Artificial Macrophytes Research Articles

Purification of aquaculture wastewater by macrophytes and biofilm systems: Efficient removal of trace antibiotics and enrichment of antibiotic resistance genes

The purification performance of aquaculture wastewater and the risk of antibiotic resistance genes (ARGs) dissemination in wetlands dominated by macrophytes remain unclear. Here, the purification effects of different macrophytes and biofilm systems on real aquaculture wastewater were investigated, as well as the distribution and abundance of ARGs. Compared to the submerged macrophytes, artificial macrophytes exhibited higher removal rates of TOC (58.80 ± 5.04 %), TN (74.50 ± 2.50 %), and TP (77.33 ± 11.66 %), and achieved approximately 79.92 % removal of accumulated trace antibiotics in the surrounding water. Additionally, the biofilm microbial communities on the surface of artificial macrophytes exhibited higher microbial diversity with fewer antibiotic-resistant bacteria (ARB) enrichment from the surrounding water. The absolute abundance of ARGs (sul1, sul2, and intI1) in the mature biofilm to be one to two orders of magnitude higher than that in the water. Although biofilms could decrease ARGs in the surrounding water by enriching ARB, the intricate network structure of biofilms further facilitated the proliferation of ARB and the dissemination of ARGs in water. Network analysis suggested that Proteobacteria and Firmicutes phyla were dominant and potential carriers of ARGs, contributing 69.00 % and 16.70 %, respectively. Our findings highlight that macrophytes and biofilm systems have great performance on aquaculture wastewater purification, but with high risk of ARGs.

Increasing chemical oxygen demand and nitrogen removal efficiencies of surface-flow constructed wetlands in macrophyte-dominant seasons by adding artificial macrophytes

The objective was to test the effectiveness of a new strategy, i.e., adding artificial macrophytes (AMs) to surface-flow constructed wetlands (SFCWs) planted with emergent macrophytes (EMs), to improve COD and N removal efficiencies (REs) at the pilot scale. During EM-dominant periods, the SFCWs with only EMs had REs of 78.2%, 59.8%, 50.8% and −54.2% for NH4+-N, NO3–-N, TN, and COD, respectively, while the SFCWs with both AMs and EMs increased the REs to 85.1%, 72.2%, 73.8%, and 2.0%. The addition of AMs to SFCWs had no significant effect on EM growth (biomass or root activity). Mutual benefit was found between EMs and AMs, i.e., AMs reduced the secondary pollution caused by withered EMs, while EMs improved AM-attached biofilm functions of chemoheterotrophy, nitrate_reduction, and nitrification. Therefore, AM addition is a useful strategy to improve COD and N REs during EM-dominant periods when pollutant removal is a challenge for SFCWs.

Mutual promotion of submerged macrophytes and biofilms on artificial macrophytes for nitrogen and COD removal improvement in eutrophic water.

Both submerged macrophytes (SMs) and artificial macrophytes (AMs) have been widely used to improve water quality in eutrophic water. However, in heavily eutrophic aquatic ecosystems, the purification function of SMs is often restricted by the poor growth state due to competition from algae, while the purification function of AMs is often restricted by the limited carbon source supply for biofilm microbes attached to the AM surface. The objective of this study was to develop a new strategy to increase pollutant removal efficiency (RE) by combining the use of SMs and AMs. Pilot-scale microcosms, including treatments with both SMs and AMs (S&A), only SMs (SO) and only AMs (AO), were established to identify the performance of the new strategy. The results suggest that treatment S&A obtained REs of 88.9% for total nitrogen (TN) and 48.1% for chemical oxygen demand (COD); as comparison, treatments SO and AO obtained REs of 77.4% and 81.2% for TN and REs of-13.7% and 39.0% for COD, respectively. Compared with SO, the S&A treatment benefited SM growth in biomass, leaf chlorophyll concentration and root activity by inhibiting algae growth. In addition, compared with treatment AO, S&A increased the biofilm microbial biomass and the relative abundance of nitrifiers of families Nitrosomonadaceae and Nitrospira attached to AM surfaces. Therefore, by the mutual promotion of SMs and biofilms on AMs, the synergic application of SMs and AMs is a useful strategy for improving TN and COD REs in eutrophic water bodies such as rivers and constructed wetlands. A strategy was developed to increase nitrogen and COD removal in eutrophic water by the mutual promotion of submerged macrophytes and biofilms on artificial macrophytes.

Predation risk and competition affect habitat use of adult perch, Perca fluviatilis

The aim of this study was to examine how the presence of a predator and an interspecific competitor influence the habitat use of adult perch (Perca fluviatilis; size: 15.1 ± 0.5 cm) when given the choice between two adjacent habitats. By conducting aquarium experiments, the habitat occupancy of P. fluviatilis was documented in the presence and absence of a predator (pike Esox lucius; size: 25.4 ± 2.1 cm) and a potential competitor (ruffe Gymnocephalus cernuus; size: 14.1 ± 0.3 cm) fish species. Two P. fluviatilis individuals generally shared the same habitat. In the presence of a conspecific, P. fluviatilis favoured the structurally more-complex, artificial macrophyte habitat over the less-structured rock and sand habitat, which in turn were used equally. In the predator- and competitor treatments, P. fluviatilis seemed to adapt their habitat use to the habitat occupancy of E. lucius and G. cernuus in the Macrophyte vs. Rock and, in the predator treatment, also in the Macrophyte vs. Sand habitat combination, by increasingly occupying a habitat that was used less by the predator or competitor species, respectively. This behaviour suggests that P. fluviatilis tried to avoid the other fish species by choosing a, in some cases less preferred, predator- or competitor-free habitat. This study emphasizes the importance of biological interactions illustrated by the potential of predation risk and competition to structure fish communities by influencing habitat use at small spatial scales.

Effects of stocking density and artificial macrophyte shelter on survival, growth and molting of juvenile red swamp crayfish (Procambarus clarkii) under experimental conditions

Optimizing stocking density and increasing shelters have been thought as strategies to enhance the performance of crayfish culture. In this study, indoor controlled experiments were carried out to determine the effects of different levels of stocking density (i.e., 50, 100 and 150 ind.·m−2) and artificial macrophyte coverage (i.e., 20%, 50% and 80%) on the survival, growth and molting of juveniles of red swamp crayfish (Procambarus clarkii). A total of nine treatments with four replications were conducted simultaneously in 36 aquariums. No significant interactions were found between stocking density and artificial macrophyte coverage (P > 0.05). The crayfish yield (total biomass, TB) at 150 ind.·m−2 was significantly higher than that at 50 or 100 ind.·m−2 (P < 0.05), while higher stocking densities showed negative effects on the growth. A stocking density of 50 ind.·m−2 led to a significantly higher survival rate (SR), total weight gain (TWG) and mean molting frequency (MMF) (P < 0.05), and the specific growth rate of weight (SGRw) at 50 and 100 ind.·m−2 was significantly higher than that at 150 ind.·m−2 (P < 0.05). The rates of cheliped injury (CIR) and shelter occupancy (SO) were not significantly correlated to stocking density. Artificial macrophyte coverage had no significant effect on the SR or CIR, while the TB and MMF at 80% coverage were significantly higher than those at 20% coverage. The TWG, SGRw and SO (daylight) at 50% and 80% coverage were significantly higher than those at 20% coverage. Overall, these results indicate that, under improved culture conditions, a stocking density of around 50 ind.·m−2 and around 50% coverage of artificial shelter can be optimal for indoor culture of juvenile P. clarkii.

Evaluating Nutrient Reduction, Grazing and Barley Straw as Measures Against Algal Growth

The aim of our study was to experimentally investigate whether it is possible to reduce nuisance growth of filamentous algae in freshwater ecosystems. We used an experimental set-up mimicking a shallow pond system and performed a field investigation in the eutrophic moat of Krapperup castle (Southern Sweden), which exemplifies an extremely impaired ecosystem with ample growth of filamentous green algae. The indoor experiment tested three treatments: I) reduced nutrient concentrations, II) invertebrate grazers and III) addition of barley straw, which may constitute measures against filamentous algal growth and thereby improve the quality of the ecosystem services provided by water bodies. Our results show a decrease in cyanobacteria and diatom abundances in all mesocosms as filamentous algae biomass increased, suggesting that the microalgae suffered from nutrient and light competition with filamentous algae. A tendency for lower filamentous algae final biomass, as well as coverage, was observed in the treatment where the concentration of nutrients was reduced. Grazers treatment showed a tendency to inhibit filamentous algae growth on artificial macrophytes towards the end of the experiment, suggesting that snails initially fed on their preferred food source (diatoms), until it was almost depleted and then started to feed on filamentous algae. Interestingly, the barley straw treatment was the only treatment promoting macrophytes growth and enhancing diatom biomass, but this did not affect filamentous algae biomass. However, the ratio between filamentous algae and macrophyte final biomasses was significantly lower in the straw treatment. In a broader context, it is likely that in a long-term perspective the positive effect of barley straw on macrophyte growth will promote a shift from dominance by filamentous algae to macrophytes as main primary producer. Moreover, our experiment shows that barley straw may be effective in reducing cyanobacterial growth, which may lead to improved water quality and thereby ecosystem services, such as supporting and cultural ecosystem services, since cyanobacteria may produce potent toxins and pose a serious risk to human and animal health. Altogether, our experimental results have important implications for the challenge of reversing nuisance filamentous algal blooms in highly eutrophic systems.

Macrophyte‐specific effects on epiphyton quality and quantity and resulting effects on grazing macroinvertebrates

Abstract Aquatic macrophytes can have a significant impact on their associated community of epiphytic algae and bacteria through the provisioning of structural habitat complexity through different growth forms, the exudation of nutrients and the release of allelochemicals. In turn, this effect on epiphytic biofilm biomass and nutrient content has a potential effect on the macroinvertebrates that depend on epiphyton as a food source. We studied the effect of living macrophytes and their growth form on biofilm development in a semi‐controlled replicated microcosm experiment. Conditions of a nutrient‐poor water layer and nutrient‐rich sediment were created to study the effects of nutrient exudation by living macrophytes. We compared biofilm quantity and quality on structurally simple (Vallisneria spiralis) versus complex (Egeria densa) living plants and artificial analogues. Subsequently, the biofilm that had developed on the plants was fed, in a laboratory growth experiment, to two species of macroinvertebrate grazers (the snail Haitia acuta and the mayfly nymph Cloeon dipterum). This enabled us to assess if and how the macrophyte‐induced effects on the epiphyton can influence macroinvertebrate grazers. Living macrophytes were found to have a significant effect on epiphytic algal cover, which was mostly expressed by a lower cover on living macrophytes compared to their artificial analogues. Additionally, epiphyton cover on artificial macrophytes was found to be higher on complex structures compared to simple ones, yet this was not observed on living macrophytes. Plant specific traits, such as the release of allelopathic substances, competition for nutrients and DIC, and the amount of CaCO3 deposition on plant surfaces might explain these results. The density of epiphytic bacteria was found to be negatively correlated with biofilm Ca content from macrophytes in every treatment except living E. densa, which differed in leaf anatomy from the other plants by possessing polar leaves. Furthermore, biofilm on living macrophytes had lower C:N:P molar ratios compared to that on artificial plants, which is likely to be explained by nutrient exudation by the living plants. Although it was expected that a more nutritious biofilm would lead to increased grazer growth, this was observed only for H. acuta on E. densa. Because biofilm quantity was not a limiting factor, this lack of effect may be caused by compensatory feeding. It can be concluded that, depending on their traits, living macrophytes can have a positive effect on macroinvertebrate grazers by providing a large surface area for colonisation by epiphytic algae and bacteria, by improving biofilm stoichiometry and by stimulating bacterial growth.

Habitat modification by invasive crayfish can facilitate its growth through enhanced food accessibility

BackgroundInvasive ecosystem engineers can facilitate their invasions by modifying the physical environment to improve their own performance, but this positive feedback process has rarely been tested empirically except in sessile organisms. The invasive crayfish Procambarus clarkii is an ecosystem engineer that destroys aquatic macrophytes, which provide a physical refuge for animal prey, and this destruction is likely to enhance vulnerability to predators. Using two series of mesocosm experiments, we tested the hypothesis that the invasive crayfish increases its feeding efficiency on animal prey by reducing submerged macrophytes, thus increasing its individual growth rate in a positive density-dependent manner.ResultsIn the first experiment, increasing crayfish density reduced both macrophytes and animal prey (dragonfly and chironomid larvae) and, importantly, increased the growth rate of individual crayfish, in accordance with our expectation. In the second experiment, we used artificial macrophytes to clarify whether the physical architecture of macrophytes itself protects animal prey and limits crayfish growth rate. Increasing the artificial macrophyte quantity not only increased the survival of animal prey, but also retarded the crayfish growth rate.ConclusionsWe conclude that macrophytes strengthen bottom-up control of crayfish, but this effect can be relaxed by increasing the density of crayfish via reduction in macrophytes. This positive feedback process may explain the crayfish outbreaks and regime shifts occasionally observed in invaded freshwater ecosystems.

Comparison of periphyton communities on natural and artificial macrophytes with contrasting morphological structures

Abstract It remains an open question whether or not artificial macrophytes are good alternatives to natural macrophytes in studies of periphyton abundance and composition in lakes. Here, a mesocosm experiment was conducted in winter (when plant growth is low) to compare simultaneously the periphyton community on three submerged macrophytes (Potamogeton lucens, Vallisneria sp. and Cabomba caroliniana) with contrasting leaf structural complexities (leaf fractal dimension = 1.12, 1.17 and 1.37, respectively) and on three types of artificial macrophytes with similar morphologies as the natural plants. We also compared intertreatment differences in phytoplankton sampled from mesocosms. Both for natural and artificial macrophytes, the periphyton chlorophyll a (Chl‐a) was positively associated with leaf fractal dimension. Although the morphological structure of natural and artificial plants and the physicochemical characteristics of the water were similar, the periphyton community differed between natural and artificial macrophytes, with the difference being dependent on the leaf structural complexity of the macrophytes. For leaves with a simple structural complexity, the abundance and composition of periphyton on natural and artificial plants were not statistically different. In addition, periphyton Chl‐a, density and biovolume were higher on the adaxial side than on the abaxial side of natural P. lucens leaves, but no differences were found between sides of the artificial leaves. For leaves with a medium structural complexity, the abundance of periphyton was lower on the natural than artificial plants, and the proportion of diatoms to the total community differed. For leaves with a high structural complexity, periphyton Chl‐a of the artificial plants was notably higher than on the natural plants, while no significant differences were found for periphyton density, biovolume, and the proportion of diatoms and green algae. Permutational multivariate analysis of periphyton genus composition confirmed that periphyton composition on the artificial plants (medium and high leaf structural complexities) was different overall from that on the natural plants. Phytoplankton Chl‐a, density, biovolume, and diversity did not show any pronounced differences among treatments. Our results suggest that artificial macrophytes cannot fully substitute for natural plants even when they are morphologically similar. Artificial macrophytes should therefore be used with caution when investigating the periphyton community on macrophytes.

The Effects of Macrophyte Stem Density and Structural Complexity on Foraging Return of Invertivorous Juvenile Largemouth Bass

AbstractAquatic vegetation can impact juvenile fishes through multiple pathways. Previous studies have found that dense macrophytes can cause a trade‐off of survival for Bluegills Lepomis macrochirus, with decreased foraging efficiency in macrophytes but also decreased predation risk. For juvenile Largemouth Bass Micropterus salmoides, this phenomenon has been proposed but never experimentally tested. In addition, little information exists on the effects of vegetation on juvenile Largemouth Bass prior to the shift to piscivory, a critical period in the recruitment process. Therefore, the objective of this study was to determine if aquatic vegetation reduces the foraging ability of invertivorous (prepiscivorous) juvenile Largemouth Bass. Twenty‐four invertivorous juvenile Largemouth Bass were tested individually at each of five stem densities (0, 125, 250, 500, and 1,000 stems/m2) of artificial macrophytes within 20.8‐liter aquaria. Five treatments of a simple stem architecture (at all stem densities) and three complex stem architecture treatments (125, 250, and 500 stems/m2) were used within this experiment. Complex stem density treatments were created using a branched design which doubled the string abundance (number of strings) within the water relative to corresponding stem densities in simple vegetation treatments. Thirty chironomid larvae were used as prey during each 15‐minute foraging bout. Foraging return (number of chironomid larvae consumed/15‐minute foraging bout) was not significantly different among all stem treatments. Therefore, our results suggest the predator avoidance advantage aquatic macrophytes provide to juvenile Largemouth Bass does not come with a trade‐off in foraging efficiency (up to 1,000 stems/m2 vegetation density), at least when Largemouth Bass are prepiscivorous.Received December 15, 2015; accepted March 23, 2016 Published online July 7, 2016

Effect of Macrophytes on Flow Conditions and Deposition of Suspended Particles in Small Streams: an Experimental Study Using Artificial Vegetation

Aquatic macrophytes are often the dominant element that influences flow conditions within streams, and are often considered as ecosystem engineers that modify their aquatic environment as a result of their physical structure and metabolic activity. The role of aquatic macrophytes on suspended matter sedimentation was studied in three shallow low-order streams in Brittany (North-western France). Field experiments were carried out in April 2007 using Callitriche-like artificial macrophytes and cylindrical sediment traps dug into the channels. Hydrodynamic characteristics (velocity profile, percentage of velocity decrease and turbulence), volume of trapped sediment, particle size characteristics, and total organic matter content were measured within the macrophytes and compared with the control traps. The aquatic macrophytes operated as sediment traps by modifying the local hydrodynamic parameters. Sedimentation of fine suspended particles within the macrophytes reached maximum values when the velocity was significantly reduced, i.e. when the initial velocity was low (less than 0.5 m∙s‑1) and the depth shallow enough for the plant to occupy the entire volume of the column water. Conversely, turbulence was generated around the macrophyte stands, which induced the resuspension of fine particles and only coarse particles were trapped. This study shows the importance of threshold values at the local scale and the highly dynamic effect of macrophytes on flow characteristics.

Rapid local adaptation mediates zooplankton community assembly in experimental mesocosms.

Adaptive evolution can occur over similar timescales as ecological processes such as community assembly, but its particular effects on community assembly and structure and their magnitude are poorly understood. In experimental evolution trials, Daphnia magna were exposed to varying environments (presence and absence of fish and artificial macrophytes) for 2months. Then, in a common gardening experiment, we compared zooplankton community composition when either experimentally adapted or D.magna from the original population were present. Local adaptation of D.magna significantly altered zooplankton community composition, leading to a suppression of abundances for some zooplankton taxa and facilitation for others. The effect size of D.magna adaptation was similar to that of adding fish or macrophytes to mesocosms, two important drivers of zooplankton community structure. Our results suggest that substantial amounts of variation in community composition in natural systems may be unexplained if evolutionary dynamics are ignored.

Stem stiffness plays a role in determining the foraging success of predators

The presence of macrophytes in the littoral zone provide prey animals with protection from predators. Two macrophyte characters, stem density and branching, are known to hinder predator foraging in macrophyte beds. Stem stiffness is a character that allows the macrophyte to withstand current power in the intertidal zone, but its effect on predator movements in macrophyte beds has not been studied to date. In this study I examined whether the foraging success of predators is constrained by stem stiffness, as well as stem density and the presence of branches. Artificial macrophytes were constructed using two types of rubber that differed in stiffness. The newt Cynops ensicauda popei and larvae of the damselfly Paracercion melanotum were used as predator and prey, respectively, in this model system. The results revealed that all three plant characters studied influenced the survival rate of prey. Stiff stems consistently increased the survival rate compared with flexible stems. Stem density had the highest positive influence on survival rate. The direct effect of branches was negative and minute, but it altered the dependency on stem density. Although stiffness did have an effect on the survival rate of prey, its magnitude was relatively low. The effect of stiffness in other settings should be examined in future studies.

Macrophyte loss drives decadal change in benthic invertebrates in peatland drainage ditches

Summary Agricultural peatlands and their associated drainage systems are often highly managed and exposed to anthropogenic pressures, such as eutrophication and stable water tables, maintained via drainage during periods of high rainfall and inlet of, alkaline‐rich, waters during dry periods. These pressures promote peat degradation, resulting in the accumulation of fine‐degraded peat particles that dramatically alter aquatic habitats by smothering surfaces and decreasing water quality. Consequential effects on benthic communities are expected but have not been investigated so far. We hypothesised that peat degradation can lead to the decline in submerged macrophytes, which are of critical importance to sustaining biodiversity of benthic invertebrate communities. To investigate this, we analysed decadal (1985–2007) changes in benthic species richness in 29 peat ditches in the Netherlands and, to determine patterns of macroinvertebrate habitat occupancy, carried out a complementary field experiment with submerged artificial macrophytes, natural sediments and emergent bank vegetation. Results from long‐term monitoring indicate that chemical conditions in agricultural peat ditches have improved slightly over the last decades; however, there has been a simultaneous decline in benthic invertebrate species richness and densities corresponding to a decline in the numbers of submerged macrophytes. The apparent dependence of macroinvertebrates on macrophytes was reinforced by our field experiment which revealed that invertebrate density was highest in submerged artificial plants, while invertebrate species richness was highest in natural emergent vegetation. Conversely, degraded peat sediments supported extremely few invertebrates. Our results clearly illustrate the strong influence of submerged macrophyte loss on macroinvertebrate assemblages in peatland waters. Furthermore, this suggests that improvements in water quality alone will not benefit invertebrates in the absence of suitable vegetative habitats.

When habitat complexity increases predation risk: experiments with invasive and neotropical native fishes

We tested the predator–prey relationships between a native piscivore (Salminus brasiliensis) and introduced and native fish species of the Paraná River, Brazil. We hypothesised that S. brasiliensis can exert biotic resistance against invasive fishes but not at the same degree for all species. Three invasive (Cichla piquiti, Oreochromis niloticus and Ictalurus punctatus) and two native (Astyanax altiparanae and Prochilodus lineatus) species were offered as prey to S. brasiliensis in 300 L aquaria trials at three levels of cover (0%, 50% and 100% of artificial macrophytes). S. brasiliensis had a greater ability to capture prey in complex habitats, so predation success did not decrease with habitat complexity and even increased on I. punctatus. Prey survival was variable through time and among species, being high for I. punctatus. The three most consumed species (P. lineatus, C. piquiti, and O. niloticus) were less active and occupied the aquaria surfaces, changing strongly their behaviour with habitat complexity. Except for P. lineatus and C. piquiti, S. brasiliensis preferably preyed on smaller individuals of the other species. Our experiments support that S. brasiliensis is an interesting candidate to resist the invasion by C. piquiti and O. niloticus but not to control the abundance of I. punctatus.

Novel Praziquantel Treatment Regime for Controlling Asian Tapeworm Infections in Pond‐Reared Fish

AbstractThe Asian tapeworm Bothriocephalus achelognathii is an intestinal fish parasite that is nonnative to but widespread throughout the southwestern United States. Praziquantel is an anthelminthic drug commonly used to treat fish for Asian tapeworm; however, it does not kill tapeworm eggs, so the water in ponds used for fish rearing must be exchanged after treatment. Our objective was to determine whether a system containing both an intermediate copepod host and a definitive fish host for Asian tapeworm could be treated without exchanging the water by using a follow‐up treatment for any tapeworms that developed from eggs released before or during the first treatment. Here, we have described a new praziquantel treatment regimen to control Asian tapeworm infections in freshwater‐reared fish. To evaluate the efficacy of this regimen, we stocked 50 red shiners Cyprinella lutrensis and an intermediate copepod host, Cyclops vernalis, into each of six pond mesocosms containing artificial macrophytes, sand, and gravel to simulate natural pools and provide suitable substrate for the copepod's life history. The test fish population had been naturally infected with B. achelognathii and had an initial infection prevalence of 14% and an infection intensity of 2.14 ± 2.19 (mean ± SD) worms per fish. Three mesocosms were treated twice, each with 2.5 mg/L praziquantel; 19 d passed between treatments to allow for possible reinfection to occur. After a 2.5‐month posttreatment period to allow any remaining tapeworms to reestablish themselves, we killed and dissected all of the remaining fish. No worms were found in treated fish; however, the control group had an infection prevalence of 18 ± 6% and an infection intensity of 3.45 ± 2.1 worms per fish. Based on these results, we concluded that the praziquantel treatment regime administered was efficacious and suggest testing it on a larger scale. We caution that praziquantel has not been approved by the U.S. Food and Drug Administration for use on fish but can be used legally in some situations.

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.

Macrophyte architecture affects the abundance and diversity of littoral microfauna

We tested the hypothesis that structural complexity is an important factor influencing the abundance and taxon richness of microfauna (e.g., rotifers, copepods, cladocerans) in littoral habitats. Research on littoral microfauna has to date focused mainly on field observations, which commonly show microfauna have preference for some macrophytes over others. However, while such studies commonly conclude that macrophyte architecture is a major determinant of these variations, independent factors may also be responsible (e.g., differences in macrophyte ages, differences in macrophyte bed densities and the depth of the respective macrophyte beds sampled). We used artificial macrophytes with three levels of complexity to keep the surface area and mass of the substrate sampled constant, and to control for confounding factors not related to the complexity of the plants. Our results support the hypothesis that structural complexity is an important factor influencing abundance and taxon richness, independent of other potential confounding factors. Microfaunal (mainly rotifer) abundance and richness were generally greater on more complex artificial macrophytes, likely a result of more complex substrates (1) providing a greater variety of habitat, (2) supporting a greater concentration or variety of food and/or (3) affording greater protection against predators. Less mobile surface-associated (i.e., benthic/periphytic) taxa were found to discriminate among substrates, whereas the abundance and richness of planktonic species were not affected by complexity level. Relatively low abundances and taxon richness of microfauna recorded in control samples, which did not contain artificial macrophytes, supports the contention that vegetated areas sustain a higher abundance and variety of species than non-vegetated areas.

Groundwater pre-treatment prevents the onset of chronic ulcerative dermatopathy in juvenile Murray cod, Maccullochella peelii peelii (Mitchell)

Chronic ulcerative dermatopathy (CUD) is a disfiguring condition affecting Murray cod cultured in untreated groundwater. This study sought to further investigate the possible etiology of the syndrome and determine whether groundwater pre-treatment could suppress the development of CUD in juvenile Murray cod. Electrolyte enrichment or pre-treatment with UV irradiation delayed the onset of CUD. In contrast, pre-conditioning of groundwater either in a vegetated earthen pond or in the presence of artificial macrophytes drastically reduced both the incidence and severity of CUD, with more than 90% of fish exhibiting no visual signs. Haematology and blood parameters were assessed for future diagnostic potential, but no changes in blood parameters were observed, even in advanced CUD-affected fish. This paper identified that the treatment of groundwater via an earthen pond and in the presence of an artificial macrophyte are two highly effective methods of preventing CUD arising in juvenile Murray cod.

Artificial macrophytes as fish habitat in a Mediterranean reservoir subjected to seasonal water level disturbances

Artificial macrophytes were experimentally deployed in Boadella Reservoir (NE Spain) and assessed for fish use throughout the first 3 months of the 2007 summer drawdown. In total, 1,832 individuals of seven fish species were recorded through visual censuses, with introduced perch Perca fluviatilis being the prevalent species. Fish richness and abundance were significantly higher in artificial macrophytes than in rocky shores and sandy beaches, displaying a unimodal variation through time. P. fluviatilis, Rutilus rutilus and Abramis brama were significantly more abundant in artificial macrophytes than in the other two naturally submersed habitats, where no individuals of these species were recorded. The abundances of Squalius laietanus, Lepomis gibbosus, Micropterus salmoides and Cyprinus carpio were overall greater in artificial macrophytes than in rocky shores but displayed significant habitat × time interactions. A decrease in predation risk was apparently the key-factor of the intensive use of artificial macrophytes by small fish (≤100 mm TL) in Boadella Reservoir, since most species reduced drastically or completely the use of submerged structures when body size increased. This study demonstrates that adding artificial structures mimicking aquatic macrophytes can be an interesting tool to mitigate the adverse effects of water level fluctuations on fish assemblages in structure-less and homogeneous ecosystems, and to understand the mechanisms affecting habitat use and species replacement.