Water, vegetation and sediment gradients in submerged aquatic vegetation mesocosms used for low-level phosphorus removal.

Gradients in phosphorus (P) removal and storage were investigated over 6 years using mesocosms (each consisting of three tanks in series) containing submerged aquatic vegetation (SAV) grown on muck and limerock (LR) substrates. Mean inflow total P concentrations (TP) of 32 µg L(-1) were reduced to 15 and 17 µg L(-1) in the muck and LR mesocosms, respectively. Mesocosm P loading rates (mean=1.75 gm(-2) year(-1)) varied widely during the study and were not correlated with outflow TP, which instead varied seasonally with lowest monthly mean values in December and January. The mesocosms initially were stocked with Najas guadalupensis, Ceratophyllum demersum, and Chara zeylanica, but became dominated by C. zeylanica. At the end of the study, highest vegetative biomass (1.1 and 1.4 kg m(-2) for muck and LR substrates) and tissue P content (1775 and 1160 mg kg(-1)) occurred in the first tank in series, and lowest biomass (1.0 and 0.2 kg m(-2)) and tissue P (147 and 120 mg kg(-1)) in the third tank. Sediment accretion rates (2.5, 1.9 and 0.9 cm yr(-1) on muck substrates), accrued sediment TP (378, 309 and 272 mg kg(-1)), and porewater soluble reactive P (SRP) concentrations (40, 6 and 4 µg L(-1)) in the first, second and third tanks, respectively, exhibited a similar decreasing spatial trend. Plant tissue calcium (Ca) near mesocosm inflow (19-30% dry weight) and outflow (23-26%) were not significantly different, and sediment Ca was also similar (range of 24 to 28%) among sequential tanks. Well-defined vegetation and sediment enrichment gradients developed in SAV wetlands operated under low TP conditions. While the mesocosm data did not reflect deterioration in treatment performance over 6 years, accumulation of P-enriched sediments near the inflow could eventually compromise hydraulic storage and P removal effectiveness of these shallow systems.

Submerged aquatic vegetation-based treatment wetlands for removing phosphorus from agricultural runoff: response to hydraulic and nutrient loading.

Submerged aquatic vegetation (SAV) communities exhibit phosphorus (P) removal mechanisms not found in wetlands dominated by emergent macrophytes. This includes direct assimilation of water column P by the plants and pH-mediated P coprecipitation with calcium carbonate (CaCO3). Recognizing that SAV might be employed to increase the performance of treatment wetlands, we investigated P removal in mesocosms (3.7 m2) stocked with a mixture of taxa common to the region: Najas guadalupensis, Ceratophyllum demersum, Chara spp. and Potamogeton illinoensis. Three sets of triplicate mesocosms received agricultural runoff from June 1998 to February 2000 at nominal hydraulic retention times (HRTs) of 1.5, 3.5 or 7.0 days. Mean total P (TP) loading rates were 19.7. 8.3 and 4.5 g/m2/yr. After eight months of operation. N. guadalupensis dominated the standing crop biomass and P storage, whereas C. demersum exhibited the highest tissue P content. Chara spp. was prominent only in the 7.0)-day HRT treatments while P. illinoensis largely disappeared. Inflow soluble reactive phosphorus (SRP) (10 163 microg/L) was reduced consistently to near the detection limit (2 microg/L) in the 3.5- and 7.0-day HRT treatments, and to a mean of 9 microg/L in the 1.5-day HRT treatment. The mean inflow TP concentration (10(7) microg/L) was reduced to 52, 29 and 23 microg/L in the 1.5-, 3.5- and 7.0-day HRT treatments, respectively. Total P concentrations in new sediment (mean= 641, 408 and 459 mg/kg in the 1.5-. 3.5-, and 7.0-day HRT mesocosms, respectively) were much higher than in the muck soil used to stock the mesocosms (236 mg/ kg). The calcium content of new sediment was twice that of the muck soil (16.5% vs. 7.6%), demonstrating that CaCO3 production and, perhaps, coprecipitation of P occurred. We observed no nocturnal remobilization of SRP despite diel fluctuations in pH and dissolved oxygen. Mean outflow TP (21 microg/L) from a 147 ha SAV wetland (4-day nominal HRT) was similar to mean outflow TP in the 3.5-day and 7.0-day HRT treatments. The mesocosms adequately mimicked P removal and other important characteristics of the larger system and can be used to address research questions regarding treatment performance of full-scale SAV wetlands. Available data suggest that the incorporation of SAV communities into the stormwater treatment areas may benefit Everglades restoration.

Phosphorus removal from Everglades agricultural area runoff by submerged aquatic vegetation/limerock treatment technology: an overview of research.

The 1994 Everglades Forever Act mandates the South Florida Water Management District and the Florida Department of Environmental Protection to evaluate a series of advanced treatment technologies to reduce total phosphorus (TP) in Everglades Agricultural Area runoff to a threshold target level. A submerged aquatic vegetation/limerock (SAV/LR) treatment system is one of the technologies selected for evaluation. The research program consists of two phases. Phase I examined the efficiency of SAV/LR treatment system for TP removal at the mesocosm scale. Preliminary results demonstrate that this technology is capable of reducing effluent TP to as low as 10 microg/L under constant flows. The SAV component removes the majority of the influent soluble reactive P, while the limerock component removes a portion of the particulate P. Phase II is a multi-scale project (i.e., microcosms, mesocosms, test cells and full-size wetlands). Experiments and field investigations using various environmental scenarios are designed to (1) identify key P removal processes; (2) provide management and operational criteria for basin-scale implementation; and (3) provide scientific data for a standardized comparison of performance among advanced treatment technologies.

The use of macrophyte-based systems for phosphorus removal: an overview of 25 years of research and operational results in Florida.

Phosphorus (P) removal from wastewaters and surface runoff using macrophyte-based systems (MBS) has been a topic of great interest in Florida for over 25 years. During this period, P removal by both treatment wetlands and floating aquatic macrophyte systems has been evaluated from both a research and operational standpoint. Several factors have contributed to the increased focus on the use of MBS for P removal. First, there exist no conventional technologies that can cost-effectively achieve the low outflow P concentrations required to protect the integrity of Florida's relatively pristine surface waters. Second, because MBSs typically provide some water storage, they can accommodate the wide ranges of flows typical for runoff sources such as agricultural drainage waters. Finally, many regions in Florida have sufficient area for deployment of the relatively land-intensive MBS technologies. The first P removal work in Florida was initiated in the mid-1970s, and involved pilot-scale research on domestic wastewater treatment by natural wetlands. Parallel studies were performed with managed (periodically harvested) floating plant systems (i.e., Eichhornia crassipes) for tertiary treatment. Since that time, the range of operational systems that have been deployed include emergent macrophyte-based and forested wetlands, managed floating plant systems, and submerged macrophyte-based systems. Waters treated by MBS include domestic effluents, agricultural runoff and eutrophic lake waters. Phosphorus removal targets for MBS in Florida have been as low as 10 microg/L. In this paper, we summarize research and operational results for MBS in Florida over the past 25 years.