed to regional remediation strategies for ecological restoration. The goal of Everglades restoration is to restore the quality, quantity, and timing of water to one of the most ecologically unique and complex systems. This study aimed to evaluate changes to water quality associated with hydrologic restoration efforts within the Feeder Canal Basin as identified by the Western Everglades Restoration Project (WERP) by demonstrating, through simulations the potential effectiveness of implementing water quality remediation measures such as Best Management Practices (BMPs) and Stormwater Treatment Areas (STAs) within the WERP area. Additionally, to understand potential phosphorus sources and pre-develpoement conditions within the basin, a pre-development baseline was developed. Using the Watershed Assessment Model (WAM) various model alternatives including the WERP tentatively selected plan (TSP) were developed for a 22 year period of simulation. The implementation of both moderate and aggressive BMPs significantly reduced total phosphorus concentrations within the canal system at various levels indicating an improvement to near-field conditions. Meanwhile, not until the moderated BMPs and operation of an STA do water quality improvement become realized further downstream. The combined strategy of BMPs and STA demonstrates the integrated nature of nutrient remediation with both near and far-field benefits to the system with hydrologic restoration. This integrated strategy of water quality and hydrologic restoration can lead to a more sustainable and resilient restored ecosystem.

Systematic Review of Best Management Practice Implementation for Agricultural Phosphorus Management in Florida: A 30-Year Overview

Highlights BMPs have been proven to be efficient in managing P loads in Florida. BMP efficiencies vary between 20% and 80%, with few higher than 50%. Integration of modeling strategies and field demonstrations could improve BMP efficiencies. Further implementation of testbeds and pilot studies could improve adoption and enhance our understanding of BMP efficiency. Abstract. In Florida, the implementation of best management practices (BMPs) has significantly reduced phosphorous (P) loads from the Everglades Agricultural Area (EAA) into the Lake Okeechobee watershed over the past two decades. While the removal of over 6,165 metric tons of total phosphorus entering the Everglades Protection Area is a notable achievement, some freshwater sources continue to exceed established limits (Naja et al., 2017), demanding a critical examination of BMP efficiency at managing low P concentrations. In this systematic literature review, we analyzed peer-reviewed journal manuscripts published over the past 30 years, focusing on the intersection of phosphorus management, water, Florida, and the environment. The dataset synthesized from our review provides insights into the efficiency of BMPs in terms of P management over time. Our findings show a predominant focus on stormwater treatment areas (STAs) and constructed wetlands as BMPs with demonstrated good efficiency. However, the variability in reported efficiencies underscores the complexity of phosphorus pollution and its impacts. Treatment trains ranged from 20% to 39% for the lower range of efficiency in phosphorus removal and from 60% to 79% for the higher range of efficiency. Focusing on strategies with higher P load reduction efficiencies could enhance future management strategies in Florida. Field-based pilot studies with well-defined control settings can facilitate long-term evaluations of P management programs that allow the implementation of BMPs. Similarly, the evaluation of new technologies, including materials, precision-smart practices, and the integration of modeling strategies with field-scale studies suggest a promising approach to engaging stakeholders in achieving higher BMP efficiencies. This systematic review highlights current BMP strategies that have demonstrated high P load reduction efficiency. There is a need for continued research that simultaneously assesses strategies to reduce P pollution before it forms and employs a multidimensional approach to P management. This approach should integrate multiple successful BMPs through field and modeling strategies. Keywords: , Agriculture, Best Management Practices (BMP), Florida, Non-point source pollutants, Phosphorus loads.

The role of calcium and alkalinity on phosphorus removal by submerged aquatic vegetation in hardwater wetlands

The Everglades Stormwater Treatment Areas (STAs) are a 25,000 ha (61,000 acre) complex of constructed wetlands built to remove phosphorus (P) from agricultural and urban runoff in support of Everglades restoration. These STAs treat inflow waters that contain P in concentrations from 100 to 200 μg L−1, average calcium (Ca) concentrations of 60–90 mg L−1, and alkalinity (Alk) in the range of 150–350 mg L−1 as calcium carbonate (CaCO3). Photosynthesis, primarily from submerged aquatic vegetation (SAV) within these STAs, increases water pH resulting in CaCO3 formation, which is thought to contribute to P removal through coprecipitation and other processes. Since 2002, the STA complex has removed over 3000 MT of P and over 400,000 MT of Ca. Because the STAs at times treat Lake Okeechobee waters that are lower in Ca and Alk (35–50 mg L−1 and 95–125 mg L−1 as CaCO3, respectively), we conducted two experiments to explore the role of Ca and Alk on wetland P removal.First, in outdoor flow-through microcosms (0.18 m2) containing Najas guadalupensis and treating artificial inflow solutions, Ca/Alk at typical STA inflow concentrations (77 mg Ca L−1/316 mg L−1 as CaCO3) resulted in significantly lower outflow P concentrations compared to lower Ca/Alk treatments (28 mg Ca L−1/67 mg L−1 as CaCO3). Based on sediment organic carbon to total P (TP) ratios, organic matter production and sedimentation was the P removal mechanism under low Ca/Alk levels, where CaCO3 precipitation did not occur, and also under high Ca/Alk, low P (31 μg L−1) conditions, despite CaCO3 formation. High Ca/Alk increased organic matter sedimentation and associated P burial at low and high (168 μg L−1) water P concentrations. Additional P removal associated with CaCO3 precipitation occurred only under high Ca/Alk, high P concentrations, and accounted for about 47% of P sedimentation in those microcosms.Second, in flow-through mesocosms (4.75 m2) containing STA muck soil and common STA SAV species, elevated Ca/Alk (94 mg Ca L−1/185 mg L−1 as CaCO3) produced significantly lower outflow TP concentrations than treatments receiving unamended Lake Okeechobee inflow water with lower Ca/Alk concentrations (44 mg Ca L−1/112 mg L−1 as CaCO3). In the same experiment, Ca/Alk levels had no effect on P removal in mesocosms containing Typha domingensis, a common STA emergent macrophyte species. SAV mesocosms produced lower outflow TP concentrations than Typha, regardless of Ca/Alk levels. In a follow-on laboratory incubation, low Ca/Alk concentrations in incubation waters did not increase P release from sediments formed under prior low or high inflow Ca/Alk concentrations in the SAV mesocosms, but anoxia sharply increased P release from both.These experiments suggest that coprecipitation with CaCO3 is a meaningful P removal process in hardwater wetlands where high P concentrations and SAV coincide. Conversely, in STA outflow regions, where surface water P has already been treated to a low level, coprecipitation with CaCO3 probably does not contribute to further P reductions. However, Ca/Alk also may contribute to inorganic-C availability which influences submerged aquatic productivity, so the role of inorganic C limitation on STA vegetation communities and P removal merits further study.

An analysis of long-term Everglades Stormwater Treatment Areas performance using structural equation models

Phosphorus (P) retention in Stormwater Treatment Areas (STAs) is influenced by various external and internal variables, making it necessary to employ multivariate techniques to understand the complex relationships among them. This study used 14 years of monthly data from four STAs (STA-1E, -1W, −2, and − 3/4) to explore the factors influencing long-term STA performance and function related to P retention. Structural Equation Model (SEM) with predictive variables of inflow total P (TP), total nitrogen (TN) and calcium (Ca) concentration, hydraulic loading rate (HLR), and water pH and temperature was applied to evaluate similarities and differences among individual STA performance. The SEM results suggested an improvement in operational performance of STAs over time. All models explained a substantial portion of the variation in retention rate of TP (R2 > 0.63), but only a small portion of the variation in outflow TP concentration (R2 < 0.24). Notable differences were observed among the four STAs. The relationship between inflow and outflow TP concentration differed among the STAs. Furthermore, inflow TN concentration was positively correlated with the TP retention rate in STA-1W, and outflow TP concentration in STA-2, while the co-precipitation of P with Ca likely played a critical role in STA-3/4. These findings highlight the complexity of P removal in STAs and emphasize the need for site-specific management strategies. Further studies should consider incorporating long-term observation of key vegetation and soil variables (e.g. vegetation coverage and soil TP content, etc.) to inform more effective approaches to optimize P removal and enhance the performance of STAs in water treatment and ecosystem restoration efforts.

Effects of increasing complexity in biogeochemistry and hydrology on variability of total phosphorus concentration in models of a low flow subtropical wetland

In south Florida, treatment wetlands known as stormwater treatment areas (STAs) are used to reduce total phosphorus (P) concentrations of agricultural and urban stormwater runoff before discharging into the Everglades Protection Area. The remobilization of P from immobile forms stored in soils and litter has the potential to negatively affect STA treatment efficiency. Phosphorus biogeochemical dynamics in one of the best performing Everglades STA flow-ways, STA-2 FW1, were modeled mechanistically and empirically using diverse streams of data (flow, total P in water, soil, floc, vegetation). We evaluated the effect of P wetland uptake and hydrologic transport on STA-2 FW1's simulated outflow P concentrations by juxtaposing varying complexities of biogeochemistry (1–2 vs 13 wetland P pools) and hydrology (uniform vs. nonuniform flow) models. The uniform flow model considers a constant velocity with each time step while velocity in the nonuniform flow model is a stochastic ensemble mimicking spatial flow variability. When coupled with uniform or nonuniform flow, the variability of water column P concentration in the most complex model was significantly different from the two simple biogeochemistry models, although it was lower compared to data in all models. Regardless of the biogeochemistry model, uniform versus nonuniform transport did not affect simulated outflow P concentration. Significant interaction effects between the choices of biogeochemistry and hydrology models were observed. The most complex model included specific compartments for vegetation, floc and soil and reproduced data trends of more P accumulated in upstream areas that decreases with increasing distance away from inflow. Modeling biotic mechanisms and tracking P in vegetation and sediments, confirms the importance of including biogeochemistry in understanding and predicting outflow P concentrations in STA-2 FW1. This modeling tool could be used to guide future STA data collection needs and inform Everglades STA performance optimization efforts.

Biogeochemical response of subtropical treatment wetlands to different flow conditions

The biogeochemical response to different flow conditions within two parallel, well-performing flow-ways (FWs) in Everglades Stormwater Treatment Area (STA)-2 was studied. Three flow events in FW1, an emergent aquatic vegetation (EAV) based system and three flow events in FW3, a submerged aquatic vegetation (SAV) dominated system, were carried out between August 2015 and July 2017. These flow events included either low or moderate flow that was preceded and/or followed by no flow. These flow scenarios were designed to simulate different hydraulic and hydrologic conditions that STAs experience during full-scale operations. Hydraulic loading rates (HLRs, flow volume per day/wetland effective treatment area) during the flow events ranged from 0.5 to 10.4 cm d−1 with resulting phosphorus loading rates (PLRs, daily load/wetland effective treatment rate) of 1 to 20 mg P m−2 d−1. The ability of these flow-ways to remove and transform P in the water column under different flow conditions was evaluated using water quality data from autosamplers and weekly grab samples collected at six monitoring stations along each flow-way.Distinct water column concentration gradients of total phosphorus (TP) from inflow to outflow at all phases of the flow events for both flow-ways were observed. The magnitude at which P is removed from the water column increased with fractional distance from the inflow and was greater for the EAV-dominated system (FW1) than the SAV-based system (FW3). This system response translated to higher TP concentration reduction in EAV than SAV. Inflow to outflow TP concentration reductions for EAV were 94, 88, and 86% for moderate (MF), low (LF), and no flow (NF), respectively. For SAV, TP concentration reductions were 73, 71, and 64% for MF, LF, and NF, respectively. A significant increase in TP concentrations [largely dissolved organic P (DOP) and particulate P (PP)] in the SAV-dominated system was observed under no flow condition. The increase was attributed to in-situ production of PP through several mechanisms that likely include phytoplankton growth, periphyton sloughing, litter fragmentation, and resuspension via entrainment or bioturbation. While soluble reactive P (SRP) was the dominant form of inflow TP removed along the EAV FW, PP accounted for most of the reduction in TP concentration along the SAV FW, under both flow and no flow conditions. Particulate P and DOP comprised the residual P pool at the outflow region of each FW with slightly higher concentrations in the SAV than the EAV FW under no flow condition. The removal of P from the water column in the EAV FW was largely attributed to biotic processes (i.e., P uptake by biota) while coupled biotic and abiotic processes regulated P reduction in the SAV FW.

Seasonal and spatial patterns of surface water quality in large-scale treatment wetlands with different vegetation communities

The Everglades Stormwater Treatment Areas (STAs), a network of large, constructed wetlands, were built to reduce phosphorus (P) in water discharged into downstream oligotrophic Everglades ecosystem. These wetlands are colonized by two main types of macrophyte communities: emergent aquatic vegetation (EAV) and submerged aquatic vegetation (SAV). The objective of this study was to compare the seasonal and spatial patterns of surface water quality within two flow-ways (FWs) in STA-2 (FW1 dominated by EAV and FW3 a mixed marsh with both EAV and SAV) and assess how surface water quality was affected by extreme storm events. Surface water samples were collected from three stations (near the inflow, middle of the FW, and near the outflow) along the flowpath of each FW four times from 2016 to 2018. Total P and its fractions generally decreased along the flowpath in both FWs while other elements did not show a consistent trend between seasons. Surface water quality in FW1 had more evident seasonal patterns and was less affected by 2017 storm events compared to that of FW3. The changes and patterns of water quality indicate that the processes (e.g., plant nutrient uptake, microbial assimilation, detritus decomposition and particle settling, etc.), which take the primary role of removing nutrients from the water column in FW1, vary not only due to the differences in plant growth but also to variations of hydraulic and nutrient loadings over wet and dry seasons. In FW3, on the contrary, our results indicate that the occlusion process facilitated by underwater photosynthesis of SAV likely enhanced the capacity of removing P and associated elements from water column and attenuated the seasonal differences. The SAV in FW3 also was susceptible to extreme events, with both short- and long-term adverse effects of storm events on surface water quality.

Assessment of a diel phosphorus pattern's potential to benefit phosphorus retention in the stormwater treatment areas

The Stormwater Treatment Areas (STAs) are constructed wetlands used to reduce the total phosphorus (TP) level in water discharged to the environmentally sensitive areas of the Everglades Protection Area. Analysis of high temporal frequency TP data reveals that TP concentrations discharged from selected STA flow-ways exhibit diel and seasonal cyclic patterns and are most affected by discharge rate and water depth. The objective of this paper is to describe the diel TP pattern and evaluate the possibility of using it to reduce TP in STA discharge. The amplitude of diel TP pattern ranged from 0.7 to 1.5 μg L−1 (3–6% deviation from daily mean) at three flow-ways, peaking in the early afternoon and reaching lowest concentrations between midnight and 4 am. To assess the possibility of further reducing TP concentration discharged from the STAs by increasing the proportion of daily flow discharged at night when diel TP is at a minimum, a General Additive Model (GAM) was created with discharge rate, water depth, year, location, day-of-year, and time-of-day as covariates. While time-of-day was a statistically significant factor in the model (p < 0.001), its average marginal effect was smaller than all other significant model covariates: discharge rate, season, and water depth. Modeling suggests that within specific ranges of discharge rate and water depth, the opportunity exists to provide additional TP reduction in STA outflow by discharging a greater proportion of the daily flow at night when TP concentrations are lower. Further research is needed to quantify the effect of increasing nighttime flow on annual flow weighted mean TP. Additionally, an evaluation of the practicality and logistical cost required to redistribute flow is needed before attempting to take advantage of this natural cyclic pattern to improve STA TP retention.

Evaluation of the impact of aquatic-animal excretion on nutrient recycling and retention in stormwater treatment wetlands

The relative contribution of aquatic animals to phosphorus (P) and nitrogen (N) cycles of water bodies can be substantial. Excretion rates can be affected by seasonal environmental conditions, physiology, and body size. Determining the importance of particular species or size classes to total fauna-driven internal recycling of nutrients requires measurements of seasonal mass-specific excretion rates (MSER). We used short-term incubation experiments to estimate the contribution of excretion and egestion by eight species of fish and one macroinvertebrate to water column P and N in Stormwater Treatment Areas (STAs) used to retain P from water flowing to the Everglades in Florida, U.S.A. To account for environmental conditions and fish physiology, excretion was estimated during day and night in the wet and dry seasons for nine of the most abundant fish and invertebrates found in the STAs. To yield measurable contributions of analytes in short-duration incubations, large-fish species were incubated individually, whereas small-fish species and invertebrates were incubated in single-species groups. For large-fish species, total excreted nutrients were positively correlated with the mass of individual fish, but MSER decreased with increased individual size. All four small species, three fish and invertebrate, excreted P and N at greater MSERs than all five large-fish species. Most MSERs were not correlated with incubated mass (stocked density) of small animals (80%), indicating that there were no density-dependent effects on excretion rates. Most species excreted more N and less P during the South Florida wet season (May to October) though this was not the case for one or more analytes for three of the large species. Most species excreted more during the day, except large herbivores that excreted more at night. Small species excreted higher proportions of particulate P and N, while the larger species had higher proportions of dissolved nutrients. Using measured MSERs and areal biomass estimates for the nine species, we estimated mass of P and N excreted annually. Based on known inflow P and N loads for four STAs, a preliminary and conservative estimate is that fish excretion can recycle between 50% to 92% of the annual total P entering the system and 15% to 29% of the annual total N entering the system. These preliminary results underestimate the actual amounts because they do not account for STA cells that were not sampled, nor known under-estimation of large-fish biomass, as well as other animal effects on nutrient cycles such as bioturbation. Though preliminary, our results suggest that altering aquatic animal abundance or species composition could improve the efficiency of STAs to remove and retain nutrients from inflowing waters.

Periphyton enzymatic activities in the water column along internal low-phosphorus nutrient gradients in the Everglades Stormwater Treatment Areas

The Everglades Stormwater Treatment Areas (STAs), important components in the Everglades Restoration efforts, were built to remove excess phosphorus (P) from stormwater runoff before it enters the Everglades Protection Area. The STAs operate within the low phosphorus domain of constructed wetlands and are mandated to achieve ultra-low P outflow concentrations to protect the receiving water bodies. Microbial processing by periphyton enzymatic activity under these low-P conditions may be critical for achieving nutrient reduction, particularly in the lower reaches of the STA treatment cells where nutrients are primarily in the dissolved organic and particulate forms. Enzymes are produced by periphyton to liberate required nitrogen (N), carbon (C), and P from dissolved organic matter under nutrient limiting conditions. Enzyme activity had not been measured within the STA treatment cells and this study was conducted to evaluate if enzyme activity occurs along the nutrient transect from inflow to outflow in well-performing treatment cells and if this activity differs within the dominant vegetation communities (i.e., emergent or submerged aquatic vegetation, EAV and SAV, respectively). The STAs receive variable hydraulic and nutrient loadings and the potential influence of these conditions on enzyme activity was evaluated under a range of loadings. The periphyton response in three different well-performing STA treatment cells (STA-2 Cells 1 and 3 and STA-3/4 Cell 3B) was evaluated using periphyton that accrued in the water column over a 7-d period. Because nutrient cycling is interrelated, a suite of enzyme activity associated with nutrient acquisition from dissolved organic material was measured. Enzymes associate with P-acquisition (phosphatase; alkaline phosphatase and bis-phosphodiesterase), C-acquisition (β-glucosidase), and N-acquisition (leucine aminopeptidase) were measured along internal transects from inflow to outflow.We found that enzyme activity occurs in the STA treatment cells and the most reactive enzymes were the phosphatase and N-acquisition enzymes; there was little activity and variability measured in the C-acquisition enzyme. Phosphatase activity increased along the nutrient gradient and the rates became more variable at the midflow and outflow transect locations. When dissolved organic P (DOP) in the surface water was above 10 μg/L, the phosphatase rates were reduced, an expected response since enzyme production occurs only during nutrient limiting conditions. When DOP concentrations were <10 μg/L, greatly increased rates were observed but there were still also times when the rates were reduced that did not appear to be influenced by surface water organic P or N concentrations or water depths. The activity for N-acquisition enzyme was also found to be variable, but this variability was between each treatment cell rather than along the transect as observed for phosphatase activity. The proportional change of each enzyme activity to the other was used to evaluate nutrient limiting conditions in the periphyton. Along the transect, both N- and P-limiting conditions were found in the periphyton at the inflow and midflow sites and solely P-limiting conditions at the outflows in all treatment cells. This implies that the mineralization processes can be disrupted under higher nutrient conditions, impacting cycling of the organic material. There were also indications that nutrient cycling may differ between periphyton from the EAV and SAV communities under low-P conditions, suggesting there may be a complementary enzymatic response within a mixed marsh configuration that may enhance nutrient cycling.

Biomass and nutrient storage of aquatic plants along phosphorus gradients in everglades stormwater treatment areas

The Everglades Stormwater Treatment Areas (STAs) are large freshwater wetlands constructed to reduce phosphorus (P) in stormwater runoff discharged to the Everglades Protection Area. The STAs consist of compartmentalized flow-ways (FWs) containing one or more treatment cells, which are managed to promote either emergent aquatic vegetation (EAV) or a mixed marsh of EAV and submerged aquatic vegetation (SAV). This study evaluated biomass, nutrient content, and nutrient storage of common EAV (Typha spp.) and SAV (Chara spp.) species within well performing STA FWs (i.e., P discharges below 20 μg/L). Samples for both vegetation types were collected at inflow, midflow, and outflow locations. Typha samples were partitioned into aboveground and belowground components. All samples were analyzed for biomass, P, nitrogen (N), and carbon. Content (g kg−1) of P and N in Typha were significantly higher in belowground components compared to aboveground for all sites, while storage (g m−2) of P and N was highest at inflow sites compared to midflow and outflow sites. In Chara, P and N content were significantly higher at some midflow sites relative to outflow sites. The ratio of N:P (based on mass storage) typically increased from inflow to outflow, though ratios at most Typha sites indicated N limitation while P limitation was observed primarily at Chara dominated outflow sites. Typha nutrient storage measured in the present study was comparable to storages within floc and recently accreted soil layers measured during concurrent studies, indicating that Typha represents a significant pool of P within the STAs. Evaluations conducted here represent recent in situ biomass and nutrient storages for the most common STA vegetation, which have not been documented thoroughly since early in STA operation, and support ongoing STA biogeochemical modeling efforts.

Evaluation of water depth and inundation duration on Typha domingensis sustainability in the Everglades Stormwater treatment areas: A test cell study

Typha domingensis is a dominant plant species in the Everglades Stormwater Treatment Areas, freshwater wetlands constructed to reduce phosphorus (P) levels in stormwater runoff prior to discharge into the Everglades. Water depth and duration of inundation can negatively affect the health of these plants. To determine the effect of water depth and inundation duration, Typha plants were grown in this study in a series of 15 test cells, 0.2 ha in area, and exposed to five water-depth treatments over 10 months: control (40 cm), shallow (61 cm), moderate (84 cm), deep (104 cm) and extremely deep (124 cm). Mean adult Typha densities declined from 14.1 shoots m−2 in the control to 10.9 and 10.5 shoots m−2 in the deep and extremely-deep treatments, respectively. However, only mean juvenile Typha densities were significantly different among water-level treatments, declining from 3.2 juveniles m−2 in the control to 1.2 and 0.6 juveniles m−2 in the deep and extremely-deep treatments, respectively. Typha leaf elongation rate (LER) was significantly higher in the deeper treatments, with the highest rates measured during the active growing season (July to October). LER increased from 6.84 cm day−1 during June baseline measurements to 8.89 cm day−1 in the deeper treatments after eight weeks of continuous inundation. Typha showed morphological plasticity to adapt to deep water through changes in morphology and biomass allocation. Typha grew significantly taller and heavier to adapt to deep-water conditions and facilitate gas exchange between the above and belowground tissues. Ramet height of Typha harvested at the end of the study averaged 343 and 374 cm in the deep and extremely-deep treatments, respectively, compared to 285 cm in the control. Likewise, the weight of individual adult ramets was significantly greater in the deeper treatments (256 g ramet−1) compared to the control (123 g ramet−1). Although none of the water-level treatments resulted in a total collapse of their Typha populations, increased LER in the deeper treatments resulted in less structural support in stems, causing substantial Typha lodging when water levels were lowered at the end of the study. These findings provide valuable insight into the effect of different water-level inundations on plant responses and overall health of Typha populations in the STAs.

Abiotic mineralization of dissolved organic phosphorus for improved nutrient retention in a large-scale treatment wetland system

The Everglades Stormwater Treatment Areas (STAs) in South Florida are large treatment wetlands that were constructed and managed to reduce phosphorus (P) loads to the Everglades Protection Area. Most P in inflow waters originates from urban, agricultural, and equestrian runoff, as well as discharges from Lake Okeechobee. To effectively reduce dissolved organic P (DOP) in the outflow of the STAs, DOP must be mineralized to soluble reactive phosphorus (SRP) (Ged and Boyer, 2013), and then removed through uptake by aquatic vegetation or co-precipitation with calcite during photosynthesis. In order to explore the effects of sunlight and photolytic processing on dissolved P mineralization within the STAs, a series of photochemical experiments were conducted. First, DOM entering the STAs was measured and found to be highly aromatic (mean SUVA = 0.38) and dominated by large molecular weight molecules (75% of bulk DOM > 10KDa) that are reactive to photolytic degradation. Model molecules, such as phytic acid, and site water from the STAs were exposed to light and produced SRP from DOP after exposure to high energy UV radiation. Additionally, analysis of excitation emission matrices (EEM) found that aromatic and humic structures were photolytically degraded during UV exposure. These findings suggest that photolysis is a significant process in DOM and P cycling in the STAs, and if leveraged in SAV dominated areas, could benefit P removal by co-precipitation with calcite. These findings suggest that vegetation management strategies could be employed to maximize UV photolytic reactions to optimize P retention.

Mesocosm studies clarify effects of external and internal phosphorus (P) loads to treatment wetlands at lower limits of P removal

This study evaluates wetland phosphorus (P) removal where P concentrations can be extremely low (∼20 μg/L or less). Stormwater Treatment Area (STAs) are large water quality improvement projects constructed for Everglades restoration, where research is underway to understand the processes that control P removal at already low P concentrations, particularly in the lower reaches of the flow-ways, upstream of the discharge structures. One concept that is being researched consists of scraping the P-enriched muck soil from the discharge region of STAs to expose the underlying limerock (LR) substrate. This has been investigated in numerous subscale platforms and results in proliferation of periphyton and submerged aquatic vegetation (SAV) communities. In principle, it also reduces internal P loads to the water column, although this point has lacked direct data support. One 40-ha field-scale periphyton-based STA (PSTA) cell has operated since 2008 and has regularly reduced ∼15 μg/L inflow TP to ∼10 μg/L (long-term flow-weighted means). This study documents two mesocosm experiments aimed at better understanding PSTA systems. The first experiment investigated the response of outflow P to external P loading rates (PLR = 0.3–1.3 g/m2/yr). Together with the field data from full-scale STAs, the results establish that PSTA systems are responsive to upstream P inputs (a reduction in external P inputs results in lower outflow P), whereas adjacent muck-based STA treatment cells receiving similar P inputs are not. The second experiment investigated differences in discharge TP and plant tissue and sediment P. Seven substrate-vegetation combinations were used: 0, 5, and 15 cm of limerock (LR) “cap” over indigenous muck with and without SAV found in the STAs; and a LR-only system inoculated with SAV but heavily dominated by periphyton. Rooted SAV produced internal P loads to the water column at a magnitude that is dependent upon access to the soil-P pool (muck only > LR cap over muck > LR only). Thus, the likely reason why PSTA achieves lower P discharges and exhibits external PLR influence is very low internal P loading rates (iPLR), where more elevated iPLR in muck-based systems appears to mask the effects of reducing external loading under low-P conditions. These results improve our understanding of external and internal P load contributions in low-P wetlands.

Controls on the composition of dissolved organic matter in treatment wetland source waters of South Florida, USA

Dissolved organic matter (DOM) plays an important role in the phosphorus (P) and nitrogen (N) cycle, but the examination of coupled biogeochemical cycling in treatment wetlands, like the Everglades Stormwater Treatment Areas (STAs), has been limited. Here, we studied eight STA inflow waters to gain insights into the production and potential photo- and biolability of DOM in surface waters flowing into these treatment wetlands. A suite of optical and fluorometric indices was coupled with amino acid and bulk elemental analyses to evaluate differences in inflow water quality for particulate organic matter (POM), DOM (<0.2 μm), and high and low molecular weight DOM (HMW-DOM and LMW-DOM). A photochemical experiment was conducted with these inflow waters to assess how solar exposure alters DOM turnover and its relationship with nutrient concentrations, and to evaluate the influence of photochemistry on potential STA performance. The DOM of STA inflow water was predominantly composed of allochthonous terrestrial sources, and the LMW-DOM fraction represented 70 to 94% of the total DOM pool. Inflow waters separated along the biophysical dimension of our PCA based on their photolability and bioavailability, driven by differences in the inflow water's DOM composition. UV exposure decreased humic and aromaticity proxies with concomitant photo-production of LMW compounds and increases in bioavailability proxies. Photo-exposure significantly decreased nitrate and dissolved organic carbon (DOC) concentrations for most of the sites and dissolved organic N (DON) for three of the eight sites, but no significant changes were noted for ammonia. One site exhibited an increase in nitrate with UV exposure and had DOM primarily from terrestrial origins. Spectroscopic indices, PARAFAC components, and amino acids provided corroborating evidence that further supported the differences in photo- and biolability at each inflow site, which is likely driven by their original DOM characteristics. PARAFAC components were correlated with dissolved organic P (DOP) and suggest that there may be adsorption of DOP with humic components in the LMW fraction. This work highlights the importance of photochemistry in the STAs, the complexity of DOM turnover in STA source waters, and the potential interactions between dissolved P and other DOM components, which may further influence the treatment efficiency of the STAs.

History and performance of the Everglades STA-3/4 periphyton-based stormwater treatment area (PSTA)

The Everglades Stormwater Treatment Areas (STAs) retain phosphorus (P) from stormwater runoff before it enters the Everglades Protection Area. These large treatment wetlands are extremely efficient in reducing total P (TP) concentrations. However, in some cases, the STAs may require additional measures to reach and/or maintain mandated discharge TP concentrations. One such option, a Periphyton-based Stormwater Treatment Area (PSTA) Cell, was constructed in 2005 on 40 ha at the outflow region of one flow-way in STA-3/4. This PSTA Cell was built to determine operational and engineering requirements associated with the implementation of this technology on a large scale. The PSTA Cell has supported an oligotrophic, periphyton-rich treatment wetland over its operational period of record. This system has consistently produced annual flow-weighted mean (FWM) outflow TP concentrations at or near the mandated outflow TP levels. The PSTA Cell maintained optimal treatment performance when subjected to hydraulic conditions observed in other full-scale STA cells (i.e., deeper water depths, pulsed flows, and major storm events). The PSTA Cell's treatment performance appeared to be better (FWM outflow concentrations ∼9 μg L−1) during continuous flows for 21 days compared to intermittent flow periods. Daily PSTA Cell outflows often were ≤ 13 μg L−1 when inflow TP concentrations were ≤ 22 μg L−1. The PSTA Cell also removed additional P from very low inflow concentrations (< 15 μg L−1). Seepage into the PSTA Cell occurred when its stage was lower than stages in adjacent water bodies, but seepage was greatly reduced when stages in the PSTA Cell were increased and the difference in stage between the PSTA Cell and adjacent bodies was reduced. TP concentrations in seepage water were usually higher than the PSTA Cell's inflow or surface water TP concentrations, therefore, the PSTA Cell performance did not benefit from seepage inflows. Over the 13 water years of operation, accretion of new soil (TP < 300 mg kg−1) did not result in nutrient gradients within the PSTA Cell that might affect P performance. Overall, the PSTA Cell study achieved its goal of providing operational and engineering insights for the implementation of this technology on a larger scale, and the PSTA Cell consistently attained ultra-low-P concentrations (mean ± standard deviation) of 10 ± 3 μg L−1 over its operational period of record.

Ultrafine (

Ultrafine particulate SRP may have an important role in the transfer of P from the Everglades Stormwater Treatment Areas (STAs), constructed wetlands designed to reduce phosphorus (P) in stormwater that is discharged to the Everglades Protection Area, to sensitive downstream water bodies due to increases in mobility associated with small particle size. P concentrations in discharges from the STAs are monitored using routine methods for particulate P (>0.45 μm) and dissolved P (<0.45 μm), an approach that does not consider ultrafine particulate P within the <0.45 μm fraction. Ultrafine particulate SRP was determined by ultrafiltration and was operationally defined as SRP associated with particles between <0.45 μm and <3 kDa. This study determined the magnitude and proportion of ultrafine particulate SRP within the <0.45 μm fraction of inflow, outflow, and in some cases, midflow waters from a selection of well-performing (discharge flow weighted mean concentration (FWMC) of P ≤ 20 μg L−1) and under-performing flow-ways (FWs) (FWMC P > 20 μg L−1) containing emergent aquatic vegetation (EAV), submerged aquatic vegetation (SAV) or a mix of EAV/SAV during the wet and dry season. During the wet season, inflow <0.45 μm SRP was predominately (60–100%) truly dissolved SRP (< 3 kDa) in all flow-ways. In the under-performing flow-ways, <0.45 μm SRP remained largely as truly dissolved, highly available SRP from inflow to outflow, whereas in the well-performing flow-ways, the percentage of <0.45 μm SRP present as ultrafine particulate SRP increased up to 100% as the distance from the inflow increased and the amount of truly dissolved SRP was very low. Two flow-ways having 100% particulate SRP at the outflow (STA-3/4 CFW and STA-5/6 FW1) contain an EAV cell followed by an EAV/SAV cell. However, in flow-ways containing more SAV (STA-2 FW3), ultrafine particulate SRP was a smaller proportion of outflow SRP (29%). During the dry season, ultrafine particulate SRP similarly increased from inflow to outflow at some well and under-performing sampling locations, but to a smaller proportion, and ultrafine particulate SRP never made up 100% of the <0.45 μm SRP fraction. These findings reveal that ultrafine particulate SRP is largely present within the <0.45 μm SRP fraction, at times making up >50% of TP, making it an important but poorly understood component of STA P budgets.

The largest constructed treatment wetland project in the world: The story of the Everglades stormwater treatment areas

The Everglades Stormwater Treatment Areas (STAs) are the largest complex of constructed treatment wetlands in the world with a current treatment area of over 25,000 ha which will total approximately 25,900 ha by the end of 2025. They are designed to remove phosphorus from agricultural and urban stormwater runoff, and when there is available capacity, they can also receive and treat water from Lake Okeechobee. STA-treated water is then sent south to the Everglades to support restoration and maintenance of this unique ecosystem. The recommendation to build treatment wetlands began in 1988 with the Lake Okeechobee Technical Advisory Committee, and the mandate to operate and maintain the STAs continues today through operating permits and consent orders issued to the South Florida Water Management District (SFWMD) by the Florida Department of Environmental Protection. The permits and consent orders set forth a stringent phosphorus water quality-based effluent limit (WQBEL) that must be met starting in 2026. SFWMD has conducted extensive research on treatment wetland function and phosphorus removal efficiency to support construction, management, and operation of the STAs to achieve the WQBEL. The most recent work has been conducted under the Restoration Strategies Science Plan. This 11-year research program began in 2013 and will continue through 2024 to address six key scientific questions aimed at assisting the SFWMD in meeting the WQBEL for discharges from the Everglades STAs. What follows in this Special Edition are articles discussing the results and conclusions from several of these studies.

Dynamics of total phosphate in the greater Everglades

Controlling and reducing excess phosphate is a fundamental objective of Everglades restoration. The immense, complex structural composition of South Florida landscapes and ecosystems, coupled with the multidecadal time scale of restoration, present a challenge to integrated assessment of restoration efforts. We address this with state space based convergent cross mapping to identify drivers and links of phosphate in response to construction and operation of stormwater treatment areas, engineered wetlands designed to sequester phosphate. Results identify drivers of phosphate dynamics, connecting dynamic response to nutrient and water management actions. Convergent cross mapping and state space dimensional reduction provide data-driven, model-free indicators of phosphate dynamics. We also identify low-flow conditions along the Everglades National Park inflow boundary as influential drivers of phosphate concentrations.

Nutrient dynamics in a eutrophic blackwater urban lake

Iraola ND, Mallin MA, Cahoon LB, Gamble DW, Zamora PB. 2022. Nutrient dynamics in a eutrophic blackwater urban lake. Lake Reserv Manage. 38:28–46. Greenfield Lake is a eutrophic blackwater urban lake in Wilmington, North Carolina, and in 2014 was declared by the state as impaired waters due to excessive chlorophyll a concentrations. Phytoplankton production is strongly nitrogen (N) limited, and the lake supports filamentous green algal blooms in spring and N-fixing cyanobacteria blooms in summer. To inform future lake restoration efforts, we quantified nutrient loads from 5 perennial streams that drain the highly impervious Greenfield Lake watershed to determine their impact on the lake’s eutrophic state. The 5 streams were sampled monthly from 2016 to 2017 during periods of dry weather and again after rain events of at least 1.2 cm to compare stormwater runoff to baseflow conditions. Two streams alone accounted for 76% of inorganic N and 63% of inorganic P surface inflow, whereas 2 other streams, despite high (53–74%) impervious coverage, had low loading. A lake-wide sediment phosphorus survey also revealed P accumulations mirrored streams of high nutrient load, while low P accumulations were in areas draining streams with large wetland and stormwater treatment areas. Drainage area was positively correlated with nitrate concentration and nitrate and ammonium loading. Unconventionally, percent impervious coverage was negatively correlated with nutrient concentrations and loads; we suspect that the presence of a large golf course in the watershed with the lowest impervious cover was a key driving factor. Our work also demonstrates the significance of catchment features: Drainages with large stormwater retention structures and significant natural wetlands in the lower reaches yielded low nutrient loadings.