IN SITU ALLELOPATHIC POTENTIAL OF MYRIOPHYLLUM VERTICILLATUM (HALORAGACEAE) AGAINST SELECTED PHYTOPLANKTON SPECIES1

The potential allelopathic impact of Myriophyllum verticillatum L. under in situ conditions was determined in a series of field and laboratory experiments. Coexistence experiments were performed in a lake dominated by M. verticillatum (Van Goor) Meffert where we exposed three unialgal phytoplankton cultures in dialysis tubes to macrophyte exudates regularly during the vegetated period. Plant content and exudation of polyphenolic compounds were determined, and the inhibitory activity of polyphenol‐containing extracts was tested in bioassays with cyanobacteria. To account for possible resource interference, we monitored growth and photosynthesis of phosphorus‐limited and unlimited cyanobacterium Limnothrix redekei in dialysis tubes exposed to M. verticillatum in aquaria. A high allelopathic potential of M. verticillatum was concluded from high tissue concentrations of total phenolic compounds of 6%–12% of dry matter, the demonstrated release of bioactive polyphenols into the surrounding medium, and the inhibition of cyanobacteria by extracts. We could not unambiguously demonstrate the exudation of polyphenols by M. verticillatum in situ due to interference with allochthonous humic compounds. In laboratory experiments, L. redekei exhibited significantly reduced maximum relative electron transport rates when co‐cultivated in dialysis tubes with M. verticillatum. The field dialysis tube experiment confirmed this result, accompanied by a decline of chl a and PSII activity for L. redekei and the diatom Stephanodiscus minutulus (Kütz.) Greve et Möller, but not for the green alga Scenedesmus armatus Chodat in August. At other times, either no effects or stimulatory effects were observed with all species. Nutrient limitation of the target species may have masked allelopathic effects, and M. verticillatum may have enhanced phytoplankton growth due to phosphorus leakage.     

Effects of Spirogyra arcta on biomass and structure of submerged macrophyte communities

The presence of algae can greatly reduce the amount of light that reaches submerged macrophytes, but few experimental studies have been conducted to examine the effects of algae on biomass and structure of submerged macrophyte communities. We constructed communities with four submerged macrophytes (Hydrilla verticillata, Egeria densa, Ceratophyllum demersum, and Chara vulgaris) in three environments in which 0 (control), 50 and 100% of the water surface was covered by Spirogyra arcta. Compared to the control treatment, the 100% spirogyra treatment decreased biomass of the submerged macrophyte communities and of all the four macrophytes except C. demersum. Compared to the control and 50% treatments, the 100% treatment significantly increased relative abundance of C. demersum and decreased that of E. densa. Therefore, the presence of S. arcta can greatly affect the productivity and alter the structure of submerged macrophyte communities. To restore submerged macrophyte communities in conditions with abundant algae, assembling communities consisting of C. demersum or similar species may be a good practice.     

Allelopathic inhibition of epiphytes by submerged macrophytes

The hypothesis that epiphytes are more vulnerable to allelochemicals released by submerged macrophytes than phytoplankton was tested by measuring growth and photosystem (PS) II activity of three common epiphytic algae and cyanobacteria in coexistence with Myriophyllum spicatum using dialysis tubes. Results were compared with earlier experiments on planktonic species. Contrary to the planktonic species, the tested epiphytes, the green algae Stigeoclonium tenue, the diatom Gomphonema parvulum and the cyanobacterium Oscillatoria limosa, were not significantly inhibited by M. spicatum. Growth and PS II activity of O. limosa were even significantly enhanced by M. spicatum, but this effect disappeared under phosphorus-deficiency due to the allelopathically induced inhibition of the alkaline phosphatase activity or phosphorus leakage by the macrophytes. My findings of a lower vulnerability of epiphytes against allelopathic substances of submerged macrophytes are supported by results of a literature survey.     

Effects of eutrophication and snails on Eurasian watermilfoil (Myriophyllum spicatum) invasion

Exotic species can invade and establish new habitats both as a result of their own traits, and as a result of the characteristics of the environment they invade. Here, we show that the abundance of the invasive submerged aquatic plant, Myriophyllum spicatum (Eurasian watermilfoil) is highly dependent on the conditions of the environment in a mesocosm experiment. M. spicatum is allelopathic towards epiphytic algae, and in the absence of algivorous snails, we found that the abundance of both algae and M. spicatum significantly increased with experimentally increased nutrient loading, while the abundance of native submerged macrophytes declined. However, when snails were present, snail biomass increased with increasing nutrient loading, and M. spicatum biomass was consistently low while native submerged macrophyte biomass was consistently high. Our results stress the importance of the interaction between species traits and environmental conditions when considering the invasiveness of certain exotic species and the invasibility of certain environments.     

Impact of submerged macrophytes on fish-zooplanl phytoplankton interactions: large-scale enclosure experiments in a shallow eutrophic lake

1. The impact of changes in submerged macrophyte abundance on fish-zooplankton-phytoplankton interactions was studied in eighteen large-scale (100 m²) enclosures in a shallow eutrophic take. The submerged macrophytes comprised Potamategon pectinatus L., P. pusillus L. and Callitriche hermaphroditica L. while the fish fry stock comprised three-spined sticklebacks, Gasterosteus acuteatus L., and roach, Rutilus rutilus L. 2. In the absence of macrophytes zooplankton biomass was low and dominated by cyclopoid copepods regardless of fish density, while the phytoplankton biovolume was high (up to 38 mm³1) and dominated by small pennate diatoms and chlorococcales. When the lake volume infested by submerged macrophytes (PVI) exceeded 15–20% and the fish density was below a catch per unit effort (CPUE) of 10 (approx. 2 fry m^?2), planktonic cladoceran biomass was high and dominated by relatively large-sized specimens, while the phytoplankton biovolume was low and dominated by small fast-growing flagellates. At higher fish densities, zooplankton biomass and average biomass of cladocerans decreased and a shift to cyclopoids occurred, while phytoplankton biovolume increased markedly and became dominated by cyanophytes and dinoflagellates. 3. Stepwise multiple linear regressions on log-transformed data revealed that the biomass of Daphnia, Bosmina, Ceriodaphmia and Chydorus were all significantly positively related to PVI and negatively to the abundance of fish or PVI x fish. The average individual biomass of cladocerans was negatively related to fish, but unrelated to PVI. Calculated zooplankton grazing pressure on phytoplankton was positively related to PVI and negatively to PVI x fish. Accordingly the phytoplankton biovolume was negatively related to PVI and to PVI x zooplankton biomass. Cyanophytes and chryptophytes (% of biomass) were positively and Chlorococcales and diatoms negatively related to PVI, while cyanophytes and Chlorococcales were negatively related to PVI x zooplankton biomass. In contrast diatoms and cryptophytes were positively related to the zooplankton biomass or PVI x zooplankton. 4. The results suggest that fish predation has less impact on the zooplankton community in the more structured environment of macrophyte beds, particularly when the PVI exceeds 15–20%. They further suggest that the refuge capacity of macrophytes decreases markedly with increasing fish density (in our study above approximately 10 CPUE). Provided that the density of planktivorous fish is not high, even small improvements in submerged macrophyte abundance may have a substantial positive impact on the zooplankton, leading to a lower phytoplankton biovolume and higher water transparency. However, at high fish densities the refuge effect seems low and no major zooplankton mediated effects of enhanced growth of macrophytes are to be expected.     

Can allelopathically active submerged macrophytes stabilise clear-water states in shallow lakes?

Inhibition of phytoplankton by allelochemicals released by submerged macrophytes is supposed to be one of the mechanisms that contribute to the stabilisation of clear-water states in shallow lakes. The relevance of this process at ecosystem level, however, is debated because in situ evidence is difficult to achieve. Our literature review indicates that allelopathically active species such as Myriophyllum, Ceratophyllum, Elodea and Najas or certain charophytes are among the most frequent submerged macrophytes in temperate shallow lakes. The most common experimental approach for allelopathic interference between macrophytes and phytoplankton has been the use of plant extracts or purified plant compounds. Final evidence, however, requires combination with more realistic in situ experiments. Such investigations have successfully been performed with selected species. In situ allelopathic activity is also influenced by the fact that phytoplankton species exhibit differential sensitivity against allelochemicals both between and within major taxonomic groups such as diatoms, cyanobacteria and chlorophytes. In general, epiphytic species apparently are less sensitive towards allelochemicals than phytoplankton despite living closely attached to the plants and being of key importance for macrophyte growth due to their shading. Light and nutrient availability potentially influence the sensitivity of target algae and cyanobacteria. Whether or not additional stressors such as nutrient limitation enhance or dampen allelopathic interactions still has to be clarified. We strongly propose allelopathy as an important mechanism in the interaction between submerged macrophytes and phytoplankton in shallow lakes based on the frequent occurrence of active species and the knowledge of potential target species. The role of allelopathy interfering with epiphyton development is less well understood. Including further levels of complexity, such as nutrient interference, grazing and climate, will extend this ecosystem-based view of in situ allelopathy.     

Loss of Submerged Macrophytes in Shallow Lakes in North‐Eastern Germany

During the 1950s, the submerged vegetation of shallow lakes in north‐eastern Germany was dominated by nutrient tolerant species, with Ceratophyllum demersum and Myriophyllum sp. being most common. Almost one third of 300 investigated lakes had already lost their submerged macrophytes at that time. Very shallow lakes showed either high or low macrophyte abundance. Increasing depth resulted in medium macrophyte abundances, which may contribute to the stabilisation of local or temporary clearwater states. Forty years later, the percentage of lakes without macrophytes had dramatically increased. Between 55 and 85% of the investigated lakes showed a low abundance. The decline was most pronounced in very shallow lakes. The majority of the investigated lakes showed summer TP concentrations below 100 μg L^–1, but no colonisation by submerged macrophytes, which indicates a resilience against re‐colonisation.     

A 2-year experimental study on nutrient and predator influences on food web constituents in a shallow lake of north-west Spain

1. A 2‐year study was carried out on the roles of nutrients and fish in determining the plankton communities of a shallow lake in north‐west Spain. Outcomes were different each year depending on the initial conditions, especially of macrophyte biomass. In 1998 estimated initial ‘per cent water volume inhabited’ (PVI) by submerged macrophytes was about 35%. Phytoplankton biomass estimated as chlorophyll a was strongly controlled by fish, whereas effects of nutrient enrichment were not significant. In 1999 estimated PVI was 80%, no fish effect was observed on phytoplankton biomass, but nutrients had significant effects. Water temperatures were higher in 1998 than in 1999. 2. In the 1998 experiment, cladoceran populations were controlled by fish and cyanobacteria were the dominant phytoplankton group. There were no differences between effects of low (4 g fresh mass m^?2) and high (20 g fresh mass m^?2) fish density on total zooplankton biomass, but zooplankton biomass was higher in the absence of fish. With the high plant density in 1999, fish failed to control any group of the zooplankton community. 3. Total biovolume of phytoplankton strongly decreased with increased nutrient concentrations in 1998, although chlorophyll a concentrations did not significantly change. At higher nutrient concentrations, flagellate algae became more abundant with likely growth rates that could have overcompensated cladoceran feeding rates. This change in phytoplankton community composition may have been because of increases in the DIN : SRP ratio. Both chlorophyll a concentration and total phytoplankton biovolume increased significantly with nutrients in the 1999 experiment. 4. A strong decline of submerged macrophytes was observed in both years as nutrients increased, resulting in shading by periphyton. This shading effect could account for the plant decline despite lower water turbidity at the very high nutrient levels in 1998.     

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 SUBMERGED MACROPHYTES ON SEDIMENTARY DIATOM ASSEMBLAGES1

Submerged macrophytes are a central component of lake ecosystems; however, little is known regarding their long‐term response to environmental change. We have examined the potential of diatoms as indicators of past macrophyte biomass. We first sampled periphyton to determine whether habitat was a predictor of diatom assemblage. We then sampled 41 lakes in Quebec, Canada, to evaluate whether whole‐lake submerged macrophyte biomass (BiomEpiV) influenced surface sediment diatom assemblages. A multivariate regression tree (MRT) was used to construct a semiquantitative model to reconstruct past macrophyte biomass. We determined that periphytic diatom assemblages on macrophytes were significantly different from those on wood and rocks (ANOSIM R = 0.63, P < 0.01). A redundancy analysis (RDA) of the 41‐lake data set identified BiomEpiV as a significant (P < 0.05) variable in structuring sedimentary diatom assemblages. The MRT analysis classified the lakes into three groups. These groups were (A) high‐macrophyte, nutrient‐limited lakes (BiomEpiV ≥525 μg · L^?1; total phosphorus [TP] <35 μg · L^?1; 23 lakes); (B) low‐macrophyte, nutrient‐limited lakes (BiomEpiV <525 μg · L^?1; TP <35 μg · L^?1; 12 lakes); and (C) eutrophic lakes (TP ≥35 μg · L^?1; six lakes). A semiquantitative model correctly predicted the MRT group of the lake 71% of the time (P < 0.001). These results suggest that submerged macrophytes have a significant influence on diatom community structure and that sedimentary diatom assemblages can be used to infer past macrophyte abundance.     

修复白洋淀镉污染水体的沉水植物筛选试验

为了筛选出适宜修复白洋淀镉(Cd)污染水体的沉水植物,该研究通过室内模拟试验,分析了四种沉水植物黑藻、狐尾藻、金鱼藻和菹草对Cd的耐受性及对底泥Cd的富集和迁移能力。结果表明:(1)通过毒性测试研究,Cd对黑藻、狐尾藻、金鱼藻及菹草的4 d-EC₅₀(半数抑制浓度)分别为0.51、0.81、0.03、0.12 mg·L^-1,狐尾藻对Cd的耐性最强,黑藻次之,金鱼藻对Cd的耐性最低; 四种沉水植物对Cd的最大富集量分别为27.89、15.28、22.54、32.74 g·kg^-1,菹草对Cd的富集能力最强,黑藻次之,狐尾藻对Cd的富集能力最低。(2)通过Cd污染底泥修复研究,黑藻、狐尾藻和菹草体内Cd富集量整体表现为根>叶片和茎(P<0.05); 地上部、根对Cd的富集能力分别表现为黑藻>菹草>狐尾藻,菹草>黑藻>狐尾藻; 三种沉水植物对Cd的迁移能力则表现为黑藻>狐尾藻>菹草。总之,黑藻对底泥中Cd富集和迁移能力均较强,且耐性较高,是最适合修复白洋淀Cd污染水体的沉水植物。     

Implications of rudd (Scardinius erythrophthalmus) herbivory on submerged macrophytes in a shallow eutrophic lake

The role of rudd (Scardinius erythrophthalmus) herbivory was studied in Kirkkojärvi, a shallow and turbid basin in Lake Hiidenvesi, Finland. The submerged species dominating in the rudd diets were Potamogeton obtusifolius, Ranunculus circinatus, Sparganium emersum, bryophytes, and filamentous algae. Plant consumption estimated with bioenergetics modelling increased with fish age, being highest in late summer concomitant with the macrophyte biomass peak. Depending on the age structure, a rudd biomass of 20 kg ha^?1 consumes 18–23 kg of macrophytes ha^?1 a^?1, while a rudd biomass of 100 kg ha^?1 results in plant consumption of 92–115 kg ha^?1 a^?1. Although, rudd seemed to feed rather unselectively on suitable-sized and edible plants, some species abundant in the littoral, such as Myriophyllum verticillatum and pleustophytic Ceratophyllum demersum, were not found in rudd guts, indicating selective plant consumption. In Kirkkojärvi, selective grazing by rudd and increased turbidity and high nutrient levels partly caused by bottom dwelling cyprinid fish, may promote the inedible and pleustophytic macrophytes, which have increased in Kirkkojärvi during the past decades.

     

Effects of land use on aquatic macrophyte diversity and water quality of ponds

1. Aquatic macrophyte diversity and water quality of 55 ponds in western Japan were related to land use and morphometric variables to identify the environmental factors that sustain biodiversity and the spatial extent at which these factors operate. 2. The relevant spatial extent for floating‐leaved macrophyte richness (500 m from pond edge) was larger than that for submerged macrophyte occurrence (10, 75 and 100 m), whereas emergent macrophyte richness was best explained at much larger extents (1000 m). Total macrophyte richness was explained at the extent of 500, 750 and 1000 m. The extents relevant for explaining the physicochemical condition of pond water (100 and 250 m) were similar to those for submerged and floating‐leaved macrophytes, suggesting that these two growth forms are more sensitive to water quality compared to emergent macrophytes. 3. Diversity of all three growth forms and that of total macrophytes collectively were inversely related to turbidity and nutrient concentration; among the three growth forms, submerged macrophytes were most affected by water quality. 4. Negative relationships were found between the proportion of urban area and the diversity of the three growth forms and that of total macrophytes and water quality. Species richness of emergent, floating‐leaved and total macrophytes decreased with depth and increased with surface area up to about 5000 m², above which it declined. 5. Urbanisation and enlargement of ponds were the two main factors that decreased aquatic macrophyte diversity in irrigation ponds. To alleviate the adverse effects of urban areas on aquatic macrophyte diversity, our results suggest that management efforts should focus on the creation of buffer zones within the relevant spatial extent from the pond edge.     

An experimental system to study ecophysiological responses of submerged macrophytes to temperature and light

An experimental growth system was devised to study the ecophysiological responses of submerged macrophytes to temperature and light. Using the system, a pilot study was conducted to compare responses to light of three representative species common to the littoral zone of Lake Biwa: Vallisneria asiatica Miki var. biwaensis Miki, which is endemic to Japan, Potamogeton maackianus A. Benn., a dominant submerged macrophyte in Lake Biwa, and Elodea nuttallii (Planch.) St. John, one of the most influential submerged exotic macrophytes naturalized in Japan. Different relative growth rate responses and other growth parameters among the species were observed. We ascertained the experimental system to be useful for the comparison of ecophysiological responses of submerged macrophytes.     

沉水植物化感作用对西湖湿地浮游植物群落的影响

通过微宇宙实验,在控制光照和营养盐浓度的条件下分别研究了苦草(Vallisneria spiralis)、金鱼藻(Ceratophyllum demersum)和穗花狐尾藻(Myriophyllum spicatum)的化感作用对采集于杭州西湖湖西湿地的藻类密度、叶绿素a浓度、群落结构、多样性指数等的影响。其结果表明,3种沉水植物对微宇宙系统中的藻类都具有明显影响,藻类密度与叶绿素a浓度受到显著抑制,3个草-藻研究系统中藻类群落结构都发生了变化。在实验末期苦草组、金鱼藻组和穗花狐尾藻组中藻类总生物量(以细胞密度计)分别较初始值降低了37.06%、78.37%和83.40%。栅藻对3种沉水植物的化感作用敏感性较弱。藻类生物多样性方面,穗花狐尾藻系统中最高,其次是金鱼藻组,最后是苦草组,其Shannon-Wiener指数(H)分别为2.76、2.06和0.72,穗花狐尾藻组中H的显著高于苦草组(P0.05)。     

Seasonal and interannual dynamics of polyphenols in Myriophyllum verticillatum and their allelopathic activity on Anabaena variabilis

In 4 successive years, we investigated the seasonal and interannual variability of the total polyphenolic pool and of the individual polyphenolic compounds in Myriophyllum verticillatum, as well as their allelopathic activity in a small eutrophic lake. We tested whether nutrient availability explained interannual and seasonal changes in the production of polyphenols. There were no strong interannual variations in plant tissue carbon, nitrogen and phosphorus concentrations, while total phenolic compounds (TPC) significantly differed between years, especially in apical meristems (range: 38–122 mg g⁻¹  dry weight (DW)). Seasonal patterns, with maxima between May and July, changed between years. Partially confirming the carbon-nutrient balance hypothesis sensu Bryant et al. [Bryant, J.P., Chapin III, F.S., Klein, D.R., 1983. Carbon/nutrient balance of boreal plants in relation to vertebrate herbivory. Oikos 40, 357–368], we found correlations between TPC and the C/N (carbon/nitrogen) ratio in some but not all years, especially in apical meristems. Plant tissue phosphorus content accounted also for the variability in TPC in some years. Crude extracts of apical meristems always inhibited the growth of Anabaena variabilis, used as a target cyanobacterium. Plant TPC concentration and allelopathic activity were significantly correlated in all years except in 2005. Bioassay-directed fractionation of M. verticillatum extracts coupled with LC–MS analyses of the respective fractions revealed several isomers of HHDP-di- and -tri-galloylglucose apparently responsible for the allelopathic effects. The individual active compounds revealed a more distinct seasonal pattern compared to the pool of phenolic compounds in M. verticillatum, with a clear maximum in May, the ecologically most relevant period for inhibitory effects of submerged macrophytes on phytoplankton.     

Allelopathic growth inhibition and colony formation of the green alga <Emphasis Type="Italic">Scenedesmus obliquus</Emphasis> by the aquatic macrophyte <Emphasis Type="Italic">Stratiotes aloides</Emphasis>

Laboratory experiments were conducted to elucidate the allelopathic effects of exudates from the aquatic macrophyte Stratiotes aloides on the growth and morphology of the green alga Scenedesmus obliquus. Both water originating from a S. aloides culture and water that had not been in contact with S. aloides was simultaneously inoculated with S. obliquus cells. In all experiments we observed a highly significant inhibitory effect of Stratiotes water on the green alga. The duration of the lag phase as well as the initial biovolume doubling time of S. obliquus were extended. The growth rate of S. obliquus was only significantly inhibited by the younger Stratiotes plants. Furthermore, the mean particle volume (MPV) of Scenedesmus increased significantly in the presence of Stratiotes water in all three experiments. Microscopic analyses confirmed that S. obliquus forms more colonies in the presence of water from a S. aloides culture. Colonies of phytoplankton have higher sinking rates than single phytoplankton cells of the same species and disappear faster from the upper water layers, reducing the competition for light between algae and macrophytes.     

Phytoplankton and Epipelon Responses to Clear and Turbid Phases in a Seepage Lake (Buenos Aires,Argentina)

Annual changes in the algal density and concentrations of chlorophyll a, total phosphorus, and organic matter were analyzed in water and sediments at four sites characterized by the presence or absence of submerged and emergent macrophytes, during turbid‐ and clear‐water conditions to determine the contribution of the algal components of the plankton and the epipelon and to identify the most typical species in each community. Three states were recognized: one turbid and two clear, with different submerged macrophyte cover. The peaks of phytoplankton and epipelon occurred in the turbid phase, whereas the highest proportion of true epipelic algae in sediments was reached in the second clear phase. The Oscillatoriaceae dominated during the turbid phase in the water and throughout the entire year within the sediments. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The contribution of epiphyton to the primary production of tropical floodplain wetlands

Tropical floodplains are one of the most productive ecosystems on earth. Studies on floodplain productivity have primarily focused on trees and macrophytes, rather than algae, due to their greater biomass. However, epiphyton—algae and bacteria attached to the submerged portion of aquatic macrophytes—is a major source of energy in many tropical floodplains. Epiphyton productivity rates are unknown for most tropical floodplain wetlands, and spatial variability is not well understood. In this study, we measured primary productivity of epiphyton in Kakadu National Park in northern Australia. We estimated the relative contribution of epiphyton to aquatic production (epiphyton, + phytoplankton + macrophytes). We sampled sites dominated by different macrophyte structural types: vertical emerging grasses, horizontal emerging grasses, submerged macrophytes, and macrophytes with floating leaves. Epiphyton productivity was highly influenced by the structural type of the macrophyte. Highest potential productivity per weight was measured from epiphyton growing on macrophytes with floating leaves and horizontal grasses (1.52 ± 0.53 and 1.82 ± 0.61 mgC/dw g epiphyton/h, respectively) and lowest in submerged macrophytes and vertical grasses (0.57 ± 0.26 and 0.66 ± 0.47 mgC/dw g epiphyton/h, respectively). When considering the areal biomass of the macrophyte and the amount of epiphyton attached, epiphyton on horizontal grasses and submerged macrophytes had productivity values approximately ten times higher (45–219 mgC/m²/d) compared to those on vertical grasses and macrophytes with floating leaves (2–18 mgC/m²/d). Epiphyton contributed between 2 to 13 percent to the aquatic production of these tropical floodplain wetlands.     

Responses of four submerged macrophytes to freshwater snail density (Radix swinhoei) under clear‐water conditions: A mesocosm study

Macrophytes play a key role in stabilizing clear‐water conditions in shallow freshwater ecosystems. Their populations are maintained by a balance between plant grazing and plant growth. As a freshwater snail commonly found in shallow lakes, Radix swinhoei can affect the growth of submerged macrophytes by removing epiphyton from the surface of aquatic plants and by grazing directly on macrophyte organs. Thus, we conducted a long‐term (11‐month) experiment to explore the effects of snail density on macrophytes with distinctive structures in an outdoor clear‐water mesocosm system (with relatively low total nitrogen (TN, 0.66 ± 0.27 mg/L) and total phosphorus (TP, 36 ± 20 μg/L) and a phytoplankton chlorophyll a (Chla) range of 14.8 ± 4.9 μg/L) based on two different snail densities (low and high) and four macrophyte species treatments (Myriophyllum spicatum, Potamogeton wrightii, P. crispus, and P. oxyphyllus). In the high‐density treatment, snail biomass and abundance (36.5 ± 16.5 g/m² and 169 ± 92 ind/m², respectively) were approximately twice that observed in the low‐density treatment, resulting in lower total and aboveground biomass and ramet number in the macrophytes. In addition, plant height and plant volume inhabited (PVI) showed species‐specific responses to snail densities, that is, the height of P. oxyphyllus and PVI of M. spicatum were both higher under low‐density treatment. Thus, compared with low‐density treatment, the inhibitory effects of long‐term high snail density on macrophytes by direct feeding may be greater than the positive effects resulting from epiphyton clearance when under clear‐water conditions with low epiphyton biomass. Thus, under clear‐water conditions, the growth and community composition of submerged macrophytes could be potentially modified by the manual addition of invertebrates (i.e., snails) to lakes if the inhibitory effects from predatory fish are minor.