Search

External inputs and internal cycling are two pathways of nutrients to the aquatic macrophytes in the wetland ecosystem. Internal sources of nutrients include plant die-off and the decomposition of organic matter. In this study, we used a stable nitrogen isotope (15N) enrichment experiment to track the timing and magnitude of cattail (Typha angustifolia) use of coontail (Ceratophyllum demersum) – derived nitrogen (CDN) in a freshwater wetland. Dry coontail powder enriched in 15N (δ15N = 1259 ± 15‰) was buried in the soil close to the roots of cattail plants in field mesocosms at the Yeyahu National Preserve, Beijing, China, from July 22 to September 30, 2014. Different parts (roots, rhizomes, stems, leaves and leaf apexes) of the cattail plants were collected at day 0, 14, 28, 42, 49, 56, 63 and 70 for stable isotope analysis. Results reveal uptake of the nitrogenous nutrient from coontail detritus by cattail four weeks after being exposed to CDN. The maximum uptake rate for excess 15N in roots, leaves, stems, rhizomes and leaf apexes were 1.55 ± 0.84, 0.69 ± 0.11, 0.33 ± 0.08, 0.14 ± 0.11, 0.06 ± 0.02 mg 15N m−2. The root system is the largest pool of nitrogen in cattails, which serves as the source of nitrogen for other plant parts. The uptake rate per unit biomass was highest in the leaf apexes.

Carbon and nitrogen stable isotopes (δ13C and δ15N) have been used as indicators for environmental changes in aquatic ecosystems. In this study, δ13C and δ15N values of select flora taxa were evaluated as indicators of human-induced eutrophication in an Everglades wetland which has been impaired by the source and quality of inflow and the alteration of hydrological pattern. Results showed that δ13C of cattail (Typha domingensis Pers.) increased with the decrease in total phosphorus (TP) concentration. Limited data for sawgrass (Cladium jamaicenses Crantz) collected at three study sites along the nutrient gradient also show a similar change in δ13C. However, δ13C of periphyton and bladderwort (Utricularia sp.) decreased with the decrease in TP concentration. The elevated δ13C values of emergent vascular plants (cattail and sawgrass) were likely the results of decreases in stomatal conductance, carboxylation and 13C fractionation under nutrient stress. This is supported by corresponding decreases in photosynthetic rate of cattail along the decreasing nutrient gradient. In contrast, δ15N values of all flora taxa increased with the increase in TP concentration. Findings from this study indicate that δ13C of emergent plants is a sensitive indicator for nutrient stress while δ15N of major flora species may serve as a robust indicator for wetland eutrophication.

Wastewater with a high nitrogen (N) loading rate and high ammonium (NH4+-N)/nitrate (NO3−-N) ratio is increasingly becoming a problem in regional environment. An experimental system was established to test the effects of plant diversity on N removal in microcosms of constructed wetlands. The microcosms were treated with simulated wastewater with NH4+-N as the sole chemical form of N, and with mixed NH4+/NO3—N (1:1) as a control, both at a loading rate of 162 g N m−2 yr−1. Results indicated that: (1) high plant species and functional group richness improved N removal efficiency under a high ammonium loading rate, (2) high species richness reduced the difference of N removal between sole NH4+-N and mixed NH4+/NO3−-N treatments, (3) the presence of Coix lacryma-jobi (C4 grass) increased both biomass and N removal, whereas the presence of Aeschynomene indica (legume) increased N removal but decreased biomass of the communities. Overall, the results showed that high plant diversity enhanced N removal efficiency regardless of the N-form ratios and N level in the ecosystem.

Variations of total mercury (THg) and methylmercury (MeHg) concentrations at the inflow and outflow of a constructed treatment wetland, Stormwater Treatment Area 2 (STA-2) in south Florida were investigated from 2000 to 2011. The linkages between inflow water quality parameters and outflow THg and MeHg concentrations were assessed. THg and MeHg concentrations at the outflow in 2001 (2.88 and 0.95 ng L −1 ) and 2002 (2.33 and 0.87 ng L −1 ) were significantly greater than those (1.14 and 0.14 ng L −1 , 1.16 and 0.12 ng L −1 , respectively) at the inflow during the same time period. This was due to dryout and rewetting which may lead to Hg release from soil and promote Hg methylation. The average concentrations (0.75 and 0.11 ng L −1 ) of THg and MeHg at the outflow from 2005 to 2011 (except 2007) were lower than those (1.00 and 0.13 ng L −1 ) at the inflow, indicating that net THg and MeHg retention occurred in STA-2 during those years, which ranged from 0.35% to 49% for THg and from 3.5% to 37% for MeHg, respectively. Stepwise regression identified several inflow water quality parameters which explained 68% and 58% of the variances in the outflow THg and MeHg concentrations from 2000 to 2011. Certain inflow quality parameters were strong predictors of the variations in outflow THg and MeHg concentrations. These parameters included sulfate, chloride, dissolved organic carbon (DOC), dissolved oxygen (DO), pH, and temperature for outflow THg; and sulfate, chloride, DOC, and DO for outflow MeHg. Inflow sulfate and DO concentrations separately exhibited significant correlations with outflow THg and MeHg concentrations only prior to 2004. The results indicate that STA-2 was a source of THg and MeHg during early years of operation and a sink for THg and MeHg when the wetland passed the initial operation period and especially as water management improved to prevent dryout from occurring. Findings from this study also underscore the important role of inflow water quality to the variations of the outflow THg and MeHg concentrations.

The transport of dissolved inorganic nitrogen in wetlands was investigated in the Yeyahu National Preserve of Beijing, China, from July to December 2011. Stable isotope of nitrogen ( 15 N) as NH 4 + was added to experimental mesocosms vegetated by seven common wetland plants. The 15 N abundance in soil and various plant parts were measured at prescribed time intervals during the 49-day experimental period. Rapid 15 N enrichments were found in soil and aquatic plants during the first sampling event at day 3 and typically increased linearly during the experimental period. Highest 15 N enrichment was found in submerged plants, followed by emergent plants and soil. No differences in δ 15 N values were found among leaves and stems of all submerged plants ( Potamogeton pusillus , Potamogeton perfoliatus , Najas marina , Ceratophyllum demersum ), suggesting that these plant structures are equally effective in nitrogen assimilation. The roots of P. perfoliatus displayed the lowest enrichment compared to the stems and leaves. Such results indicate that submerged plant roots may have a relatively weak role in nitrogen assimilation. The two emergent plants ( Phragmites australis and Typha orientalis ) showed no differences in δ 15 N values among all plant parts, suggesting rapid translocation of nitrogen from roots to other portions. This experiment demonstrates that wetland soil and plant stems and leaves are major sinks for dissolved nitrogen from the surface water and provides valuable insight into nitrogen retention and transport pathways in various wetland plants.

a b s t r a c t Plant species richness and hydraulic retention time (HRT) are two important variables in mediating pollu- tant removals of constructed wetlands, but their collective effects have not been clear. Pollutant removals in vertical flow mesocosms were investigated using both plant species richness and HRT as fixed factors. Both BOD5 and COD removals were enhanced by the longer HRT and the significant species richness × HRT interaction, as shown by two-way ANOVA. Differently, the removals of the NH4-N, NO3-N and phosphate were improved by the species richness, HRT, and the species richness × HRT interaction. The present findings highlight the importance of the interaction between the plant species richness and the HRT in improving the pollutant removals in vertical flow constructed wetlands.

Concerns over the impacts of water pollution and a need for sustainable development have led to the exploration of various approaches to mitigating the nutrient enrichment in surface waters. An integrated floating island system consisted of aquatic vegetation near riversides and mosaic floating island with adsorptive biofilms was constructed to purify eutrophic river water in Jiaxing City, Zhejiang Province. This study indicated that average removal rates for total nitrogen (TN), NH4+-N, NO3−-N NO2−-N, total phosphorus (TP) and chlorophyll a in summer–autumn season were 36.9%, 44.8%, 25.6%, 53.2%, 43.3% and 64.5%, respectively, which were 16.2%, 18.4%, 12.8%, 25.8%, 26.3% and 58.7% higher than those respective values in winter–spring season. In addition, it also effectively reduced the concentrations of total suspended substance (TSS), Escherichia coli and heavy metals. Due to greater biomass, alligator flag (Thalia dealbata) showed the greatest element uptake, with 60.9 g N m−2, 8.2 g P m−2, 856.6 g C m−2 and 6.2 g S m−2 respectively. The tested hydrophytes contained abundant crude protein ranging from 128 g kg−1 to 255 g kg−1 and Ca, Mg, Fe and Mn. Feasibility of the plant biomass used as animal feed to meet nutritional and safety requirements is discussed.

Eutrophication of shallow freshwater lakes is a severe ecological problem all over the world. Phosphorus is one of the main triggering nutrients responsible for eutrophication of shallow freshwater lakes. Constructed wetlands provide an effective means of phosphorus removal from enriched waters. The cost of a constructed wetland depends mainly on the substrates filled within it. Here we used three inexpensive substrates including loess, cinder and limestone to construct a wetland along the shore of Lake Taihu, and then tested the total phosphorus (TP) removal ability of the wetland, the chemical and physical characteristics, and the abilities of phosphorus adsorption and inhibition of substrates. We found that the artificial wetland constructed with layered loess, cinder, and limestone but without hydrophytes and mesophytes had high phosphorus removal ability during the 44-day test, especially in the first 10 days. The average removal rate for TP was 41% for the overall testing time. Chemical properties of the substrates had stronger impacts on phosphorus removal than physical properties did. Among the three substrates, limestone had the highest phosphorus removal and inhibition ability due to its highest calcium content. This suggests that more attention should be paid to chemical composition during the selection and assembly of substrates for both constructed wetland study and its engineering applications.

The Everglades Nutrient Removal Project (ENRP) was a 1544 ha constructed wetland built by the South Florida Water Management District as part of Everglades restoration efforts. The limnology of this wetland is characterized over its 60-month operational history. The ENRP received agricultural runoff containing high levels of C, N, P and other dissolved constituents; had moderately high alkalinity with a circumneutral pH; and had low to moderate DO. The ENRP provided substantial treatment (concentration reduction from inflow to outflow) for Al, Fe, NH4, NOx, SRP, TP, TSS and turbidity (high-treatment variables), while Secchi depth increased markedly. These changes were judged biologically significant. Dissolved oxygen, and water temperature had well defined annual cycles, while some level of seasonality was noted for Al, alkalinity, Ca, conductivity, DOC, Fe, hardness, K, Mg, Mn, Na, NH4, pH, Secchi depth, SiO2, TOC, TN, turbidity, and TSS. The ENRP was P limited based on TN:TP molar ratios. Dissolved ions were dominated by Ca, Cl, Na, and HCO3; the stoichiometric balance of both major and minor ions was similar throughout the wetland. The downstream settling of TSS was associated with increased light penetration, but did not appear important in sediment accretion. The adsorption of P to Ca, and perhaps Al and Fe, precipitates is thought to have been an important nutrient removal mechanism. Although there was little net reduction in DOC, we speculate that some incoming material was degraded and replaced by new DOC produced within the wetland.

A tracer study (Rhodamine-WT dye) was performed on a 147-ha submerged aquatic vegetation (SAV)-dominated free-water surface treatment wetland in south Florida that received agricultural drainage waters (ADW). Two dimensional, time series plots of the dye concentrations revealed that a disproportionate amount of tracer flowed along the eastern and western levees of the cell. The tracer response curve developed from the outflow data indicated a prominent short circuit, which conveyed 44% of the volumetric flow based on model analysis. This large fraction of flow bypassed the SAV community and exited the wetland with only partial treatment for total phosphorus (TP) removal. The volumetric efficiency of the wetland was high (∼100%), but the hydraulic efficiency was low due to the low tanks-in-series number (N = 1.3). Even though the short-circuit channels promoted near-plug flow, the dispersion number for the wetland was nevertheless still high ( D = 1.25), due to the wide distribution of detention times among the multiple flow paths. A dynamic hydraulic model based on a tanks-in-series network with mixing between the short-circuited and shallower, vegetated zones simulated the tracer response curve to an accuracy of 0.99 (as the coefficient of determination). The amount of time that a parcel of water resided within the wetland affected TP removal. Hydraulic retention times (HRTs) > 2.5 days consistently produced TP concentrations less than 35 μg/L, or 64% TP removal based on the inflow concentration of 96 μg/L. “Steady-state” TP concentrations among the internal and outfall stations during the tracer study enabled direct analysis of the impact of short-circuits on P concentrations and removal effectiveness. Based on longitudinal P concentration gradients, faster flowing short-circuit channels that contain little SAV were less effective in reducing TP concentrations (k = 0.24 day−1) than the more densely SAV-occupied areas (k = 0.50 day−1) that characterize 56% of the wetland. This was likely due to both the shorter detention time and the sparse SAV community within the channels. The lack of SAV contributed to an environment that reduced the sedimentation rate, direct plant uptake, and the pH, all of which contribute to removal and retention of P in the more quiescent, shallower SAV zones. Our results suggest that tracer studies, coupled with internal P concentration gradient analyses, are useful for determining hydraulic efficiency of a wetland and its effect on P removal. These types of investigations can also serve as the basis for modeling, optimization, and design studies.

The performance of a mesocosm-scale experimental wetland system (9-cm deep raceways with a limerock substrate followed by a limerock filter) that received effluent from a treatment wetland in south Florida was monitored for 19 months. The raceways were operated at a constant hydraulic load of 11 cm per day, resulting in a nominal hydraulic retention time (HRT) of 0.8 days. The vegetation community that developed in these systems varied along the longitudinal phosphorus (P) gradient and became dominated by eutrophic periphyton at the inflow (Cladophora and Microspora) and oligotrophic taxa along the raceway's mid-regions and outflow (Schizothrix and Scytonema). The inflow total P (TP) concentration (mean = 18 μg P L−1) was reduced to a mean of 10 μg P L−1 at the raceway outflow and 8 μg P L−1 exiting the limerock filters. Total P removal for the entire system was 0.32 g m−2 per year, or 46% of the inflow TP load, and uptake rates (k values) were 24 and 32 m per year for the raceway and the raceway/limerock filter system, respectively. Most TP removal occurred within the first 3 m of the raceways at an HRT of 1.3 h. Both soluble reactive and particulate P were reduced to their analytical detection limits, while dissolved organic P was removed less efficiently. There was little evidence for a diel trend in P removal. Following a 1-week dry-out, the raceways exported P for only 24 h after rehydration, after which treatment performance quickly returned to pre-desiccation levels. Nutrient ratios suggest that the raceways were never N-limited and that P limitation increased with distance from the inflow ports. Accrued sediment had a high percentage of non-labile P (>70%). A number of operational and construction issues need to be resolved before this technology could be scaled up for use in conjunction with Everglades restoration.

Anthropogenic phosphorus (P) loads have been implicated in eutrophication of lakes and wetlands throughout Florida. One technology that holds considerable promise for controlling these loads in a cost-effective manner is the use of treatment wetlands. Preliminary research in south Florida on the use of submerged aquatic vegetation (SAV) as the dominant vegetation in these treatment wetlands is reporting higher P removal performance than wetlands dominated by rooted, emergent plants. This research has been based to-date primarily on relatively small-scale mesocosms (5–2000 m2) and on a larger scale treatment wetland (148 ha) that has been operated for about 7 years. Considering the magnitude of engineering decisions and project costs to implement P control in the Everglades Agricultural Area and elsewhere in Florida, it is prudent to look for additional confirmation of P removal performance from other existing SAV-dominated systems in Florida that have a longer operational period. This paper describes an analysis of existing data collected from a number of SAV-dominated, flow-through lakes and rivers in Florida with characteristics similar to the proposed SAV treatment systems. While these existing input–output data were not specifically collected for the purpose of preparing mass balances and P removal rate estimates, they can be judiciously applied to that analysis. The overall conclusion of this analysis is that SAV-dominated lakes and rivers do typically remove P from the water column. The likely long-term sink for this P is the newly accreted sediment. The long-term average P removal rate for 13 SAV-dominated lake and river systems in Florida was 1.2 g/m2 per year. This result compares favorably with an average net sediment P accumulation rate of 1.2 g/m2 per year reported by others for 11 SAV-dominated Florida lakes. These estimated long-term P removal rates are higher than those for full-scale wetlands dominated by emergent vegetation (Treatment Wetlands (1996); Wetlands Ecol. Mgmt. 4 (1997) 159). Average first-order P removal rate constants for SAV-dominated lakes (15 m/year) and rivers (46 m/year) are generally less than those estimated in SAV-dominated mesocosms (60–140 m/year) and similar to a large-scale SAV-dominated stormwater treatment area (STA) (40 m/year). P removals in all of these SAV-dominated systems are influenced by inlet P loading rates, with removal rates positively correlated to both P inlet concentration and hydraulic loading rate (HLR). Based on this analysis, caution is recommended when extrapolating the P removal results from relatively short-term or small-scale mesocosm studies to the design of full-scale, long-term operating SAV-dominated wetlands.