Macrophyte Treatments Research Articles

Effects of two submerged macrophyte species on microbes and metazoans in rooftop water-storage ponds with different labile carbon loadings

Nature-based solutions including rooftop-water storage ponds are increasingly adopted in cities as new eco-designs to address climate change issues, such as water scarcity and storm-water runoff. Macrophytes may be valuable additions for treating stored rooftop waters and provisioning other services, including aquaponics, esthetic and wildlife-conservation values. However, the efficacy of macrophyte treatments has not been tested with influxes of different labile carbon loadings such as those occurring in storms. Moreover, little is known about how macrophytes affect communities of metazoans and microbes, including protozoans, which are key players in the water-treatment process. Here, we experimentally investigated the effectiveness of two widely distributed macrophytes, Ceratophyllum demersum and Egeria densa, for treating drained rooftop water fed with two types of leaf litter, namely Quercus robur (high C lability) and Quercus rubra (low C lability). C. demersum was better than E. densa at reducing water conductivity (by 10 ̶ 40 µS/cm), TDS (by 10–18 mg/L), DOC (by 4–5 mg/L) and at increasing water transparency (by 4–9%), water O2 levels (by 19–27%) and daylight pH (by 0.9–1.3) compared to leaf-litter only microcosms after 30 days. Each treatment developed a different community of algae, protozoa and metazoa. Greater plant mass and epiphytic chlorophyll-a suggested that C. demersum was better at providing supporting habitat than E. densa. The two macrophytes did not differ in detritus accumulation, but E. densa was more prone to develop filamentous bacteria, which cause sludge bulking in water-treatment systems. Our study highlights the superior capacity of C. demersum and the usefulness of whole-ecosystem experiments in choosing the most adequate macrophyte species for nature-based engineered solutions.

The shift from macrophytic to algal particulate organic matter favours dissimilatory nitrate reduction to ammonium over denitrification in a eutrophic lake

Abstract In eutrophic lakes, the shift from a macrophyte‐dominated state to an algae‐dominated state changes the particulate organic matter (POM) sources, which in turn alters organic matter quality that is released and that sinks to the benthos. However, the influences of this shift on denitrification and dissimilatory nitrate reduction to ammonium (DNRA) are unclear in eutrophic lakes. Here, we elucidated how various POM sources influenced these nitrate reduction pathways in a eutrophic lake. The lake sediments from August were incubated under algae, macrophyte, and soil treatments in aquarium tanks. Potential denitrification and DNRA rates and related functional gene abundances were measured periodically. Meanwhile, the n‐alkanes composition of POM and fluorescence excitation‐emission matrices of dissolved organic matter were measured before and during incubations, respectively, to aid in the analysis of the potential mechanisms by which organic matter quality affect nitrate reduction. The results indicated that algal detritus was more labile and decomposed faster than other POM, coupled with higher DNRA rates and nrfA gene abundances. Macrophyte treatments resulted in nitrate accumulation and the increase of gene abundances related to denitrification, but no increase in denitrification rates was observed. In soil treatments, nitrate reduction processes were not significantly influenced by the addition of POM. Overall, compared with other sources, algae‐derived POM can produce high‐quality organic carbon, low dissolved oxygen and nitrate concentrations, which is more conducive to DNRA than denitrification. A loss of macrophytes and increase in algal biomass will induce changes in autochthonous POM quality and potentially drive nitrogen recycling in eutrophic lakes.

The combined effects of macrophytes and three road salts on aquatic communities in outdoor mesocosms

Because of environmental and societal concerns, new strategies are being developed to mitigate the effects of road salt. These include new deicers that are alternatives to or mixtures with the most common road salt, sodium chloride (NaCl), improved techniques and equipment, and biotic mitigation methods. Using outdoor mesocosms, we investigated the impacts of NaCl and two common alternatives, magnesium chloride (MgCl2) and calcium chloride (CaCl2) on freshwater communities. We also investigated the mitigation ability of a common macrophyte, Elodea. We hypothesized that road salt exposure reduces filamentous algae, zooplankton, and macrocrustaceans, but results in increases in phytoplankton and gastropods. We also hypothesized that MgCl2 is the most toxic salt to communities, followed by CaCl2, and then NaCl. Lastly, we hypothesized that macrophytes mitigate some of the effects of road salt, specifically the effects on primary producers. We found that all three salts reduced filamentous algal biomass and amphipod abundance, but only MgCl2 reduced Elodea biomass. MgCl2 had the largest and longest lasting effects on zooplankton, specifically cladocerans and copepods, which resulted in a significant increase in phytoplankton and rotifers. CaCl2 increased ostracods and decreased snail abundance, but NaCl increased snail abundance. Lastly, while we did not find many interactions between road salt and macrophyte treatments, macrophytes did counteract many of the salt effects on producers, leading to decreased phytoplankton, increased filamentous algae, and altered abiotic responses. Thus, at similar chloride concentrations, NaCl alternatives, specifically MgCl2, are not safer for aquatic ecosystems and more research is needed to find safer road management strategies to protect freshwater ecosystems.

Effects of the manipulation of submerged macrophytes, large zooplankton, and nutrients on a cyanobacterial bloom: A mesocosm study in a tropical shallow reservoir

Biomanipulation is an efficient tool to control eutrophication and cyanobacterial blooms in temperate lakes. However, the effects of this technique are still unclear for tropical ecosystems. Herein, we evaluated the effects of the biomanipulation on cyanobacterial biomass in a tropical shallow reservoir in Northeast Brazil. A mesocosm experiment was conducted in Tapacurá reservoir (Pernambuco) with eight treatments, in which we factorially manipulated the presence of submerged macrophytes (Ceratophyllum demersum), large herbivorous zooplankton (Sarsilatona serricauda), and nutrients (0.4 mg L−1 of nitrogen and 0.5 mg L−1 of phosphorus). On the first, fifth, and tenth days, we analyzed the total biomass of cyanobacteria, and the morphotypes coccoid, heterocyted filamentous, and non-heterocyted filamentous cyanobacteria; these components were compared through a three-way ANOVA. The bloom was composed mainly of five Microcystis morphospecies (coccoids) and Raphidiopsis raciborskii (heterocyted filaments). On the fifth day of the experiment, the combined addition of macrophytes and zooplankton was more efficient at controlling cyanobacterial biomass. On the tenth day, all macrophyte treatments showed significant cyanobacterial biomass reduction, decreasing up to 84.8%. On the other hand, nutrients and zooplankton, both isolated and combined, had no significant effect. Macrophytes also reduced the biomass of coccoids, heterocyted filaments, and non-heterocyted filaments when analyzed separately on the tenth day. Ceratophyllum demersum was more efficient at controlling the bloom than the addition of large herbivorous zooplankton, which could be related to allelopathy since cyanobacterial biomass was also reduced when nutrients were added. The addition of submerged macrophytes with allelopathic potential, associated with the increase of large herbivorous zooplankton, proved to be an efficient technique for controlling tropical cyanobacterial blooms.

Macrophytes moderate the taxonomic and functional composition of phytoplankton assemblages during a nutrient loading experiment

Abstract Ponds and shallow lakes are sensitive to eutrophication, which often increases turbidity and produces toxic algal blooms. Previous research has found that the presence of macrophytes in shallow water bodies can mitigate the effects of nutrient loading by promoting water clarity and inhibiting phytoplankton growth. However, there is still little known about how macrophyte abundance modifies the response of phytoplankton taxonomic assemblages, particularly their functional composition, to nutrient loading. We investigated how macrophyte abundance altered phytoplankton assemblages using experimental ponds that were treated with a nutrient press. Natural communities of phytoplankton, zooplankton, and fish were seeded in ponds with one of three levels of macrophyte abundance that received weekly nutrient additions. A total of nine ponds (each treatment in triplicate) were monitored for 76 days. Analyses of Shannon's diversity and functional dispersion revealed significant variation in phytoplankton assemblage composition among macrophyte treatments and through time. Phytoplankton biovolume, community metrics, and the dominant traits were generally more stable in the high macrophyte treatment ponds. Although nutrient loading did not induce a regime change to a turbid, phytoplankton‐dominated state, phytoplankton assemblages did shift to a dominance of larger, inedible taxa and metaphyton became more abundant in ponds with macrophytes and zooplankton assemblages shifted to smaller species. Our study demonstrates how functional traits and functional diversity metrics can provide richer insights into the dynamics and causes of shifts in shallow freshwater ecosystems. Macrophyte densities clearly played a significant role in shaping phytoplankton assemblages. Based on the functional traits of phytoplankton distinguished among treatments, we inferred that competition for light, grazing, and, probably, allelochemicals were mechanisms through which macrophytes mediated changes in phytoplankton assemblages.

Shifts in algal dominance in freshwater experimental ponds across differing levels of macrophytes and nutrients

AbstractExcess nutrient loading into ponds and shallow lakes can lead to undesirable algal growth and a shift to a turbid state. Previous work has suggested that such an ecosystem transition may be mediated by the biotic constituents of the habitat and food web; however, earlier experiments have been conducted at coarse temporal resolution and have typically used a single initial density of macrophytes, a key structural component of ponds and shallow lakes. To address these gaps, we tested the hypotheses that experimental ponds with lower macrophyte densities and more rapid increases in nutrient loading would shift to phytoplankton dominance, whereas higher macrophyte densities and slower, lower concentration nutrient inputs would maintain a clear state. Ponds containing plankton and juvenile fish were assigned to treatments with none, low, or high macrophyte densities, and weekly, high or biweekly (i.e., fortnightly), low nutrient inputs. Using additive mixed‐effects models, we demonstrated that temporal trajectories of phytoplankton biomass were explained by macrophyte density in interaction with biomass of important zooplankton grazers (Bosminidae, Sididae, and Daphniidae), as well as with pH and time. Phytoplankton biomass followed a convex unimodal trajectory in ponds with no or low macrophytes (muted in the latter), and minimal increases in high macrophyte treatments. Declines in phytoplankton attributable to top‐down control likely freed resources for periphyton and metaphyton, which subsequently became abundant in ponds without and with macrophytes, respectively. Our results demonstrate that high densities of macrophytes, combined with herbivory and competition for light between phytoplankton and metaphyton, enhance resilience of the clear water state to the undesirable effects associated with eutrophication.

Enhanced phosphorus reduction in simulated eutrophic water: a comparative study of submerged macrophytes, sediment microbial fuel cells, and their combination

ABSTRACTThe phosphorus reduction in water column was attempted by integrating sediment microbial fuel cells (SMFCs) with the submerged macrophyte Vallisneria spiralis. A comparative study was conducted to treat simulated water rich in phosphate with a control and three treatments: SMFC alone (SMFC), submerged macrophytes alone (macophyte), and combined macrophytes and fuel cells (M-SMFC). All treatments promoted phosphorus flux from the water column to sediments. Maximum phosphorus reduction was obtained in proportion to the highest stable phosphorus level in sediments in M-SMFC. For the initial phosphate concentrations of 0.2, 1, 2, and 4 mg/L, average phosphate values in the overlying water during four phases decreased by 33.3% (25.0%, 8.3%), 30.8% (5.1%, 17.9%), 36.5% (27.8%, 15.7%), and 36.2% (0.7%, 22.1%) for M-SMFC (macrophyte, SMFC), compared with the control. With macrophyte treatment, the obvious phosphorus release from sediments was observed during the declining period. However, such phenomenon was significantly inhibited with M-SMFC. The electrogenesis bacteria achieved stronger phosphorus adsorption and assimilation was significantly enriched on the closed-circuit anodes. The higher abundance of Geobacter and Pseudomonas in M-SMFC might in part explain the highest phosphorus reduction in the water column. M-SMFC treatment could be promising to control the phosphorus in eutrophic water bodies.

Combined ozonation and aquatic macrophyte (Vallisneria natans) treatment of piggery effluent: Water matrix and antioxidant responses

The efficiency of the combined process of ozonation with Vallisneria natans to improve the quality of piggery effluent was investigated in this study, and the effects of ozonated water on the antioxidant responses of V. natans were explored. The effluent was treated with ozone at 10, 30 and 50mgL−1 for 30min in combination with V. natans for 28−d. Combined treatment was successful at removing inorganic nutrients and organic compounds from piggery effluent. Low ozone concentration treatment of water was optimal for the growth of V. natans, while exposure to high levels for long periods of time increased reactive oxygen species (ROS) and damaged their antioxidant systems. The results demonstrate that combined ozonation and submerged macrophytes treatment, can remove inorganic nutrients and organic compounds from piggery effluent, reducing the pollution into aquatic ecosystem.

Complex Interactions Between the Macrophyte Acorus Calamus and Microbial Fuel Cells During Pyrene and Benzo[a]Pyrene Degradation in Sediments.

This study investigated the interaction of the macrophyte Acorus calamus and sediment microbial fuel cells (SMFC) during the degradation of high molecular weight-polycyclic aromatic hydrocarbons (HMW-PAHs) in sediments. Over 367-days, the combination of macrophyte and SMFC led to an increase in pyrene and benzo[a]pyrene degradation rates by at least 70% compared to SMFC or macrophyte alone. While either the macrophyte or SMFC increased redox potential in sediments, redox potentials near the anode (approximately 6 cm depth) in the macrophyte-SMFC combination were markedly lower than that in the only macrophyte treatment. Moreover, rhizospheric bacterial communities in macrophyte-SMFC and macrophyte treatments were distinctly different. Aerobic genera (Vogesella, Pseudomonas, Flavobacterium and Rhizobium) and anaerobic genera (Longilinea, Bellilinea, Desulfobacca and Anaeromyxobacter) became dominant in the rhizosphere in macrophyte and macrophyte-SMFC treatments, respectively. In addition, the macrophyte-SMFC combination improved the numbers of not only aerobic but anaerobic PAHs degraders in sediments. So, the SMFC employment facilitated the formation of anoxic zones in sediments with oxygen loss and exudates from the roots. As a result, cooperation of anaerobic/aerobic microbial metabolism for accelerating HMW-PAHs removal occurred within sediments after combining macrophytes with SMFC.

Submerged macrophytes mitigate direct and indirect insecticide effects in freshwater communities.

Understanding how ecological interactions mitigate the impacts of perturbations such as pesticides in biological communities is an important basic and applied question for ecologists. In aquatic ecosystems, new evidence from microcosm experiments suggests that submerged macrophytes can buffer cladocerans from pulse exposures to the widely used insecticide malathion, and that mitigation increases with macrophyte density. However, whether these results scale up to more complex aquatic communities where ecological interactions such as competition can alter toxicity is unknown. Further, macrophyte abilities to mitigate different insecticide exposure scenarios (i.e. single versus repeated pulses) have never been tested. To address these gaps, we performed a factorial mesocosm experiment examining the influence of four macrophyte treatments (0, 10, 50, or 100 Elodea Canadensis shoots planted per mesocosm) crossed with three malathion exposure scenarios (no insecticide, single pulse, repeated pulses) on aquatic communities containing zooplankton, phytoplankton, periphyton, two snail species, and larval amphibians. In the absence of macrophytes, single malathion pulses caused short-term declines in cladoceran abundance followed by their rapid recovery, which precluded any indirect effects (i.e. trophic cascades). However, repeated malathion pulses caused cladoceran extinctions, resulting in persistent phytoplankton blooms and reduced abundance of one snail species. In contrast, with macrophytes present, even at low density, malathion had no effect on any taxa. We also discovered novel effects of macrophytes on the benthic food web. In the two highest macrophyte treatments, we observed trends of reduced periphyton biomass, decreased abundance of one snail species, and decreased amphibian time to and mass at metamorphosis. To our knowledge, this is the first evidence of negative submerged macrophyte effects on amphibians, a taxa of global conservation concern. Our findings suggest that facilitating macrophytes could be an important strategy for buffering freshwater communities from insecticides, though consideration of their impacts on animal species is necessary.

Constructed wetlands for treatment of harvest effluents from grow-out ponds of the Amazon river prawn

Effluent discharges from aquaculture can reduce water quality in receiving water bodies and that strategies or practices to reduce this are necessary. One possibility is to reduce, or eliminate, water renewal in grow-out ponds. In this study, we eliminated water renewal in grow-out ponds associated with the culture of 40 individuals m−2 of Amazon river prawn (Macrobrachium amazonicum). At the end of the culture period it was, however, necessary to drain the pond to harvest the prawns. An experiment was performed in triplicate, in which the water supply characteristics and harvest water characteristics of ponds were evaluated. To reduce these concentrations of total N and P, an aquatic macrophyte (Eichhornia crassipes, water hyacinth) treatment system (CWs) was adopted. The water characteristics in the CWs were evaluated after 1, 3, 7, 14 and 21 days. The water supply of ponds presented the average concentrations of 0.67 ± 0.32 mg L−1 and 17.4 ± 14.7 μg L−1 of total-N and total-P respectively. The harvest effluent of ponds had elevated concentrations of different forms of nitrogen (4.44 mg L−1 of total-N) and phosphorous (100.9 μg L−1 of total-P). After 1 day of the experiment we found the following reductions in key nutrients in treatment system containing E. crassipes: 90%, 78% and 45% reductions in the concentrations of particulate matter, orthophosphates and nitrates respectively. We noted that after 3 days the nitrates had been reduced by 53%. We concluded that 3 days of this treatment was sufficient for the removal of the additional nutrients that had accumulated in the Amazon river prawn ponds.

Cytotoxic and genotoxic evaluation and chemical characterization of sewage treated using activated sludge and a floating emergent-macrophyte filter in a municipal wastewater treatment plant: a case study in Southern Brazil

This study was conducted to evaluate the chemical parameters and the cytotoxic and genotoxic potential of raw domestic sewage and effluents from treatment with activated sludge and a floating emergent-macrophyte filter from a domestic wastewater treatment plant in the city of Novo Hamburgo, Rio Grande do Sul, Brazil. The physicochemical analysis revealed that both treatment systems achieved the legal emission pattern for biochemical oxygen demand, chemical oxygen demand, and suspended solids, but ammoniacal nitrogen and E. coli values were above the limits in the macrophyte treatment effluent. Phosphorous values were above the maximum permitted for both treatments. The results obtained from the Allium cepa test and the micronuclei test in fish did not demonstrate any significant differences in both cytotoxicity (mitotic index) and genotoxicity (chromosome aberration and micronucleus) endpoints between the negative control group and the exposed groups. However, the comet assay in fish revealed a DNA damage increase in animals exposed to the 30 % concentration of the macrophyte effluent and two concentrations of the activated sludge treatment effluent (10 and 75 %), which suggests that these two treatment systems may increase wastewater genotoxicity.

Mitigation of malathion's acute toxicity by four submersed macrophyte species

Some submersed macrophyte species rapidly sorb some insecticides from the water, potentially reducing exposure for aquatic species. The rates at which macrophytes remove insecticides, however, can differ widely among plant species. Furthermore, few studies have examined how much macrophytes actually influence insecticide toxicity to sensitive animals. The authors quantified the ability of several macrophyte species to mitigate insecticide toxicity by comparing the survival of the aquatic herbivore, Daphnia magna, following exposure to a factorial combination of 3 malathion concentrations (0 µg/L, 3 µg/L, and 24 µg/L) and 7 macrophyte treatments (no macrophytes, 4 different macrophyte monocultures, and 2 inert substrates: plastic plants and polypropylene rope). The authors also quantified the rate that different macrophytes reduced malathion's toxicity by exposing D. magna to water samples collected from each treatment after 2 h, 8 h, and 48 h of exposure. The results revealed that whereas 3 µg/L and 24 µg/L of malathion decimated D. magna in the no-macrophyte, plastic plant, and rope treatments, all 4 macrophyte species strongly mitigated these effects. When the authors compared the rate at which malathion's toxicity decreased, they found that all macrophytes negated malathion's toxicity within 2 h, whereas it took more than 8 h in the absence of macrophytes or in the presence of inert substrates. These results demonstrate that numerous macrophyte species can equally and strongly mitigate insecticide toxicity, whereas inert substrates cannot.

Constructed Wetlands With Floating Emergent Macrophytes: An Innovative Stormwater Treatment Technology

The treatment of urban stormwater poses numerous technical and operational challenges, particularly due to the intermittent and highly variable nature of hydrologic and pollutant inputs. Floating emergent macrophyte treatment wetlands (FTWs) are a hybridization of ponds and wetlands that offer potential advantages for treatment of these highly variable flows. FTWs utilize rooted, emergent macrophytes growing on a mat or raft floating on the surface of the water rather than rooted in the sediments. Thus, they can tolerate the widely fluctuating water depths typical of stormwater systems, without the risk of the plants drowning. The roots hang beneath the floating mat and provide a large surface area for biofilm attachment. The authors provide a review of the FTW concept, structure, function, and treatment efficiency reported to date and discuss the potential advantages of this emerging technology for stormwater applications. Although still limited, the available data from mesocosm and pilot studies on removal of key pollutants such as organic matter, suspended solids, nutrients, and metals shows that they can significantly enhance performance of pond systems, and provide similar or better performance than surface flow wetlands for a range of polluted waters. Further studies are needed to verify the apparent potential of FTWs treating stormwater at full scale.

Biological control of phytoplankton by the subtropical submerged macrophytes Egeria densa and Potamogeton illinoensis: a mesocosm study

1. In temperate regions, submerged macrophytes can hamper phytoplankton blooms. Such an effect could arise directly, for instance via allelopathy, or indirectly, via competition for nutrients or the positive interaction between submerged macrophytes and zooplankton grazing. However, there is some evidence that the positive interaction between submerged macrophytes and zooplankton grazing is less marked in warmer regions, where the interaction is less well studied, and that negative effects of higher water plants on phytoplankton biomass are weaker. 2. We carried out two consecutive mesocosm experiments in Uruguay (subtropical South America) to study the effects of two common submerged macrophytes from this region (Egeria densa and Potamogeton illinoensis) on phytoplankton biomass, in the absence of zooplankton grazing. We compared phytoplankton development between different macrophyte treatments (no macrophytes, artificial macrophytes, real Egeria and real Potamogeton). We used artificial macrophytes to differentiate between physical effects (i.e. shading, sedimentation and competition with periphyton) and biological effects (i.e. nutrient competition and allelopathy). 3. In Experiment 1, we found no evidence for physical effects of macrophytes on phytoplankton biomass, but both macrophyte species seemed to exert strong biological effects on phytoplankton biomass. Only Egeria affected phytoplankton community structure, particularly tempering the dominance of Scenedesmus. Nutrient addition assays revealed that only Egeria suppressed phytoplankton through nutrient competition. 4. We performed a second mesocosm experiment with the same design, but applying saturating nutrient conditions as a way of excluding the effects of competition for nutrients. This experiment showed that both macrophytes were still able to suppress phytoplankton through biological mechanisms, providing evidence for allelopathic effects. Our results indicate that both common macrophytes are able to keep phytoplankton biomass low, even in the absence of zooplankton grazing.

The influence of plant-associated filter feeders on phytoplankton biomass: a mesocosm study

Low phytoplankton biomass usually occurs in the presence of submerged macrophytes, possibly because submerged macrophytes enhance top-down control of phytoplankton by offering a refuge for efficient grazers like Daphnia against fish predation. However, other field studies also suggest that submerged macrophytes suppress phytoplankton in the absence of Daphnia. In order to investigate these mechanisms further, we conducted an outdoor mesocosm experiment to study the effect of submerged macrophytes (Elodea nuttallii) on phytoplankton and zooplankton biomass. The experiment combined four nutrient addition levels (0, 10, 100, and 1000 μg P l−1; N/P ratio: 16) with three macrophyte levels (no macrophytes, artificial macrophytes, and real macrophytes). We inoculated the tanks with species-rich inocula of phytoplankton and zooplankton but excluded fish or macro-invertebrates. Probably due to the lack of predators in the mesocosms, potential grazing rates of pelagic zooplankton (estimated from zooplankton biomass) did not differ between the macrophyte treatment combinations. Compared to the treatment combinations without macrophytes, lower phytoplankton biomass occurred in the treatment combinations with real macrophytes at all the nutrient addition levels and in those with artificial macrophytes at all the nutrient levels except the highest. Significantly, higher abundances of plant-associated filter feeders (Simocephalus vetulus and Ceriodaphnia spp.) occurred in the treatment combinations with real and artificial macrophytes. The estimated potential grazing rate of these plant-associated filter feeders indicated that these filter feeders could be responsible for the lower phytoplankton biomass in the presence of real and artificial macrophytes. Our results suggest that the plant-associated filter feeders may be significant grazers in vegetated shallow lakes.

Survey of plant species present on Constructed Wetlands in Opalenica Municipality

Survey of plant species present on Constructed Wetlands in Opalenica Municipality The aim of this study was to analyse over fifty constructed wetlands established in two villages in Opalenica Municipality (Wielkopolska, Poland) differing in building time, prevailing hydrological and microclimatic conditions, appearance of macrophytes species and chosen plantation method. Two subsystems of constructed wetlands that are complementing one another, based on emergent macrophyte treatment system with vertical subsurface flow in plant filter and open water in denitrification ponds give the opportunity to the full-scale wastewater treatment performance. The main conclusions that can be drawn from this study regard the considerable role of the presence of macrophytes that significantly improve microbial activity, insulate against frost during winter, prevent from clogging and odour occurrence and improves value of the landscape. The conducted analyses proved that the establishment of this kind of semi-natural wastewater treatment system is becoming widely implemented into environmental protection schemes for rural communities.

Allelopathic effects of the submerged macrophyte Potamogeton malaianus on Scenedesmus obliquus

Allelopathic effects of the submerged macrophyte Potamogeton malaianus on Scenedesmus obliquus were assessed using a twophase approach under controlled laboratory conditions. In the co- culture experiment ( phase I), the growth of S. obliquus at two different initial cell densities was significantly inhibited by P. malaianus. Moreover, the growth inhibition was dependent on the biomass density of P. malaianus. Antioxidant enzymes ( SOD, CAT and POD), MDA, APA, total soluble protein, protein electrophoretic pattern and morphology of S. obliquus were determined after the coculture experiment was terminated. The activities of SOD, CAT, POD and APA at the low initial cell density were stimulated, the contents of MDA and total soluble protein were increased, and some special protein bands disappeared in P. malaianus treatments. The macrophyte had no effect on the activities of SOD and APA at the high initial cell density, but significantly influenced other physiological parameters of S. obliquus with the increase of biomass density. The morphology of S. obliquus showed no difference in the macrophyte treatments and the controls, and the cultures were dominated by 4- celled coenobia. The results indicated P. malaianus had significant allelopathic effects on the growth and physiological processes of S. obliquus. Moreover, the allelopathic effects depended on initial algal cell density, biomass density of the macrophyte, and their interaction. In the experiment of P. malaianus culture filtrates ( phase II), filtrates from combined culture of plant and S. obliquus at the low initial cell density exhibited no apparent growth inhibitory effect on S. obliquus. The result showed that initial addition of growth- inhibiting plant filtrates had no allelopathic effect on S. obliquus. We concluded that the allelopathic effects on S. obliquus were found in the presence of P. malaianus, but not in P. malaianus filtrates. However, the absence of allelopathic effect on S. obliquus might be due to the very low concentrations of allelochemicals in the filtrates.

Importance of Nutrient Competition and Allelopathic Effects in Suppression of the Green Alga Scenedesmus obliquus by the Macrophytes Chara, Elodea and Myriophyllum

Possible allelopathic effects of substances released from the macrophytes Chara globularis, Elodea canadensis, Myriophyllum spicatum on the common green alga Scenedesmus obliquus were tested in the laboratory with plastic plants and untreated medium as controls. A two-phase approach was used in which first the effects of physical presence of plants was studied (phase I) followed by the effects of plant culture filtrates (phase II). In the presence of plastic plants growth was reduced only marginally, but strong growth inhibition of Scenedesmus occurred in the physical presence of all macrophytes. In contrast, filtrates from Chara had no growth inhibitory effect on Scenedesmus. Myriophyllum filtrate reduced particle-based growth rate by 7% compared to filtration controls, while Elodea culture filtrate reduced volume-based growth by 12%, chlorophyll-based growth by 28% and particle-based growth by 15%. Photosystem II-efficiency of Scenedesmus was reduced in all three macrophyte treatments in phase I, but not in filtrates from macrophyte cultures (phase II). Thus, while enzyme activity or other physiological aspects may have been affected, the current study yielded no proof for allelopathically active compounds being directed at photosynthesis. Mean particle volume (MPV) of Scenedesmus was not influenced by macrophyte exudates and cultures remained dominated by unicells. The strong growth inhibitory effects found for Scenedesmus in the physical presence of macrophytes, but not in plastic controls, and no or weaker response in nutrient-enriched filtrates, suggest nutrient competition was a more powerful driving factor than allelochemicals. However, the experimental design does not exclude disappearance of allelochemicals during the filtration process.

Differential response of stream periphyton and invertebrate grazers to habitat modification by the emergent macrophyte Justicia americana

An instream experiment was conducted using artificial substrate tiles during summer to examine the relative importance of shading by the emergent macrophyte Justicia americana and top-down control by stream grazers on summer periphyton accrual. Macrophyte treatments included removal of above-ground stems, removal of stems and rhizomes and an unmanipulated control, whereas grazer treatments included snail-accessible and snail-exclusion tiles. Above-ground Justicia structures reduced both sunlight to the stream bed and velocity by almost 50%. Abundance of the dominant snail, Elimia ucheensis (Pleuroceridae), on snail-exclusion tiles was significantly lower than on snail-accessible tiles only during the first week of the experiment; therefore, the barriers were ineffective over the entire experiment. Despite ineffective reductions of Elimia abundance over the entire experiment, periphyton accrual was higher on snail-exclusion than snail-accessible tiles irrespective of macrophyte treatment. Among the macrophyte treatments, periphyton biomass (as ash-free dry mass) was significantly lower on tiles in the unmanipulated control than treatments where Justicia stems were removed. Higher mean differences and relative magnitude of effects were associated with the macrophyte treatments when compared with the grazer treatments. Periphyton appeared to be primarily light-limited by Justicia and secondarily controlled by grazing invertebrates; however, the effects of grazing may have been underestimated because snail-exclusion barriers were ineffective at reducing grazer abundance over the experiment. Removal of Justicia canopy did not result in higher abundance of any invertebrate grazers on tiles; however, the abundance of the freshwater limpet (Ferrissia mcneili) was twice as high on tiles within the more shaded Justicia control sections compared with either of the two open-canopy treatments. Habitat modification by Justicia can negatively affect benthic primary producers and influence the distribution of some, but not all, primary consumers.