effects of plant community and phosphorus loading rate on ...

WETLANDS, Vol. 28, No. 1, March 2008, pp. 81–91 ’ 2008, The Society of Wetland Scientists

EFFECTS OF PLANT COMMUNITY AND PHOSPHORUS LOADING RATE ON CONSTRUCTED WETLAND PERFORMANCE IN FLORIDA, USA Binhe Gu and Thomas Dreschel Everglades Division, South Florida Water Management District 3301 Gun Club Road West Palm Beach, Florida, USA 33406 E-mail: [email protected] Abstract: We evaluated the effectiveness of constructed wetlands with varying plant communities for phosphorus (P) reduction from the Everglades Agricultural Area runoff in south Florida. Weekly or biweekly water samples from the inflow and outflow regions of 11 test cells (2,000 m2) were analyzed for various forms of P and other selected water quality variables between January 2002 and August 2004. Test cells located at the north site received water with a high average total P (TP) concentration (72 mg L21), while test cells located at the south site received water with a lower average TP concentration (43 mg L21). These test cells were dominated by an emergent vascular plant-cattail (Typha latifolia), submerged aquatic vegetation or SAV (Najas guadalupensis, Chara sp., Ceratophyllum demersum, and Hydrilla verticillata), or algal periphyton (mixed with Eleocharis cellulosa and Utricularia spp. in the south site only). Under a constant hydraulic loading rate (9.27 m yr21), these test cells removed P effectively, with removal efficiencies of 56%–65% at the north site and 35%–62% at the south site. The mass removal rate and rate constant at the north site were also higher than at the south site. Soluble reactive P (SRP) and particulate P were the major forms at inflow and were removed effectively by all of the test cells. The removal of dissolved organic P was significant (,60%) in the cattail and periphyton test cells, but no removal was detected in the SAV test cells. At the north site, P removal efficiency of the cattail test cells was slightly higher than that of the SAV test cells. At the south site, periphyton test cells were the best performers. The removal of SRP was positively correlated with the removal of calcium in the majority of the test cells, pointing to the potential importance of co-precipitation of calcium carbonate and SRP. Direct plant uptake, wetland filtering, microbial degradation, and co-precipitation with calcium carbonate were mechanisms thought to be responsible for P removal in these wetlands. Outflow TP concentration, an important measure for restoration performance, increased continuously with the increases in the TP mass loading rate at the north site, but peaked at approximately 30 mg L21 when the TP mass loading rate reached 0.5 g m22 yr21 at the south site. Key Words: calcium, cattail, hydraulic loading rate, periphyton-based stormwater treatment area, removal efficiency, submerged aquatic vegetation, test cells, total phosphorus

INTRODUCTION

Other functions of aquatic plants in the constructed wetlands include attenuating flow and changing local chemical environments (Dodds 2003), reducing suspended particles (Barko and James 1997), and providing wildlife habitat (Brix 1994). Emergent plants have been the most common species used in the treatment wetlands (Mitsch et al. 1995, Kadlec and Knight 1996, Nungesser and Chimney 2001). Other aquatic plants or plant assemblages have been tested to improve nutrient removal. These plants or assemblages include submerged aquatic vegetation or SAV (Dierberg et al. 2001, Gu et al. 2001), floating macrophytes (Vermaat and Hanif 1995, Perniel et al. 1998), and periphytic algae (Vymazal 1988, Thomas et al. 2002, DeBusk et al. 2004). A submerged aquatic vegetation treatment system consists of various submerged vascular plants,

Agricultural runoff is an important source of nutrients that affect water quality, biological diversity, and the functioning of the receiving water bodies (Carpenter et al. 1998, Beman et al. 2005). During the past several decades, constructed wetlands have been used as a restoration tool for ecosystem protection and rehabilitation (e.g., Brix 1994, Mitsch et al. 1995, Kadlec 2005, Scholz and Lee 2005). Free surface flow constructed wetlands are being used to treat stormwater runoff from agricultural areas (Coveney et al. 2002, Chimney and Goforth 2006, Scholz et al. 2007). These wetlands are typically vegetated with aquatic plants to assimilate and filter nutrients (Gersberg et al. 1986, Brix and Schierup 1989, Kao et al. 2003). 81

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WETLANDS, Volume 28, No. 1, 2008 mick et al. 1996, Gaiser et al. 2004, McCormick et al. 2006). Therefore, the periphyton-based wetlands are designed to be polishing systems that receive discharges with low TP concentrations (Bays et al. 2001). In south Florida, free surface flow treatment wetlands, called Stormwater Treatment Areas (STAs), have been constructed in recent years to remove P from the Everglades Agricultural Area runoff (Chimney and Goforth 2001). Here we report on a comparative study of TP removal efficiency in small experimental wetlands dominated by emergent, SAV, and periphyton communities, respectively. The objective of this study was to assess treatment performance by these plant communities under constant hydraulic loading and various inflow TP concentrations. MATERIALS AND METHODS Site Description

Figure 1. Schematic diagram showing location and structure of test cells.

macroalgae, and the associated periphyton. Recent studies have documented high removal rates for total phosphorus (TP) in wetlands dominated by SAV under high or moderate hydraulic and TP loadings (Dierberg et al. 2001, Gu et al. 2001, Juston and DeBusk 2006). This is attributed to the fact that SAV can assimilate nutrients effectively from the water column and sediments (Carignan and Kalf 1980, Rattray et al. 1991, Barko and James 1997). Direct P removal from the water column is especially important as the goal of the free surface flow treatment wetlands is to sequester contaminants from the inflow water. Furthermore, CaCO3 coprecipitation with P promoted by SAV (Dierberg et al. 2002) is not likely in emergent wetlands where photosynthesis largely takes place above the water. The flora community in periphyton-based treatment wetlands consists of several types of algae, emergent plants, and SAV, in varying proportions. Although these plants may be found in the SAV and the emergent wetlands, periphyton-based wetlands in south Florida possess greater periphyton biomass and coverage, especially calcareous algae, than the other wetlands (Bays et al. 2001, DeBusk et al. 2004, McCormick et al. 2006). Sparsely vegetated macrophyte beds provide physical structures that support periphyton growth. Calcareous periphyton in the Everglades, Florida is found in a hard water environment with low TP concentrations (McCor-

The test cells were small (2,000 m2) rectangular wetlands located in STA-1, west of south Florida (26u389N and 80u259W). These test cells were arranged into two banks of 15 cells (Figure 1). Research described in this paper was conducted in four north test cells (NTC) and seven south test cells (STC). Each test cell was isolated by a full liner to prevent seepage from adjacent test cells. Inflow water was first pumped into an elevated storage cell and then delivered in parallel fashion to the test cells via a pipe distribution system. Inflow rate was regulated by changing calibrated orifice caps fitted to the end of the distribution pipes. Distribution manifolds were installed to deliver water at the top of the test cells (Newman and Lynch 2001). During this study period, the major hydrologic regimes were fixed, with a water depth of 0.3 m, a flow rate of 73.4 m3 d21, a hydraulic loading rate of 9.27 m y21, and a residence time of 8.2 days. The north and south test cells received water with high (60–150 mg L21) and relatively low TP concentrations (30–50 mg L21), respectively. The average difference in inflow TP concentration of approximately 30 mg L21 between the north and south site is not only statistically significant (paired t-test, p , 0.01), but also ecologically significant for constructed wetlands designed to treat agricultural runoff with a typical range of TP concentration from 100–200 mg L21 (Chimney and Goforth 2006). A unique suite of vegetation types was stocked in two to three test cells at each site. Test cells with cattail (Typha latifolia) included NTC-5 and 10, and STC-1 and 15 (Figures 1 and 2). Test cells with SAV (Figure 2) included NTC-1 and 15, and STC-4 and

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with Chara spp. in STC-4 and STC-9. Test cells with cattail and SAV had a peat substrate over the liner. In addition, two SAV cells, STC-9 and NTC-15, had limerock berms constructed near the outflow areas to test the role of a limerock barrier on wetland performance (Figure 2). These berms of crushed limerock (,2.5 cm diameter) were constructed across the width (and perpendicular to the direction of flow) at a distance approximately 88% of the flowpath length, downstream from the inflow. Three test cells at the south site (STC-3, 8, and 13) were managed to encourage periphyton growth (Figure 2). These periphyton-based wetlands are called Periphyton Stormwater Treatment Areas (PSTAs). Floating calcareous periphyton mats were visible with sparse emergent Eleocharis cellulose and Chara spp. Originally, all of these cells had a peat substrate over the cell liner approximately two years before this study was initiated. The peat layer was replaced by 1 m of sand fill and 30 cm of locally mined shellrock in STC-3 and 8. The peat substrate in STC-13 was replaced by 80 cm of sand fill, 30 cm of shellrock, and 30 cm of peat taken from a local unflooded, former agricultural lands area. Sampling and Analyses

Figure 2. Test cells with different vegetation communities: emergent (cattail) system (top), SAV (Submerged Aquatic Vegetation) system with a limerock berm (middle), and PSTA (Periphyton Stormwater Treatment Area) showing emergent plant (Eleocharis sp.) and floating calcareous periphyton mats (bottom).

9. These cells were occupied by various species of SAV, with Najas guadalupensis as the major species and Ceratophyllum demersum and Hydrilla verticillata of secondary importance in NTC-1 and 15, and

Total P, soluble reactive P (SRP), and total dissolved P (TDP) in the test cells were measured in weekly or biweekly grab samples from the common inflow and outflow from each test cell, from January 2002 to August 2004. Particulate P (PP) was calculated by the difference between TP and TDP; dissolved organic P (DOP) was calculated by the difference between TDP and SRP. Weekly grab samples were also collected for alkalinity, calcium (Ca), aluminum (Al), magnesium (Mg), ammonium (NH4+), and nitrate and nitrite (NOx2 5 NO32 + NO22). All analyses were conducted following standard methods (APHA 1998). Water temperature, pH, and dissolved oxygen (DO) were recorded in the field with a Hydrolab (Hydrolab-Hach Co., Loveland, Colorado, USA) on each sampling date. To evaluate the treatment performance of these wetlands under the TP and hydraulic loading conditions, several performance measures were determined using the following equations: The removal efficiency (RE, %) for various forms of P was calculated as: RE ~

(Ci { Co ) | 100 Ci

ð1Þ

where Ci and Co are the P concentrations (g m23) at the inflow and the outflow.

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Table 1. Average inflow and outflow values of selected environmental variables in the north and south test cells. SAV 5 Submerged Aquatic vegetation; PSTA 5 Periphyton Stormwater Treatment Area. Temperature uC

pH SU

North Test Cells (N 5 32 months) Inflow 23.6 7.2 Cattail 22.3 7.2 SAV 21.7 7.6 South Test Cells (N 5 26 months) Inflow 23.8 7.8 Cattail 21.7 7.2 SAV 21.6 7.7 PSTA 22.5 7.5

DO mg L21

Alkalinity

Ca

mg CaCO3 L21

mg L21

Al mg L21

Mg mg L21

NOx2 mg L21

NH4+ mg L21

1.3 0.8 1.5

278 275 214

88 83 58

61.5 14.6 23.1

26.8 26.2 26.4

100 5 10

295 170 220

1.4 0.7 1.1 1.6

266 257 220 216

82 75 51 55

11.4 19.5 10.6 12.8

26.9 25.9 27.8 26.4

44 10 10 7

130 150 205 167

The mass removal rate (MRR, g m22 yr21) for TP is calculated as: Q ð2Þ MRR ~ (Ci { Co ) | A where Q was the flow rate (m3 d21) and A is the surface area (m2) of test cells. The settling rate constant (k, m yr21) for TP is calculated as: Ci { C  k ~ ln ( )|q ð3Þ Co { C  where C* is the background TP concentration (g m23) where a value of 0 g m23 was used. The term q is the annualized hydraulic loading rate (m yr21). All data analyses were performed using the monthly means calculated from the weekly or biweekly data. Statistics were conducted with JMP (Version 5, SAS Institute Inc., Cary, North Carolina, USA). The level of significance (a) was set at 0.05 for all analyses. RESULTS Environmental Conditions The average water temperature at the inflow during the study period was 24uC, reflecting the subtropical climate. Inflow water was characterized by high alkalinity, high Ca concentrations, aboveneutral pH, and low DO concentrations (Table 1). Ammonium was the dominant form of dissolved combined nitrogen. Low NOx2 concentrations at both the inflow and outflow were attributed to poor oxidation states in the inflow and in the test cells. There were some differences in these variables between the north and south test cells. The inflow to the south test cells had a higher pH, while alkalinity, Al, Ca, and dissolved nitrogen were lower than in the inflow to the north test cells (t-test, all p , 0.05). Wetland processing was the cause for the differences in these variables between the two sites

because the north test cells received water prior to treatment by a full-scale constructed wetlands and the south test cells received water from a treatment wetland (Newman and Lynch 2001). Magnesium and DO concentrations showed little differences between the two inflows (p . 0.05).

Inflow TP Concentration and Removal Total P concentrations at the north inflow ranged from 28–116 mg L21 with an average of 72 mg L21 (Figure 3). The changes in inflow TP concentrations were associated with the seasonal hydrological cycle. High TP concentrations were found in the wet season (May to August) with low TP concentrations occurring in the dry season (October to April). Outflow TP concentrations generally followed the inflow TP pattern, but did not vary as much as the inflow. Average outflow TP concentrations were 25 and 30 mg L21 for the cattail and the SAV test cells, respectively. The average TP mass removal rate for the cattail test cells was 0.63 g m22 yr21 with a removal efficiency of 64% (Table 2). This is compared to the mass removal rate and removal efficiency for the SAV test cells of 0.57 g m22 yr21 and 55%. The rate constants for the cattail and SAV cells were 10 and 9 m yr21, respectively (Table 2). Total P concentrations at the south inflow ranged from 18–84 mg L21 with an average of 43 mg L21 (Figure 4). The seasonal pattern for the inflow TP concentrations at the south site displayed greater monthly fluctuations than the north site. However, the outflow concentrations for all the south test cells remained consistently low. The PSTA test cells had the lowest average outflow TP concentration (18 mg L21) and the highest removal efficiency (Table 2) compared to the cattail and SAV cells (paired t-test, p , 0.01). The cattail cells in the south site marginally outperformed the SAV cells (p 5 0.12)

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Figure 3. Time series and box plot of inflow and outflow concentrations of various P species in the north test cells. All units are mg L21.

as was observed in the north site. Rate constants ranged from 5–9 m yr21 for the south test cells. The north test cells received higher inflow TP concentrations and thus higher mass loading rates

than the south test cells, under the same hydraulic loading rates. As a result, the mass removal rate and removal efficiency for TP at the north site were higher than at the south test cells for the cattail and

Table 2. Average and standard deviation (value in parentheses) of inflow and outflow TP concentrations, mass loading and removal, percent removal and rate constants for north and south test cells. Concentration System

21

mg L

North Test Cells (N 532 months) Inflow 72(26) Cattail 25(10) SAV 30(14) South Test Cells (N 5 26 months) Inflow 43(17) Cattail 24(8) SAV 26(12) PSTA 18(6) * Mass loading rate.

Mass removal 22

gm

21

yr

Percent removal %

Rate constant m yr21

0.96(0.35)* 0.63(0.30) 0.57(0.36)

NA 64(15) 55(26)

NA 10(4) 9(5)

0.57(.022)* 0.32(0.22) 0.29(0.29) 0.37(0.21)

NA 49(24) 33(51) 59(19)

NA 7(4) 5(4) 9(4)

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Figure 4. Time series and box plot of inflow and outflow concentrations of various P species in the south test cells. All units are mg L21.

the SAV systems (Table 2). The average inflow TP concentration at the north site was nearly double that of the south site, yet, the north cattail system was able to reduce TP concentration to virtually the same average concentration as in the south outflow. The average outflow TP concentration at the north SAV test cells was 4 and 5 mg L21 higher than from SAV and cattail for the south and north test cells, respectively (p , 0.05). Phosphorus Speciation and Removal Concentrations and removal efficiencies for all P species in each system are presented in Table 3. Overall, 47% of the TP was SRP in the inflow to the north site, followed by PP (43%) and DOP (10%). Both cattail and SAV systems removed high percentages (71% and 79%) of SRP, and 60% and 54% of PP, respectively. Removal of DOP was dramatically different between these systems, with a

removal efficiency of 64% in the cattail and -29% in the SAV system. As in the north inflow, SRP at the south inflow was the largest component (55%) of TP, followed by PP (33%) and DOP (12%). Removal of SRP again was high (57% to 74%), but compared to the north site, removal of PP significantly decreased in the south cattail system, and no removal was found in the SAV system. For DOP, the cattail and PSTA systems showed similar removal rates of ,60%, while the SAV system also showed some removal. The PSTA had the highest removal rate for all of the P species in the south test cells (Table 3). DISCUSSION Phosphorus Speciation and Removal Pathways Phosphorus in the inflow water to the test cells was dominated by soluble reactive and particulate

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Table 3. A summary of the average inflow and outflow concentrations (mg L21) and removal efficiency (RE, %) of various phosphorus species in each plant system. SRP North Test Cells (N 5 32 months) Inflow 34 Cattail 10 SAV 7 South Test Cells (N 5 26 months) Inflow 22 Cattail 10 SAV 7 PSTA 6

RE

DOP

RE

PP

RE

47 71 79

7 3 9

10 64 -29

31 13 16

43 60 54

51 57 68 74

9 4 7 5

21 61 22 64

14 11 14 7

33 25 0 43

forms. Several mechanisms have been proposed as the means for P reduction in wetlands (Figure 6). These include direct plant uptake, microbial degradation, filtering, co-precipitation with CaCO3, and UV oxidation (Moran and Zepp 1997, Dierberg et al. 2001, 2002, Kuhn et al. 2002). Soluble reactive P can be removed by plant uptake and chemical precipitation. Removal efficiencies for SRP ranged from 57%–74% in the test cells and were highest among all P species. Aquatic plants, particularly SAV, are capable of high SRP assimilation (Carignan and Kalf 1980, Dierberg et al. 2002). Similar results have also been reported in mesocosms and treatment wetlands vegetated by cattail and SAV receiving a source of stormwater runoff in a fashion similar to the test cells in this study (Gu et al. 2001, Nungesser and Chimney 2001, White et al. 2006). Calcium carbonate precipitation occurs along with anions such as PO423 when CaCO3 reaches saturation at high pH (Otsuki and Wetzel 1972, Kleiner 1988). A saturation index (SI) is often used to describe the degree of saturation within the solution with respect to calcite: SI ~

½Ca2z ½HCO3 { =½H z  | 10{10:33 Ks

ð4Þ

The solubility product (Ks) is 1028.35 for pure calcite (Stumm and Morgan 1981). A SI value above zero denotes saturation with reference to CaCO3. Using pH and Ca data from the inflow, we calculated the SI as 0.02 for the north site and 0.34 for the south site, indicating that both inflows were saturated with CaCO3. Calcium carbonate precipitation in the test cells was evidenced by the reduction of calcium from the inflow to the outflow (Table 1). The SAV test cells removed the greatest amount of calcium (30 mg L21), followed by the PSTA (20 mg L21) and the cattail test cells (, 10 mg L21). The amount of calcium removal was related to the rate of underwater photosynthesis and was positively correlated with the amount of SRP removal in some test cells

(Figure 5). We estimated that, on average, approximately 3 mg L21 of calcium was removed for every 1.0 mg L21 of SRP removal. Dierberg et al. (2002) found that approximately 0.5 mg L21 of calcium was removed for every 1.0 mg L21 of SRP removal during a mesocosm experiment stocked with SAV. Similarly, Newman and Pietro (2001) found a significant correlation between SRP and Ca concentration in a full-scale constructed wetland in south Florida. Coprecipitation with CaCO3 was observed to be a dominant pathway for SRP reduction in a marl lake and as much as 70% of SRP was removed from the water column (Otsuki and Wetzel 1972). The removal of SRP via CaCO3 coprecipitation was also significant in highly productive lakes (e.g., Murphy et al. 1983, Weidemann et al. 1985, Neal 2001). Microbial organisms convert organic P to SRP by extracellular enzymes such as alkaline phosphatase, to acquire energy for metabolism (Moran and Zepp 1997). The reduction in the DOP concentration observed at the outflows was likely the result of bacterial lyses of DOP. This process was system dependent in our study, with high removal rates in PSTA (64%) and cattail test cells (62%), and low removal rates in the SAV test cells (-29% to 22%). Recent studies show that phosphatase activity increases with decreasing SRP concentrations in periphyton mats and the roots of sawgrass (Cladium jamaisens) and cattail (Typha sp.) from the Everglades (Kuhn et al. 2002, Newman et al. 2003). Under low TP mass loading during this study, the supplies of SRP may not have met the requirements for plant growth; thus, a secondary P source such as DOP was needed to satisfy system demand. This may explain why DOP removal (average 5 17%) of the north test cells receiving high TP loading was lower than that (average 5 48%) of the south test cells receiving low TP loading. Dissolved organic compounds can form complexes with phosphatase and reduce hydrolytic activity (Boavida and Wetzel 1998). Data for dissolved organic carbon (DOC) are

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Figure 6. Conceptual diagram for phosphorus removal pathways in constructed wetlands.

explain the difference in DOP reduction between the north and south test cells. Wetland filtration is an important mechanism for inflow containing high particulate P concentrations and over 40% and 30% of the inflow TP was PP at the north and south sites, respectively. The removal efficiency from the north test cells (54%–60%) was similar to the performance (30%–67%) by a Florida filtration wetland fed with lake water containing TP concentration ranging from 80–380 mg L21 with mostly particulate organic P (Coveney et al. 2002). Our test cells and this filtration wetland also shared similar hydraulic retention times (eight and seven days, respectively). The low removal efficiency in the south test cells was related to the low inflow PP concentration. Phosphorus Removal and Wetland Characteristics

Figure 5. Relationship between TP mass loading rate and outflow TP concentration.

not available during this study period. However, data from 1999 to 2001 show that DOC concentrations of the inflows to the north and south sites were high (39.6 and 40.5 mg L21), which may have inhibited phosphatase activity; however, this cannot

This study demonstrated that nearly all test cells removed TP effectively under constant low hydraulic loading, especially in the north site, which received high TP loading. The average TP concentration at the inflow of the north site was nearly 30 mg L21 higher than the south site. The removal efficiency observed during this study was comparable to the results from studies conducted in mesocosms, other test cells, and full-scale STAs (Dierberg et al. 2001, Gu et al. 2001). However, additional performance measures such as the mass removal rates and rate constants were considerably lower because these measures are functions of hydraulic and TP loading. Richardson et al. (1997) proposed that the long-term removal rate of P for a mature constructed wetland is about 1 g m22 yr21. If this is true, then our test cells were under-loaded both with flow and nutrients, and had yet to attain their removal potentials. Differences in removal efficiency, mass removal rates, and rate constants were also found among test cells with different plant communities. Treatment wetlands dominated by SAV have been considered a

Gu & Dreschel, PLANT TYPE AND PHOSPHORUS REMOVAL better system than wetlands dominated by emergent plants for TP removal in south Florida (Gu et al. 2001, Juston and DeBusk 2006). These studies were conducted under relatively high hydraulic and TP loading rates. In contrast, removal efficiency, mass removal, and rate constants for the SAV test cells were lower than those of the cattail test cells in the north site, and also were lower than the cattail and PSTA test cells in the south site, under constant low hydraulic and TP loadings. The SAV system may require a higher threshold of TP loading to attain higher removal performance than a system populated by emergent plants and periphyton. This hypothesis will require further testing using controlled experiments. In the south site, under relatively low average TP loadings, the PSTA system outperformed the cattail and SAV systems by 10% and 20%, respectively. This demonstrated the high P removal efficiency of the periphyton-based wetlands under low TP and hydraulic loading rates. Bays et al. (2001) reported that inflow TP concentrations of 23 mg L21 in PSTA test cells on shellrock soil were reduced to as low as 11 mg L21. DeBusk et al. (2004) reported that the inflow TP concentration of 18 mg L21 from a shallow periphyton-dominated treatment system was reduced to 10 mg L21. Despite the differences in inflow TP concentrations in these studies, the removal efficiencies were consistent at 50%–60% and thus these periphyton-dominated systems were capable of reducing the low inflow TP to 10–20 mg L21. There are two possible advantages in the PSTA systems over other systems. First, the mixed plant community with periphyton, SAV, and emergent species, compared to the monoculture of cattail or SAV in other systems, favor higher P removal. Although cattail and SAV systems also contained periphyton, the PSTA had a considerably higher biomass and coverage of calcareous periphyton than the other systems. Second, the peat substrates in STC-3 and 8 were replaced by shellrock beds. The outflow TP concentrations at the two test cells were ,50% lower than that of STC-13, which only received a lime treatment. The shellrock beds might have served as a barrier to P flux from the sediment to the water column. The dissolution of CaCO3 from the shellrock may have further helped precipitate SRP from the sediments. However, the limerock berm (constructed near the downstream ends of NTC-15 and STC-9 SAV test cells) did not provide significant improvements on P removal during this study. The outflow TP concentration, removal efficiency, and mass removal for these cells were only slightly better than observed for NTC-1 and STC-4, which did not possess a limerock berm.

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Figure 7. Relationship between calcium removal and SRP removal in north and south test cells with different plant types.

Total Phosphorus Mass Loading and Outflow Concentration Outflow TP concentration is often an important criterion for wetland restoration and performance evaluation. One of the ultimate goals of the Everglades restoration is to return the system to a pristine condition with a water column TP concentration below 10 mg L21 (Sklar et al. 2005). Our data revealed increases in outflow TP concentration with the increases in TP mass loading rate, especially in the north sites (Figure 7). This indicates that these test cells cannot maintain constantly low outflow TP concentrations under elevated mass loading rates. Nevertheless, the outflow TP concentration was generally less than 30 mg L21 when the TP mass loading rate was at or below 1 g m22 yr21 at the south site. Recently, Juston and DeBusk (2006) found that at or below a mass loading rate of 1.3 g m22 yr21, the outflow TP concentrations from several large-scale wetlands in south Florida were less than 30 mg L21. They also found a lower outflow TP concentration (20 mg L21) in a SAVbased wetland at the same TP mass loading rate. Our study revealed the lowest outflow TP concentration (at or below 20 mg L21) in most dates when TP mass loading was at or below 1 g m22 yr21 in the

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PSTA system. These findings, which are consistent with the previous studies (Bays et al. 2001, DeBusk et al. 2004), demonstrate that periphyton-based wetlands are the best green technology in order to achieve low outflow TP concentration under low TP mass loading rates. Since the TP mass loading rate is a function of both hydraulic loading and the TP concentration, to reach a target TP concentration, one measure would be to control the hydraulic loading rate and the TP concentration at the inflow. CONCLUSIONS Small constructed wetlands based on emergent, submerged macrophytes, and periphyton were capable of effective removal of P under a low, constant hydraulic loading rate. Between 35% and 65% of TP from the inflow was retained in the wetlands resulting from plant uptake, microbial degradation, filtering, and co-precipitation with CaCO3. The relative importance of various P removal pathways needs further research to better understand the dominant removal mechanisms in the constructed wetlands. The PSTA system demonstrated the highest removal efficiency, mass removal rate, and rate constant, and the lowest outflow TP concentrations under the low inflow TP concentrations. This indicates that a polyculture of various types of aquatic plants with the dominance of calcareous periphyton is the most effective combination of vegetation for P removal. The SAV wetlands did not show the same high removal rates that had been observed in previous studies with high hydraulic and P loading rates. The cattail wetlands demonstrated a similar removal performance under both high and low TP loading. Soluble reactive P was the major form present in the inflow and was also the most utilized P by all the wetlands. Particulate P removal was as effective as SRP removal from the inflow with high TP. Removal of DOP was effective in the cattail and PSTA wetlands. Outflow TP concentration was generally a function of mass loading rate at the north site, but was significantly lower in the south site, especially in the PSTA test cells. From this study, we recommend that, as a final polishing step, PSTA systems be used downstream from other constructed wetland vegetated with emergent and submerged plants, to provide further removal of P. ACKNOWLEDGMENTS We thank the staff of the SCADA Hydro Data Management and Environmental Resource Assess-

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