Functional differences in epiphytic microbial communities in nutrient-rich freshwater ecosystems: an assay of denitrifying capacity

1. The denitrifying capacity of epiphyton was used to evaluate differences in the function of epiphytic microbial communities on submersed macrophytes in nutrient-rich freshwater ecosystems. The denitrifying capacity of epiphyton on Potamogeton perfoliatus shoots of different age and with different epiphytic abundances from a eutrophic lake was investigated in laboratory microcosms in the light and dark. Additionally, differences between epiphyton on shoots of Potamogeton pectinatus grown under different in situ nutrient and hydraulic conditions were investigated by examining their denitrifying capacity.
2. Denitrification was registered in well-developed epiphytic layers on both mature and senescent shoots in the dark, with activities 3- to 10-fold higher in the epiphytic communities of senescent shoots. No activity was detected on young shoots with sparse epiphyton or on shoots from which loosely attached epiphyton had been removed. Denitrification never occurred during illumination.
3. Even though the epiphytic abundance was similar in magnitude, the denitrifying capacity of epiphyton adapted to high nutrient loadings was about a hundred times higher than that of epiphyton adapted to lower nutrient levels. Additionally, epiphytic abundance and denitrifying capacity were higher at sites less exposed to wave turbulence or water currents, than at sites with more water turbulence.
4. The results illustrate how the hydraulic and nutrient conditions of the surrounding water affect both the quantity and function of epiphytic microbial communities in nutrient-rich freshwater ecosystems.     

Growth of macrophytes and ecosystem consequences in a lowland Danish stream

SUMMARY. 1. The River Suså is a small, nutrient-rich stream situated in an open landscape with clayish subsoil under intensive cultivation. Discharge was variable daily and seasonally due to low groundwater input. Above-ground development of submerged macrophytes was restricted to late May to November, when water velocity and depth were low. Dominant macrophytes were rooted Potamogeton pectinatus and Sparganium emersum and unrooted Cladophora . Biomass development was closet) related to light availability.
2. Growth rates of macrophytes were linearly related to light availability when self-shading was accounted for. Potamogeton pectinatus grew rapidly m May-June, concentrated the biomass at the water-surface during July-August, and then declined exponentially when the shoots became basally senescent. Sparganium emersum had linear, flexible leaves that were continuously replaced from a basal meristem. Sparganium emersum was less susceptible to high water velocities than Potamogeton pectinatus and the biomass declined later and at lower rates during autumn. Sparganium emersum also regrew after culling that left its meristem intact in the sediment. Unrooted Cladophora developed a high biomass during sunny periods and subsequently disappeared at high discharges. The summer biomass of rooted macrophytes was greater in years with high summer discharge, whereas the biomass of Cladophora and of the epiphytic microbial community was lower due to scouring.
3. Submerged macrophytes played a key role in structure and functioning of the ecosystem. They reduced water velocities two to four fold during summer and promoted extensive organic sedimentation. The biomass of benthic diatoms declined parallel to increased macrophyte shading and sedimentation. In addition, submerged macrophytes formed a large substratum for macroinvertebrates and for a microbial community.     

Floating meadow epiphyton: biological and chemical features of epiphytic material in an Amazon floodplain lake

SUMMARY. 1. Quantities and the chemical composition of epiphyton on the roots of floating aquatic macrophytes were measured in Lake Calado, an Amazon floodplain lake. Growth of epiphytic algae following physical disturbance and losses of epiphyton due to grazing and storms were investigated.
2. Deposition of silt from invading river water decreased chlorophyll and nutrient content (%C, %N, %P) of epiphyton during rising water. N:P ratios of epiphyton indicated that proximity to the river increased supplies of phosphorus. Attached algal biomass per unit root tissue was higher overall during the falling water period, when light was greater, storms less frequent, and new host plant tissue produced more slowly.
3. Epiphytic algal biomass at the margins of floating meadows exceeded that of the phytoplankton in the open water on a per unit area basis. Increases in attached algal chlorophyll ranged from two- to ten-fold over 1 week. Artificial denudation of roots was followed by rapid regrowth of attached algae, leading, after I week, to four-fold increases in chlorophyll over the pre-denuded state.
4. Wind-blown macrophytes experienced an episodic loss of 70% of epiphytic material in less than 1 h. Particulate material lost from roots grazed by snails included root tissue and contained significantly more carbon than material lost from ungrazed roots.     

Interaction effects of flow velocity and oxygen metabolism on nitrification and denitrification in biofilms on submersed macrophytes

Effects of water flow velocityon nitrification, denitrification, andthe metabolism of dissolved oxygen andinorganic carbon in macrophyte-epiphytoncomplexes were investigated in the presentstudy. The metabolic rates were measured inmicrocosms containing shoots of Potamogeton pectinatus L. with epiphyticbiofilms in the light and dark with no flow orwith the flow velocities of 0.03 and 9 cms^–1. Photosynthesis and respirationincreased with increasing water flow velocity.Rates of oxygen respiration were positivelycorrelated to the oxygen concentration of thewater. Nitrification was not significantlyaffected by flow velocity, but nitrificationwas higher in light than in dark at 0.03 cms^–1, but not at 9 cm s^–1.Denitrification was higher in stagnant waterand at 9 cm s^–1 than at 0.03 cm s^–1 inthe absence of oxygen, possibly due to complexeffects of water flow velocity on the supply oforganic matter to the denitrifying bacteria.Denitrification was always inhibited in light,and negatively correlated to the oxygenconcentration in dark. Epiphyticdenitrification occurred only at low oxygenconcentrations in flowing water, whereas instagnant water, denitrification was present inalmost oxygen saturated water. Therefore,because there are little of water movements andhigh oxygen consumption in dense stands ofsubmersed macrophytes, significant rates ofepiphytic denitrification can probably be foundwithin submersed vegetation despite high oxygenconcentrations in the surrounding water. Inconclusion, this study shows that the waterflow and oxygen metabolism within submersedvegetation have minor effects on nitrification,but significantly affect denitrification inbiofilms on submersed macrophytes.     

Factors influencing the stability of nutrient-enriched freshwater macrophyte communities: the role of sticklebacks Pungitius pungitius and freshwater snails

1. Enclosures, with the following characteristics, were used to investigate the effects of increased nutrient (nitrate and phosphate) loading on freshwater communities: (a) controls with normal densities of invertebrates, and including macrophytes, molluscs and fish (sticklebacks), and communities similar to the controls but with (b) much reduced snail densities and (c) increased fish densities. 2. The addition of nutrients resulted in increases in the biomass of phytoplankton and epiphyton, as well as zooplankton counts, in all the experimental enclosures. 3. A reduction in the snail population density resulted in a significant decrease in phytoplankton density and an increase, compared to controls, in the density of epiphytic algae growing on the leaves of the submerged macrophytes, Potamogeton crispus and Groenlandia densa. It also resulted in a significant increase (3.5 times that in the controls) in chlorophyll-a values of epilithic algae growing on glass slides, an increase in senescent tissue in macrophytes and a decline in the mass of Groenlandia densa by the end of the experiment. 4. An increase in the fish population resulted in a significant decline, compared to the controls, in the densities of zooplankton (including Chydorus sphaericus and copepods), Lymnaea peregra (from May to mid-June), Asellus meridianus, Crangonyx pseudagracilis, Culex (P.) molestus and Physa fontinalis. It also resulted in a change in the composition of the phytoplankton, with an increase in the large colonial Eudorina elegans and a decline in smaller planktonic algae, including Ankistrodesmus and Cryptomonas spp. 5. The possible mechanisms responsible for the effects are evaluated and their relevance to conservation and management of freshwater macrophyte communities are discussed.     

Biomass and oxygen dynamics of the epiphyte community in a Danish lowland stream

SUMMARY. 1. We examined the abundance and oxygen metabolism of epiphytic organisms on the dominant macrophyte, Potamogeton pectinatus , in headwaters of the eutrophic River Suså. Microbenthic algae were abundant in the stream during spring and macrophytes during summer.
2. The low macrophyte biomass in spring supported a dense epiphyte cover whereas the high macrophyte biomass during summer had a thin epiphyte cover of 10–100-fold lower abundance per unit area of macrophyte surface. The epiphyte community was dominated by microalgae in spring and by heterotrophs, probably bacteria, during summer. This seasonal shift was shown by pronounced reductions of the chlorophyll a content (from 2–3% to 0.1–0.7% of organic DW), the gross photosynthetic rate (from 20–85 to 3–15 mg O₂, g-¹ organic DW h⁻¹) and the ratio of gross photosynthesis to dark respiration in the epiphyte community (from 5–18 to 1). The reduced contributions of epiphytic microalgae correlated with reduced light availability during summer.
3. Both the density and the photosynthetic activity of epiphytic algae were low on a stream area basis relative to those of microbenthic algae and macrophytes. Rapid variations in water velocity and extensive light attenuation in water and macrophyte stands probably constrained the development of epiphytic algae. The epiphyte community was more important in overall stream respiration, contributing c. 10% to total summer respiration and c. 20% to summer respiration within the predominantly heterotrophic microbial communities on sediments and macrophyte surfaces.     

螺类与着生藻类的相互作用及其对沉水植物的影响

浅水湖泊的富营养化常导致水生植被的退化与浮游藻类的爆发［１０,１８,２９］。可利用光通常是决定沉水植物分布、生物量和生产力的最重要因子,因此,伴随高营养负荷的浮游藻类繁殖,极大地削弱了沉水植物的光合能力［２０］。然而,Ｐｈｉｌｉｐｓ等人［２８］认为,...     

INTERACTIONS BETWEEN EPIPHYTES, MACROPHYTES AND FRESHWATER SNAILS: A REVIEW

Epiphyton-feeding snails are often a conspicuous feature ofthe invertebrate fauna associated with submerged freshwatermacrophytes. In this paper I review the different interactionstaking place between snails, epiphyton and macrophytes. Studies on grazing by freshwater snails show that snails havea great impact on the biomass, productivity and species compositionof epiphytic communities. Direct effects of grazing on livingmacrophytes are probably of minor importance, but snails havea significant indirect effect on macrophytes by reducing thedetrimental impact of epiphyton (e.g. shading and competitionfor nutrients). Predators of snails can have a mediating effecton snail-epiphyton-macrophyte interactions, both through a directpredatorprey relationship (reducing the density of snails) andby inducing a habitat displacement of the snails. In a studyon the effects of predation by the pumpkinseed sunfish (a specializedsnail predator) it was found that predation indirectly affectsthe biomass and species composition of epiphytic algae by regulatingthe density of snails.     

Seasonal succession of lotic epiphyton: effects of macrophyte cover and grazing by chironomid larvae

The epiphyton is a community of periphyton on the aquatic plants. The seasonal dynamics of biomass, production and community structure of epiphytic algae on Potamogeton perfoliatus was studied at the Moscow River reach with low flow velocity from May to October 2000. The relative importance of physical-chemical environmental variables and herbivorous pressure in structuring the epiphyton community was accessed using multivariate redundancy analysis. Algal composition was mainly determined by the macrophyte cover, suggestion the overriding importance of the underwater light availability in seasonal development of epiphyton. There was a significant positive relationship between biomass of attached diatom Cocconeis placentula and abundance of chironomid larvae (r = 0.57; p < 0.01). This indicates that increase in herbivorous pressure caused the shift in epiphyton community structure. However, the intensity of herbivorous impact on epiphyton was ultimately determined by light availability in macrophyte stands. At low irradiance level and consequent slow growth of algae the upper layer of epiphyton was seriously destroyed by chironomids that resulted in dominance of C. placentula. As irradiance level and epiphyton productivity increased chironomids were unable to prevent biomass accumulation of the upper layer thus inhibiting the growth of C. placentula. The high concentrations of nutrients and relatively stable discharge determined the crucial role of light regime in the regulation of trophic interactions between epiphyton and chironomid larvae.     

Sources of nutrients to rooted submerged macrophytes growing in a nutrient-rich stream

1. The relative contribution of roots and leaves to nutrient uptake by submerged stream macrophytes was tested in experiments where plants were grown in an outdoor flow-channel system. Water was supplied from a nutrient-rich stream with inorganic nitrogen and phosphorus concentrations typical of Danish streams.
2. Four submerged macrophyte species were tested, Elodea canadensis , Callitriche cophocarpa , Ranunculus aquatilis and Potamogeton crispus, and all species were able to satisfy their demand for mineral nutrients by leaf nutrient uptake alone. This was evident from manipulative experiments showing that removal of the roots had no negative impact on the relative growth rate of the plants. Further, the organic N and P concentrations of the plant tissue was constant with time for the de-rooted plants.
3. Enrichment of water and/or sediment had no effect on the relative growth rate of two species, E. canadensis and C. cophocarpa , indicating that in situ nutrient availability was sufficient to cover the needs for growth. Despite the lack of a response in growth rate, a reduced root/shoot biomass ratio was observed with nutrient enrichment of water and/or sediment, and an increased tissue-P concentration in response to open-water enrichment.
4. The open-water nutrient concentrations of the stream in which the experiments were performed are in the upper part of the range found for Danish farmland streams (the majority of Danish streams). Still, however, the negligible effect of nutrient enrichment on the growth of submerged macrophytes observed suggests that mineral nutrient availability might play a minor role in controlling macrophyte growth in most Danish streams.     

Nutrient dependent effects of consumer identity and diversity on freshwater ecosystem function

1. Over the past decade, ecologists have tried to determine how changes in species composition and diversity affect ecosystem structure and function. Until recently, the majority of these studies have been conducted in terrestrial ecosystems and have not taken into account environmental variability. The purpose of this research was to determine how species identity and diversity in the freshwater zooplankton affected biomass of algae and zooplankton at two levels of nutrient enrichment.
2. Several species of cladocerans were grown alone and together in microcosms at both ambient and raised phosphorus concentrations to determine if the effects of consumer identity and diversity were nutrient dependent.
3. Total zooplankton biomass was greater, while algal biomass was lower, in mixed culture than in monoculture. The effects of zooplankton diversity on algal biomass, however, were only observed at raised phosphorus concentrations, suggesting that diversity effects were nutrient dependent. Specifically, diversity effects appeared to be related with biological mechanisms such as complementarity in resource use and/or facilitation.
4. More diverse communities of zooplankton appear to be better able to control algae than single species of zooplankton at high nutrient concentrations; therefore, zooplankton diversity may provide a buffer against eutrophication in freshwater ecosystems.     

Morphological responses of a submerged macrophyte to epiphyton

Epiphyton might have distinctive influence on the morphology of substrate macrophyte. In this article, we evaluate the influence of epiphyton on the morphological characteristics of their substrate submerged macrophyte, Potamogeton perfoliatus under two light intensities. The experiment was carried out for a period of 84 days in 12 glass aquaria under laboratory conditions. It was based on a 2 × 2 factorial design with epiphyton status (present or absent) and light intensity (200 or 80 μE m⁻² s⁻¹). Both epiphyton and light intensity had significant effects on the morphology and biomass allocation of the experimental plants. The average number of leaves, total length of newly recruited shoots and diameter of stems were greater in the epiphyton-free control plants than in the epiphyton-colonized plants under low light conditions. The plants with epiphyton allocated more biomass in their rhizomes and roots (% relative to total biomass basis) when compared to the control plants in both light intensities. There were also significant epiphyton–light interactions. The control plants under low light intensity showed higher internodal elongation in their main shoots when compared to the plants under high light intensity as an adaptation mechanism. Whereas the plants with epiphyton did not show such an adaptation. The new shoots of the control plants under low light intensity did not show any internodal elongation as observed in the main shoots. Furthermore, the length of the leaves of main shoots was larger in control plants with epiphyton and high light intensity than in plants with epiphyton and low light intensity, but such a variation was absent in the new shoots. We conclude that the long-term colonization by epiphyton and their shading effects induced the observed morphological changes in plants.     

Diatom ecological guilds display distinct and predictable behavior along nutrient and disturbance gradients in running waters

Three diatom ecological guilds were distinguished based on their potential to tolerate nutrient limitation and physical disturbance, i.e. a low profile, high profile, and motile guild. The guild distributions were examined along nutrient and flow disturbance gradients and across habitats in two extensively sampled streams. The guilds showed distinct distributional patterns, i.e. the low profile guild was favored in nutrient-poor and high disturbance habitats; the high profile guild reached a maximum in nutrient-rich sites and in conditions of low flow disturbance; and the motile guild increased along the nutrient gradients and decreased along the disturbance gradient. Guild distribution was habitat-specific: the low profile guild dominated the epipsammon, the high profile guild showed preference for epilithon and epiphyton, and the motile guild—for epipelon. The highest guild diversity was observed at high nutrient levels across all habitats, at higher flow disturbance levels, and in the epipelon and epiphyton. Comparisons of species, guild, and environmental distances, derived from species counts, guild abundance, and physico-chemical data, respectively, revealed high congruence between species–environment and guild–environment correlations. The predictable behavior of the three ecological guilds along nutrient and disturbance gradients, and across major benthic habitats elucidates the functional value of different diatom growth morphologies in species–environment interactions and suggests a potential use in ecological assessments of human-impacted ecosystems.     

Watershed urbanization alters the composition and function of stream bacterial communities

Watershed urbanization leads to dramatic changes in draining streams, with urban streams receiving a high frequency of scouring flows, together with the nutrient, contaminant, and thermal pollution associated with urbanization. These changes are known to cause significant losses of sensitive insect and fish species from urban streams, yet little is known about how these changes affect the composition and function of stream microbial communities. Over the course of two years, we repeatedly sampled sediments from eight central North Carolina streams affected to varying degrees by watershed urbanization. For each stream and sampling date, we characterized both overall and denitrifying bacterial communities and measured denitrification potentials. Denitrification is an ecologically important process, mediated by denitrifying bacteria that use nitrate and organic carbon as substrates. Differences in overall and denitrifying bacterial community composition were strongly associated with the gradient in urbanization. Denitrification potentials, which varied widely, were not significantly associated with substrate supply. By incorporating information on the community composition of denitrifying bacteria together with substrate supply in a linear mixed-effects model, we explained 45% of the variation in denitrification potential (p-value<0.001). Our results suggest that (1) the composition of stream bacterial communities change in response to watershed urbanization and (2) such changes may have important consequences for critical ecosystem functions such as denitrification.     

Antibacterial compounds in estuarine submersed aquatic plants

Estuarine seagrasses often exist in nutrient-rich waters and thus might benefit from mechanisms to control exterior growth of epiphytic microorganisms. In this study, we tested extracts from three estuarine seagrass species, Potamogeton pectinatus L. (sago pondweed), Potamogeton perfoliatus L. (redhead grass) and Ruppia maritima L. (wigeon grass), for antibacterial activity. Methanolic extracts of axenic cultured plants were effective against all gram-positive bacteria tested, inhibiting growth of 12 species in the genera Micrococcus, Staphylococcus, Streptococcus, Bacillus, Aerococcus, Mycobacterium and Corynebacterium. Some gram-negative species in the genera Vibrio, Listonella and Pasteurella were also sensitive. Other gram-negative bacteria were resistant. Antibacterial activity was detected in field-collected plants in spring, but appeared absent in plants collected in fall. These seagrass species thus appear to possess an antibacterial agent. It has not been identified and may consist of one or multiple compounds. The antibacterial agent in P. pectinatus was released into the water column in vitro at concentrations sufficient to inhibit or kill sensitive bacterial species. The agent was stable, with an in vitro half-life of 12 days at 25 °C and 6.6 days at 37 °C. It is possible that antibacterial production has ecological effects upon the bacterial and faunal communities associated with seagrass ecosystems.     

Response of epiphytic algae to nutrient loading and fish density in a shallow lake: a mesocosm experiment

Studies on the effect of eutrophication on the ecology of shallow lakes, usually pay scant attention to changes within the epiphytic algal community, though the contribution of this to the ecosystem dynamics is transcendental. In order to test the influence of nutrient loadings and fish densities in the structure of algal epiphyton in a shallow lake, an experiment was performed using in situ mesocosms. Nutrient additions were related to significant decreases in the total epiphyton biovolume and that of bacillariophyceans and zygnematophyceans, but with increases in the abundance of cyanobacteria. The different response of algal groups at the higher nutrient concentrations (increases or decreases in their abundance and/or biovolume) can be related to their ecophysiological constraints such as different resistance to toxicant ammonium accumulation. Plant-associated macroinvertebrates numbers were positively correlated with total numbers of epiphyton. The presence of planktivorous fish enhanced the abundance and biovolume of all algal groups, except cyanobacteria. Fishes enhanced the abundance of plant-associated animals and of total epiphyton. Fish indirect effects (e.g., nutrients release) and their dietary particularities were among the factors that together with nutrients influenced epiphyton growth. The role of indirect effects of fishes and the importance of their dietary particularities are stressed as key factors to understand the processes controlling epiphyton ecology and the food web structure of shallow lakes. Handling editor: D. Ryder.     

Regulation of submersed macrophyte biomass in a temperate lowland river: Interactions between shading by bank vegetation, epiphyton and water turbidity

Growth of aquatic vegetation is often controlled by light supply, which is potentially decreased by bank vegetation, water turbidity and epiphytic biofilm. To understand the relative importance of these shading factors and the interactions between them we analysed the seasonal course of macrophyte biomass, shading by bank vegetation, turbidity of the water column and epiphytic light absorption in shaded and sunny sections of a temperate eutrophic lowland river. At a shaded site, bank vegetation decreased the light supply by 79%, 0.5 m water column by 45% and 2-week-old epiphyton by 28% during the vegetation period. Growth of submersed macrophytes, but not of epiphyton, was light-limited in the shaded sections. We found a saturation-type correlation between light supply and macrophyte biomass. Therefore, the additional light absorption of the water column or epiphyton only shortened the period of optimum light supply at the sunny site, but was crucial for macrophyte development at the shaded site. Light absorption of phytoplankton was most important in spring and that of epiphyton in late summer. Submersed macrophytes effectively retained particles and thus improved light supply of downstream stands, but this positive feedback effect was only relevant for shaded sections in summer.     

不同初始螺类密度对沉水植物刺苦草Vallisneria spinulosa Yan及其附着藻类的影响

大型水生植物及其附着藻类是浅水湖泊中的重要初级生产者。淡水螺类作为重要的初级消费者,其密度对沉水植物及其附着藻类的影响存在争议。本研究设置4种初始螺类密度（0、40、80、240 ind·m^-2）,研究淡水螺类（椭圆萝卜螺Radix swinhoei H.Adams）对刺苦草（Vallisneria spinulosa Yan）及其附着藻类的直接牧食作用和螺类种群的变化。结果显示,在添加螺处理中,刺苦草和人工基质表面附着藻类的生物量显著降低,同时沉水植物的生长显著增加,在较高初始密度螺类处理中刺苦草产生更多的分株。到实验结束时,螺类的死亡率较高,但3个有螺处理间螺类鲜重无显著差异,而高初始密度螺类条件下的最终密度仍较高,同时个体重量（均重）也较小。在中富营养条件下淡水螺类可以直接牧食沉水植物叶片,但对植物生长的抑制作用不显著,有可能是因为沉水植物并不能作为唯一的食物来源维持螺类种群,同时螺类的种群结构受到水体营养水平等因素的制约。     

Global change and plant-ecosystem functioning in freshwaters

Freshwater ecosystems are of worldwide importance for maintaining biodiversity and sustaining the provision of a myriad of ecosystem services to modern societies. Plants, one of the most important components of these ecosystems, are key to water nutrient removal, carbon storage, and food provision. Understanding how the functional connection between freshwater plants and ecosystems is affected by global change will be key to our ability to predict future changes in freshwater systems. Here, we synthesize global plant responses, adaptations, and feedbacks to present-day and future freshwater environments through trait-based approaches, from single individuals to entire communities. We outline the transdisciplinary knowledge benchmarks needed to further understand freshwater plant biodiversity and the fundamental services they provide.     

Epiphyton as a niche for ammonia-oxidizing bacteria: detailed comparison with benthic and pelagic compartments in shallow freshwater lakes

Next to the benthic and pelagic compartments, the epiphyton of submerged macrophytes may offer an additional niche for ammonia-oxidizing bacteria in shallow freshwater lakes. In this study, we explored the potential activities and community compositions of ammonia-oxidizing bacteria of the epiphytic, benthic, and pelagic compartments of seven shallow freshwater lakes which differed in their trophic status, distribution of submerged macrophytes, and restoration history. PCR-denaturing gradient gel electrophoresis analyses demonstrated that the epiphytic compartment was inhabited by species belonging to cluster 3 of the Nitrosospira lineage and to the Nitrosomonas oligotropha lineage. Both the ammonia-oxidizing bacterial community compositions and the potential activities differed significantly between compartments. Interestingly, both the ammonia-oxidizing bacterial community composition and potential activity were influenced by the restoration status of the different lakes investigated.