Structure of the diatom community of the River Adige (North-Eastern Italy) along a hydrological gradient

Physical constrains such as water discharge, suspended solids and turbidity act as dominant factors in driving the planktonic diatom assemblages of the River Adige (North-Eastern Italy). Two sampling stations, characterised by different hydromorphological features (Cortina all’Adige and Boara Pisani, with torrential and more potamal characteristics, respectively) were sampled fortnightly following an integrated approach encompassing physical, chemical and biological measurements and aiming at identifying the dominant factors controlling the temporal development of the community. A morpho-functional approach was used to classify the diatom assemblages where Morpho-Functional Diatom Groups (MFDG) were defined for diatom genera, according to their morphology, habitat selection and modality of adhesion to river substrate. In the two sampling points, algal growth was never limited by nutrients or zooplankton. The irregular development of MFDG was determined by the stochastic hydrological events and changes in variables related to water discharge (suspended solids and light attenuation). Tychoplanktonic, benthic and drifted taxa (such as Diatoma spp., Encyonema spp., Navicula spp. and Nitzschia spp.) were dominant in the torrential station (Cortina all’Adige), while the contribution of euplanktonic unicellular centric taxa (such as Cyclotella spp., and Stephanodiscus spp.) was higher in the potamal station (Boara Pisani).     

Changes in riverine benthic diatom community structure along a chloride gradient

The application of salt is the primary means of deicing roads and highways in colder regions of north-eastern North America. This has increased the chloride concentrations of many lake and stream ecosystems. While this salinization has been documented, less is known about how increased salinity alters benthic communities in downstream ecosystems. Natural thresholds, at which there are large scale changes in community composition, have not yet been established for many types of contaminants, including chloride. The diatom community, which is sensitive to small changes in the ambient environment, has the potential to be a strong indicator of salinization effects on stream ecosystems. In this study, we sampled diatom communities in 41 streams across a salinity gradient throughout south central Ontario, Canada. We sampled benthic diatom assemblages in early May following complete snowmelt, when stream water chloride concentrations ranged from 5 to 502 mg/L. Based on redundancy analysis, we confirmed a strong association between the species composition of the diatom community and water conductivity, a commonly used index of stream salinity. Taxa indicator threshold analyses (TITAN) indicated the community changed substantially at chloride concentrations greater than 35 mg/L. We also found that, an indicator taxa, Meridion circulare, was extremely sensitive to high concentrations of salt and negatively correlated with chloride. In a wide synoptic survey of streams of our region, we found that streams in most developed watersheds exceed tolerance thresholds for benthic diatom communities. This work suggests that current chloride concentrations in urban watersheds are greatly exceeding the benthic community thresholds, for which improved management and regulatory practices are needed. Salinization thus appears to be an important feature of urban streams and needs to be considered as an important ecological driver in future studies.     

Genetic diversity patterns of microbial communities in a subtropical riverine ecosystem (Jiulong River,southeast China)

Prokaryotic and eukaryotic microbes are key organisms in aquatic ecosystems and play pivotal roles in the biogeochemical cycles, but little is known about genetic diversity of these communities in subtropical rivers. In this study, microbial planktonic communities were determined by using denaturing gradient gel electrophoresis (DGGE) analysis from the Jiulong River, southeast China, and their relationships with local environmental factors were studied. The Betaproteobacteria (26%) and Dinophyceae (26%) were the most dominant taxa in prokaryotic and eukaryotic clones derived from DGGE bands, respectively. Further, both cluster and ordination analyses of prokaryotic and eukaryotic DGGE fingerprinting resulted in three identical groups from the 15 sites, which were closely related with the environmental factors. Partial redundancy analysis (partial RDA) revealed that agricultural pollution (phosphorus and nitrogen) and saltwater intrusion (conductivity and salinity) were the main factors impacting microbial community composition, by explaining more than two-thirds of the total variation in both prokaryotic (67.0%) and eukaryotic (70.5%) communities. Moreover, the robust and quantifiable relationship between DGGE results and environmental variables indicated that the community-level molecular fingerprinting techniques could support the physicochemical assessment of riverine water quality and ecosystem health.     

Aquatic Habitat Dynamics along a Braided Alpine River Ecosystem (Tagliamento River, Northeast Italy)

Aquatic habitat change caused by flooding was quantified along the Fiume Tagliamento, a morphologically intact gravel-bed river ecosystem in northeast Italy. Five different geomorphic reaches (each around 1.5 km), ranging from near the headwaters at 800 m above sea level (a.s.l.) to near the mouth at 5 m a.s.l., were studied over a 1-year period. All floodplain water bodies in each reach were delineated in August 1999 using a differential global positioning system. Each reach was remapped twice (in December 1999 and August 2000) to investigate the impact of autumn and spring flood seasons on aquatic habitat composition and configuration. A high degree (nearly 62%) of aquatic habitat turnover was documented in a braided headwater floodplain. The degree of aquatic habitat turnover decreased with decreasing elevation to approximately 20% turnover in a meandering reach at 5 m a.s.l. In contrast to turnover, braiding, sinuosity, and aquatic habitat composition changed little in response to flooding in all reaches. Location of aquatic habitats in floodplains changed considerably (turnover), whereas habitat configuration and composition remained relatively stable. These results support the applicability of the shifting mosaic steady-state model to riverine floodplain environments.     

Application of the microbial community coalescence concept to riverine networks

Flows of water, soil, litter, and anthropogenic materials in and around rivers lead to the mixing of their resident microbial communities and subsequently to a resultant community distinct from its precursors. Consideration of these events through a new conceptual lens, namely, community coalescence, could provide a means of integrating physical, environmental, and ecological mechanisms to predict microbial community assembly patterns better in these habitats. Here, we review field studies of microbial communities in riverine habitats where environmental mixing regularly occurs, interpret some of these studies within the community coalescence framework and posit novel hypotheses and insights that may be gained in riverine microbial ecology through the application of this concept. Particularly in the face of a changing climate and rivers under increasing anthropogenic pressures, knowledge about the factors governing microbial community assembly is essential to forecast and/or respond to changes in ecosystem function. Additionally, there is the potential for microbial ecology studies in rivers to become a driver of theory development: riverine systems are ideal for coalescence studies because regular and predictable environmental mixing occurs. Data appropriate for testing community coalescence theory could be collected with minimal alteration to existing study designs.     

Salinity responses of benthic microbial communities in a solar saltern (Eilat, Israel)

The salinity responses of cyanobacteria, anoxygenic phototrophs, sulfate reducers, and methanogens from the laminated endoevaporitic community in the solar salterns of Eilat, Israel, were studied in situ with oxygen microelectrodes and in the laboratory in slurries. The optimum salinity for the sulfate reduction rate in sediment slurries was between 100 and 120 per thousand, and sulfate reduction was strongly inhibited at an in situ salinity of 215 per thousand. Nevertheless, sulfate reduction was an important respiratory process in the crust, and reoxidation of formed sulfide accounted for a major part of the oxygen budget. Methanogens were well adapted to the in situ salinity but contributed little to the anaerobic mineralization in the crust. In slurries with a salinity of 180 per thousand or less, methanogens were inhibited by increased activity of sulfate-reducing bacteria. Unicellular and filamentous cyanobacteria metabolized at near-optimum rates at the in situ salinity, whereas the optimum salinity for anoxygenic phototrophs was between 100 and 120 per thousand.     

Dynamics and control of continuous microbial propagators to subject substrate inhibition

Inhibitory substrate levels are common in industrial fermentations and in biological waste-water treatment of many industrial wastes. Continuous microbial cultures are unstable to certain disturbances, such as shock loading by inhibitory substrates. Two feedback proportional control strategies are analyzed and compared for a simple model culture assumed represent able by the culture concentrations of biomass and a single rate-limiting and growth-limiting nutrient (substrate). One control strategy, the well known turbidostat, consists of adjusting culture holding time (e.g., by flow rate adjustment) in response to deviations in turbidity or some other measure of culture biomass concentration. The other control strategy is to adjust holding time in response to deviations in limiting nutrient concentrations in the culture. This second control strategy, termed the nutristat, can be superior to the turbidostat in many applications. The sign and magnitude of the dimensionless group {(X /YD )[dμ/dS]s }, is shown to be an important determinant, in the behavior of the open loop and the two closed loop processes. This characteristic group is positive when the specific growth rate is increased by increases in the nutrient concentration, zero when the growth rate is unaffected by the nutrient concentration, and negative in the presence of nutrient or substrate inhibition. The effects of process modifications and of modeling assumptions on the control of the process are discussed and more sophisticated control schemes are also proposed.     

Oligochaeta of the middle Po River (Italy): principal component analysis of the benthic data

The application of principal component analysis to two types of habitat (the benthos of macrophytes and of central river bed) enabled us to single out some of the factors that affect the dynamics and the structure of the oligochaete population and its various reactions to environmental conditions. As regards macrophytes, the distribution of the variables on the basis of the first component is correlated, to a certain extent, with a seasonal factor without any significant differences among sites. The largest population is most closely correlated with the summer months. In fact, we found that the Naididae and Tubificidae species generally develop in larger numbers at higher temperatures. For the Tubificidae, we could detect a precise seasonal cycle. In the central river bed habitat, the first component was correlated with the river discharge, which determines the granulometric characteristics of the sediment; we noticed a correlation among the sites that have the same characteristics, regardless of sampling site or date. The species which correlate most closely among themselves are the Tubificidae Limnodrilus hoffmeisteri, Tubifex tubifex, L. udekemianus and L. profundicola, which are very characteristic of environments that contain abundant organic matter. The second component is correlated with temperature, and hence with the availability of oxygen, which determines the presence and the abundance of more sensitive species.     

Seasonal fluctuations in ionic concentrations drive microbial succession in a hypersaline lake community

Microbial community succession was examined over a two-year period using spatially and temporally coordinated water chemistry measurements, metagenomic sequencing, phylogenetic binning and de novo metagenomic assembly in the extreme hypersaline habitat of Lake Tyrrell, Victoria, Australia. Relative abundances of Haloquadratum-related sequences were positively correlated with co-varying concentrations of potassium, magnesium and sulfate, but not sodium, chloride or calcium ions, while relative abundances of Halorubrum, Haloarcula, Halonotius, Halobaculum and Salinibacter-related sequences correlated negatively with Haloquadratum and these same ionic factors. Nanohaloarchaea and Halorhabdus-related sequence abundances were inversely correlated with each other, but not other taxonomic groups. These data, along with predicted gene functions from nearly-complete assembled population metagenomes, suggest different ecological phenotypes for Nanohaloarchaea and Halorhabdus-related strains versus other community members. Nucleotide percent G+C compositions were consistently lower in community metagenomic reads from summer versus winter samples. The same seasonal G+C trends were observed within taxonomically binned read subsets from each of seven different genus-level archaeal groups. Relative seasonal abundances were also linked to percent G+C for assembled population genomes. Together, these data suggest that extreme ionic conditions may exert selective pressure on archaeal populations at the level of genomic nucleotide composition, thus contributing to seasonal successional processes. Despite the unavailability of cultured representatives for most of the organisms identified in this study, effective coordination of physical and biological measurements has enabled discovery and quantification of unexpected taxon-specific, environmentally mediated factors influencing microbial community structure.     

Effects of water-level fluctuations on the littoral benthic fish community in lakes: a mesocosm experiment

Changes in the water level of lakes, either natural or man-made,are important environmental perturbations for eulittoral benthicfish communities. In outdoor mesocosm experiments, we testedthe effects of decreasing shelter availability due to autumnlake-level decrease on the behavior and the growth of two littoralbenthic dwellers, the juvenile burbot and the stone loach. Thetwo species showed significantly different changes in behaviorwhen shelter availability decreased. Burbot built up a distincthierarchy when shelter became sparse, with larger fish beingsignificantly more successful in competing for suitable shelterthan smaller conspecifics. The hierarchy, however, ceased whenshelter availability decreased below a certain level. The largestfish then increasingly abandoned shelter use, while smallerfish persisted with their sheltering behavior. Stone loach,in contrast, showed no hierarchical order or size-related shelteruse. Only burbot showed a significant relationship between theability to occupy the preferred shelter and the somatic growth.These two species provide an example of different strategiesto deal with environmental perturbations like lake-level decrease,with the stenoecious, dominant strategy of the burbot and theeuryoecious, evasive strategy of stone loach. The results demonstratethe importance of conducting behavioral studies under as naturalconditions as possible.     

Inundation Dynamics in Braided Floodplains: Tagliamento River, Northeast Italy

The relationships among water level, inundated area, and shoreline dynamics were investigated in a bar-braided and an island-braided floodplain of the Tagliamento River in northeast Italy. Ground-based surveys with a differential global positioning system (aGPS) unit were used to delineate all aquatic-terrestrial interfaces (shorelines) in the active floodplain at different water levels. Despite complex inundation patterns, a highly significant (P < 0.00001) linear relationship between water level and arcsine square root of inundated area was found in both reaches (y = 0.49x + 0.07). A highly significant (P < 0.00009) second-order polynomial relationship occurred between water level and shoreline length (y = 87.83 − 65.85x² + 169.83x). Using these relationships as simple predictive models, we converted several years of water-level data into predictions for degree of inundation and shoreline length. The plot of the simulated degree of inundation strongly resembled the actual hydrograph. Complete inundation of the active floodplains occurred one or two times per year; however, the degree of inundation at lower water levels was highly dynamic during most of the year. Simulated shoreline length averaged 171 m ha⁻¹ (13.6 km km⁻¹), with a maximum of 197 m ha⁻¹ (15.6 km km⁻¹) occurring during periods with intermediate water levels. The corresponding values determined with GPS were somewhat higher, with an average value of 181 m ha⁻¹ (14.4 km km⁻¹) and a maximum of 214 m ha⁻¹ (16.3 km km⁻¹). During major flood events, actual shoreline length decreased to 28 m ha⁻¹ (2.1 km km⁻¹). Braiding index and upstream surface hydrologic connectivity were positively related to water level, whereas total area of isolated water bodies was negatively related to water level. The number of nodes remained high most of the time during the 2-year study period.     

Annual cycles of benthic community oxygen uptake in a deep oligotrophic lake (Attersee,Austria)

Benthic community oxygen uptake of Lake Attersee sediments was measured between 1976 and 1979, along two profiles at 25, 50 and 100 m depth. Profile I was situated in the bay of Unterach into which the main tributary, Mondsee-Ache, discharges a high load of organic matter. Profile II was chosen at Weyregg to avoid the eutrophying effect of Mondsee-Ache. Oxygen uptake rates of Unterach sediments at 25 and 50 m depth were found to be higher when compared to the other sites (mean rates: Unterach 25 m = 15.56, 50 m = 11.05 mg O₂ · m⁻² · h⁻¹; Weyregg 25 m = 6.43, 50 m = 5.14 mg O₂ · m⁻² · h⁻¹). Organic content of the uppermost sediment layer was also higher in the bay of Unterach than at Weyregg. Oxygen uptake rates of undisturbed sediment cores vary considerably throughout the year, but no simple correlation existed with variations in organic content of the sediments. Peaks of organic matter were found to concur with following peaks of oxygen uptake rates, which implies that a certain time span is necessary for transforming freshly sedimented organic matter into a state digestable for the benthic community. The retardation between increasing organic matter of the sediment and the corresponding increase of benthic oxygen uptake was different at Unterach and Weyregg respectively, which is explained by the different quality of sedimenting material.     

Multidecadal stability of benthic community structure in a high-Arctic glacial fjord (van Mijenfjord,Spitsbergen)

Long-term change in benthic community structure may have significant impact on ecosystem functions. Accelerating climate change and increased human activity in the Arctic suggest that benthic communities in this region may be expected to exhibit change over time scales coinciding with these potential stressors. In 2000 and 2001, we resampled the soft-sediment communities of van Mijenfjord, a semi-closed (silled) fjord system on the west coast of Spitsbergen, following initial surveys in 1980. Multivariate community analyses and biodiversity indices identified distinct regions within the fjord. The communities characteristic of two regions were very similar to those sampled 20 years earlier. Regions corresponded with fjord basins and to community patterns and diversity gradients identified for many other Arctic fjords. Benthic communities in open (unsilled) fjords in the area have recently been shown to respond to decadal scale climatic fluctuation. We suggest that semi-closed fjords may be less susceptible to this type of environmental variability, and that communities are shaped by an interaction of impacts from local topography, glacial runoff, local circulation patterns, and faunal life-history traits. Open and closed fjords may respond to climatic warming trends in different ways, resulting in a subsequent divergence in spatial patterns of resident communities.     

Reproductive biology of four large riverine fishes (Cyprinidae) in a man-made lake,Orange River,South Africa

The reproductive biology ofBarbus holubi, B. kimberleyensis, Labeo capensis andL. umbratus was examined in a large reservoir on the Orange River, South Africa. The findings are integrated into the existing knowledge on largeBarbus andLabeo species, which coexist in most river systems in Africa and Asia.LargeBarbus spawn on gravel beds within the river channel during floods in spring or summer. In continuously flowing regulated rivers, time of spawning is governed by water temperatures. They have moderate fecundity; large eggs, incubation time of several days and the larvae are initially immobile with large yolk sacs. In the impoundment, they spawn in the inflowing regulated river withB. kimberleyensis spawning four to six weeks later than the more cold-tolerantB. holubi, the dominant largeBarbus. Survival is generally good and the juveniles disperse throughout the lake, but unseasonal release of cold water from an upstream impoundment may cause poor reproductive success.In contrast,Labeo species generally spawn on newly flooded ground, usually leaving the main river channel. Spawning may or may not be preceded by a longitudinal migration. Labeos are relatively fecund with small (30%Barbus size) eggs which hatch quickly and the larvae swim in bursts up into the water column before sinking down again.L. capensis does not require a longitudinal spawning migration and breeds throughout the lake, depending on local conditions. Large temporal variation in gonadal development within the population can result in more than one spawning. Dispersal within the lake is poor.L. umbratus uses larger inflowing tributaries for spawning thanL. capensis and its juveniles have a much greater power of dispersal. Early dependence on external feeding and undependable occurrence of conditions for spawning and juvenile feeding makes for variable reproductive success.     

Resource quantity affects benthic microbial community structure and growth efficiency in a temperate intertidal mudflat

Estuaries cover <1% of marine habitats, but the carbon dioxide (CO(2)) effluxes from these net heterotrophic systems contribute significantly to the global carbon cycle. Anthropogenic eutrophication of estuarine waterways increases the supply of labile substrates to the underlying sediments. How such changes affect the form and functioning of the resident microbial communities remains unclear. We employed a carbon-13 pulse-chase experiment to investigate how a temperate estuarine benthic microbial community at 6.5°C responded to additions of marine diatom-derived organic carbon equivalent to 4.16, 41.60 and 416.00 mmol C m(-2). The quantities of carbon mineralized and incorporated into bacterial biomass both increased significantly, albeit differentially, with resource supply. This resulted in bacterial growth efficiency increasing from 0.40 ± 0.02 to 0.55 ± 0.04 as substrates became more available. The proportions of diatom-derived carbon incorporated into individual microbial membrane fatty acids also varied with resource supply. Future increases in labile organic substrate supply have the potential to increase both the proportion of organic carbon being retained within the benthic compartment of estuaries and also the absolute quantity of CO(2) outgassing from these environments.     

Dynamics of microbial community during bioremediation of phenanthrene and chromium(VI)-contaminated soil microcosms

The combined effect of phenanthrene and Cr(VI) on soil microbial activity, community composition and on the efficiency of bioremediation processes has been studied. Biometer flask systems and soil microcosm systems contaminated with 2,000 mg of phenanthrene per kg of dry soil and different Cr(VI) concentrations were investigated. Temperature, soil moisture and oxygen availability were controlled to support bioremediation. Cr(VI) inhibited the phenanthrene mineralization (CO₂ production) and cultivable PAH degrading bacteria at levels of 500–2,600 mg kg⁻¹. In the bioremediation experiments in soil microcosms the degradation of phenanthrene, the dehydrogenase activity and the increase in PAH degrading bacteria counts were retarded by the presence of Cr(VI) at all studied concentrations (25, 50 and 100 mg kg⁻¹). These negative effects did not show a correlation with Cr(VI) concentration. Whereas the presence of Cr(VI) had a negative effect on the phenanthrene elimination rate, co-contamination with phenanthrene reduced the residual Cr(VI) concentration in the water exchangeable Cr(VI) fraction (WEF) in comparison with the soil microcosm contaminated only with Cr(VI). Clear differences were found between the denaturing gradient gel electrophoresis (DGGE) patterns of each soil microcosm, showing that the presence of different Cr(VI) concentrations did modulate the community response to phenanthrene and caused perdurable changes in the structure of the microbial soil community.     

重庆五布河壶穴形态及底栖动物群落特征

壶穴是水流与河床相互作用的产物,既是河床微地貌的组成部分,又是河流生态系统中特殊生境之一。本文对重庆五布河壶穴形态特征和壶穴内部底栖动物进行调查,统计分析山地河流壶穴形态特征、底栖动物群落结构及壶穴面积与底栖动物丰度的关系。结果表明:壶穴长短轴比介于1.0~1.2的比例最高,壶穴口径大多分布于10~20 cm,壶穴长轴和短轴、口径和深度存在较强的(对数)正相关关系;壶穴中共采集底栖动物34种,优势类群为四节蜉科、划蝽科、细蜉科;底栖动物功能摄食类群之间存在显著性差异,收集者数量最多,而撕食者最少。壶穴面积与底栖无脊椎动物丰度存在显著的正相关关系。     

Analysis of a pioneer community of Dittrichia viscosa (L.) Greuter in a mediterranean environment in southern Italy

Abstract

A pioneer community dominated by Dittrichia viscosa (L.) Greuter in a Mediterranean environment in southern Italy is described from a floristic point of view. Three main aspects are defined in a ploughed area in relation to substrate and soil depth. The pioneer role of D. viscosa is discussed in relation to its occurrence limited to below-ground disturbance.     

Long-term dynamics of catabolic plasmids introduced to a microbial community in a polluted environment: a mathematical model

The long-term dynamics of mobile plasmids in natural environments are unclear. This is the first study of the long-term dynamics of introduced plasmids with xenobiotic degradation abilities using a mathematical model that describes the horizontal gene transfer (HGT) of plasmids into indigenous bacteria via conjugation. We focussed on negative feedback between the spread of plasmids and their selective advantage, i.e. the severe competition between plasmid-bearing and plasmid-free bacteria resulting from a decrease in xenobiotic concentration caused by the gene expression of plasmids, favoring plasmid-free bacteria. Two types of HGT enhanced the persistence of plasmids and the degradation of the xenobiotic in different conditions: a relatively low rate of 'intergeneric HGT' from introduced to indigenous bacteria and a high rate of 'intraindigenous HGT' from indigenous to indigenous bacteria. In addition, when the indigenous resource supply rate was high and when the cost of bearing plasmids was low, both types of HGT made large contributions to xenobiotic degradation compared to the contribution of vertical transfer via plasmid replication within the introduced host population. Initial conditions were also important; a higher initial density of introduced plasmid-bearing bacteria led to a lower degradation rate over a long time scale.