Epilithic Communities Research Articles

The relationship between epilithic biofilm stability and its associated meiofauna under two patterns of flood disturbance

Habitat stability is an important driver of ecological community composition and development. River epilithic biofilms are particularly unstable habitats for the establishment of benthic communities because they are regularly disturbed by floods. Our aim was to determine the influence of habitat instability on meiobenthic organisms. We hypothesized that hydrologic variables are the most important predictors of meiofauna distribution. We monitored epilithic communities (meiofauna and microalgae) with a high sampling frequency during 2 sampling periods with contrasting hydrodynamic patterns in a temperate river (the Garonne, France). Nematodes and rotifers dominated meiofaunal assemblages. The critical flow velocity threshold for their maintenance in the biofilm was ,30 cm/s, a result suggesting that meiofauna can resist higher flow velocity within the biofilm than within sediments. Nematode distribution was primarily influenced by the duration of undisturbed periods, whereas rotifer distribution was also correlated with the thickness of the biofilm. During the periods after floods, rotifers were faster colonizers than nematodes. Collectively, our results show that flow regime was an essential driver for biofilm community development.

Temporal changes in periphytic meiofauna in lakes of different trophic states

Meiofaunal organisms in the periphyton of stony hard-substrates (epilithon) were studied in three Swedish lakes with different trophic states (oligo-, meso- and eutrophic) with respect to seasonal successions in abundance, biomass, and production. Over a period of 2 years, the meiofaunal population of all three lakes fluctuated greatly, with densities varying up to nine-fold within a season. In the oligotrophic lake, a significant decrease in meiofauna in winter was striking, whereas in the other two lakes, richer in nutrients, there was a pronounced peak in early summer. Although the lakes, on average, did not differ in epilithic organic and inorganic material, the differences in meiofaunal abundance, biomass, and production were significant. Correlation analysis revealed that altogether the meiofaunal biomass was positively related to the lakes’ trophic state (total phosphorus), while the meiofaunal abundance and production along the trophic spectrum displayed a humped-shape distribution, with maximum values measured in the mesotrophic Lake Erken (1324 ind cm-2 and 2249 mg DW cm-2 y-1). Nematodes were the dominant meiofaunal group in the epilithon of all three lakes, accounting for up to 58% in abundance, 33% in biomass and 55% in production of the whole meiofaunal community. However, their relative importance tended to decrease with increasing trophic state. Beside nematodes, rotifers, oligochaetes, copepods and tardigrades were also found in large numbers in the epilithon. Overall, the results demonstrated that, due to their high abundance, biomass, and production, meiofaunal organisms play an important role in epilithic communities.

Composition dynamics of epilithic intertidal bacterial communities exposed to high copper levels

Copper has a dual role for organisms, both as micronutrient and toxic element. Copper mining activities have an enormous ecological impact because of the extraction process and the consequent release of copper-containing waste materials to the environment. In northern Chile, mainly in the Chañaral coastal area, this phenomenon is clearly evident. The released waste material has caused a strong modification of the area, and copper enrichment of beaches and rocky shores has provoked a decrease in the richness and diversity of many species of macroorganisms. However, the effects that copper enrichment has on microbial (e.g. bacterial epilithic) communities associated with the rocky shore environment are poorly understood. Using a culture-independent molecular approach, field sampling and laboratory microcosm experiments, we determined the effects of copper enrichment on bacterial communities inhabiting the rocky shore environment. Field samples showed a strong effect of copper on the structure of the natural bacterial epilithic communities, and microcosm experiments demonstrated rapid changes in bacterial community when copper is added, and reversibility of this effect within 48h after copper is removed.

Photosynthetic activity of epilithic algal communities in sections of the Torna stream (Hungary) with natural and modified riparian shading

The photosynthetic activity of epilithon grown in the Torna-stream (Hungary) was studied. Samples were collected from natural sections with closed canopy and natural stream bed and from modified sections with trapezoid river bed and shores lacking riparian vegetation. We assumed that human modifications would have a significant impact on epilithic photosynthesis, and therefore expected to observe corresponding changes in photosynthesis–irradiance (P–I) parametric. The laboratory P–I measurements were carried out monthly between April 2008 and September 2009 with epilithon after 3 weeks of in situ colonization. The maximum rate of the photosynthesis (Pmax) correlated positively with the chlorophyll a content of the samples. Natural sites had lower biomass and Pmax values than modified sites, and the biomass differed substantially in spring and summer, the Pmax differed significantly only in spring. Natural sections had higher biomass-specific photosynthesis values than modified sections in all seasons, but the difference was significant only in summer–autumn: in summer the samples from the natural sections had significantly higher photosynthetic efficiency. In spring and summer, the photoadaptation parameters (Ik) of communities grown in modified sections were substantially higher than in natural sections. Light availability appeared to be the major factor regulating the seasonal photosynthetic activity of epilithic communities.

Epilithic community development on artificial reefs deployed along a cross-shelf environmental gradient off Paraná state, southern Brazil

Concrete modules were deployed on the bottom of the 11, 18 and 30 meters isobaths along a cross-shelf hydrographic gradient off Paraná State, Southern Brazil, with the purpose of studying the colonization of sessile epilithic macroinvertebrates on artificial surfaces. After one year of submersion a total of 63 species of epilithic organisms were identified, dominated by Ostrea puelchana, Chthamalus bisinuatus, Balanus cf spongicola, Astrangia cf rathbuni, Didemnum spp, poryphers and bryozoans. Diversity index and percent cover at reef stations placed at 11, 18 and 30 meters isobaths were respectively 2.28 and 66.7%, 2.79 and 96.6% and 1.66 and 77.4%. Differences of general community structure among the three assemblages were not clearly related to the general environmental conditions at the bottom layers near the reef stations. Turbidity and larval abundance are discussed as important factors affecting colonization processes. Results indicate that depths between 15-20 meters are more suitable for the implementation of large scale artificial reef systems in the inner shelf off Paraná and, possibly, throughout the inner shelves off southern Brazil with similar hydrographic conditions.

Warming leads to higher species turnover in a coastal ecosystem

AbstractThe responses of ecological communities and ecosystems to increased rates of environmental change will be strongly influenced by variation in the diversity of community composition. Yet, our understanding of how diversity is affected by rising temperatures is inconclusive and mainly based on indirect evidence or short‐term experiments. In our study, we analyse the diversity and species turnover of benthic epilithic communities within the thermal flume of a nuclear power plant at the Swedish coast. This flume covers the range of predicted future temperature rises. Species composition was significantly different between control sites and sites with higher temperatures. We found that temperature had little effect on the number of species in three functional groups (macroinvertebrates, benthic diatoms, and macrophytes, which here comprise multicellular algae and macroscopic colonies of unicellular algae and cyanobacteria), neither at single sampling dates nor summed for the entire observation year. However, species turnover significantly increased with increasing temperature for diatoms, macrophytes and invertebrates. Different temperature regimes resulted in significantly different species composition and indicator species. Thus, increasing temperatures in the thermal flume increased temporal beta‐diversity and decreased compositional stability of communities, although observed richness did not change at any point in time. We highlight the need to investigate the consequences of such declines in compositional stability for functional stability of ecosystem processes.

Epilithic Algae Distribution Along a Chemical Gradient in a Naturally Acidic River, Río Agrio (Patagonia, Argentina)

The epilithic algae distribution along a pH gradient and the relationship between the chemical gradient and biomass development were studied in Río Agrio, a naturally acidic river located in Patagonia (Argentina). The epilithic community was monitored during the summer of three consecutive years in sites located above and below the entrance of tributaries. The epilithic community showed differences between sites based on the chemical composition of the water and the precipitates that appear on the streambed of the river. The lowest biomass, diversity, and number of species were found at the most extreme part of the river in terms of pH (ca. 2) and element concentrations. Euglena mutabilis was the dominant species in this section of the river. As pH increased (ca. 3), the community changed to be dominated by filamentous green algae (Ulothrix spp., Mougeotia sp., Klebsormidium sp.) showing luxuriant growths in terms of biomass. With the inflow of a neutral tributary, the pH of Río Agrio increased above 3, and the precipitates of orange-red iron hydroxides appeared. The algal community was not affected by these precipitates or the low P concentrations, along the next 30 km of river downstream from this site. The apparent physical stress that the precipitates impose on algae is in fact a dynamic reservoir of P because diel cycle of Fe could be promoting precipitation and redissolution processes that binds and releases P from these precipitates. Where the pH increased above 6, precipitates of aluminum hydroxides appeared. At this site, the epilithic biomass and density decreased, some algae species changed, but the diversity and the number of species in general remained consistent with the upstream values. The physical stress of the Al precipitates on the algae is added to the chemical stress that represents the sequestering of P in these precipitates that are not redissolved, resulting P a limiting nutrient for algae growth.

Seasonal variations of metal concentrations in periphyton and taxonomic composition of the algal community at a Yenisei River littoral site

AbstractThe concentrations of metals K, Na, Ca, Mg, Fe, Mn, Zn, Cu, Ni, Pb, Co and Cr, in the water and periphyton (epilithic algal communities) were studied at a site in the middle stream of the Yenisei River (Siberia, Russia) during three years using monthly sampling frequencies. Despite considerable seasonal variations in aquatic concentrations of some metals, there was no correlation between metal contents in the water and in periphyton. Seasonal concentration variations of some metals in periphyton were related to the species (taxonomic) composition of periphytic microalgae and cyanobacteria. Enhanced levels of Ni and Co in periphyton in late autumn, winter, and early spring were likely caused by the predominance of cyanobacteria in the periphytic community, and annual maximum levels of K in periphyton in late spring and early summer were attributed to the domination of Chlorophyta, primarily Ulothrix zonata.

Viability of differentiated epilithic bacterial communities in the River Garonne (SW France)

Epilithic bacterial community viability was assessed on natural biofilm assemblages from environmentally contrasting locations over a 17-months period to determine if it reflects environmental conditions or conditions within the biofilm assemblage. Vital state was assessed by membrane integrity using LIVE/DEAD® BacLight™ staining kit. Samples were regularly collected in a large river, up and downstream of a large urban centre. Epilithic biomasses were similar between sites irrespective of the distinct water quality but varied temporarily, peaking up to 48 g AFDM m−2. Bacterial community composition assessed by 16S rDNA based PCR-DGGE significantly differed between sites. Bacterial densities (median of 2.5 × 1011 cell g AFDM−1) were stable whatever the sample origin or biomass. Viable bacterial fractions ranged between 13 and 83% of the total bacterial densities and were correlated with hydrological stability indicators (average of 41.9% during stable water periods, 62.4% during intermediate flow regimes and 50.0% during flow instability) and seasonal parameters. At the river section and epilithic community scales, consistent bacterial densities per unit of biomass could reflect a biofilm assemblage carrying capacity while variable membrane integrity likely integrates changes in the vital state of the community under changing environmental conditions.

Effects of elevated pCO2 on dissolution of coral carbonates by microbial euendoliths

Eight‐month‐old blocks of the coral Porites lobata colonized by natural Hawaiian euendolithic and epilithic communities were experimentally exposed to two different aqueous pCO2 treatments, 400 ppmv and 750 ppmv, for 3 months. The chlorophyte Ostreobium quekettii dominated communities at the start and at the end of the experiment (65–90%). There were no significant differences in the relative abundance of euendolithic species, nor were there any differences in bioeroded area at the surface of blocks (27%) between pCO2 treatments. The depth of penetration of filaments of O. quekettii was, however, significantly higher under 750 ppmv (1.4 mm) than under 400 ppmv (1 mm). Consequently, rates of carbonate dissolution measured under elevated pCO2 were 48% higher than under ambient pCO2 (0.46 kg CaCO3 dissolved m−2 a−1 versus 0.31 kg m−2 a−1). Thus, biogenic dissolution of carbonates by euendoliths in coral reefs may be a dominant mechanism of carbonate dissolution in a more acidic ocean.

Atmospheric nitrogen‐deposition may intensify phosphorus limitation of shallow epilithic periphyton in unproductive lakes

Summary1. We tested whether increasing atmospheric nitrogen (N) deposition along a north–south gradient intensifies epilithic phosphorus (P) limitation in oligotrophic Swedish lakes from the north to the south. We examined the epilithic community at a shallow depth from seven northern and six southern Swedish lakes, and also compared the results with a lake located geographically between the two groups. We determined lake nutrient state, epilithic nutrient ratios and epilithic algal composition, as well as grazer N : P ratios, grazer‐epilithon N : P imbalance, and N : P cycling ratios.2. Epilithic communities appear to be generally more N‐limited in the northern lakes and more P‐limited in the southern lakes. Lake water total N (Tot‐N) and epilithic N : P ratios were lower in northern than in southern lakes and the proportion of N2‐fixing cyanobacteria was higher in northern than in southern lakes.3. Gastropod grazers had lower N : P imbalances and cycled less N relative to P in northern than in southern lakes.4. Atmospheric N‐deposition showed a strong positive correlation with lake water Tot‐N and a much weaker positive correlation with epilithon N : P ratios. Atmospheric N‐deposition also correlated negatively with the proportion of N2‐fixing cyanobacteria.5. There are indications that increased atmospheric N‐deposition towards the south might intensify P‐limitation of epilithic algae and invertebrate grazers, although more studies are needed to show the strength and generality of our findings.

Changes in abiotic and biotic phosphorus uptake across a gradient of stream condition

AbstractDegraded streams have been shown to retain fewer nutrients than un‐modified streams. The aim of this project was to investigate the relative importance of abiotic and biotic pathways of phosphorus uptake by epilithic communities in un‐modified and modified streams. This was investigated through a series of filterable reactive phosphorus (FRP)‐uptake experiments in two streams of the Torrens River Catchment, South Australia. Total benthic FRP uptake was assessed as the loss of FRP to un‐sterilized epilithic communities (kT), abiotic uptake was the loss to sterilized epilithic communities (kA) and biotic uptake (kB) was the difference between total and abiotic uptake. It was hypothesized that un‐modified reaches would have higher kA and kB than degraded and engineered reaches. Overall, kT, kA and kB were greatest in un‐modified reaches, but this pattern was not consistent across seasons. kT and kB were greatest in the un‐modified reaches in autumn–winter and late spring, but not in winter–spring. Differences in kB were best explained by phosphorus availability in the water column and the period of continuous flow. kA was greatest in the un‐modified reaches in autumn–winter, greatest in the degraded reaches in winter–spring, but similar in the un‐modified reaches and degraded reaches in late spring. kA was most dependent upon the background FRP concentration, but also the attached organic matter in the un‐modified reaches. The project demonstrated that several impacts of changes in land‐use can alter the affinity of biotic and abiotic processes for phosphorus, which will have implications for in‐stream nutrient availability and downstream ecosystems. Copyright © 2009 John Wiley & Sons, Ltd.

A microbiological study of the self-cleaning potential of oily Arabian Gulf coasts

Due to the active production and transport of crude oil in the Arabian Gulf region, the Arabian Gulf coasts are routinely polluted with oil. Therefore, such coasts have been subject of studies aiming at assessing the roles of indigenous microbial consortia in cleaning these environments. In the present study, epilithic microbial communities along Kuwait coasts were studied for their oil degradation potential. Gravel particles coated with deep green biofilms were collected from four coastal sites in autumn, winter, and spring. Phototrophs in these consortia were determined in terms of their chlorophyll a contents and identified by their morphological characteristics. Total bacteria were counted microscopically and cultivable bacteria by the dilution plating method on nutrient agar as well as on inorganic medium containing oil as a sole source of carbon and energy. The bacterial community structures were also characterized and compared by denaturing gradient gel electrophoresis (DGGE). Epilithic biomass samples from the four sites in the three seasons were rich in diatoms and picocyanobacteria as well as total bacteria. Direct counting gave bacterial numbers per square centimeter gravel surface of 2 to 6 x 10(7) cells depending on the sampling site and season. Cultivable bacterial numbers on nutrient agar and crude oil as a sole source of carbon were 3 x 10(3) to 8 x 10(4) and 1 x 10(3) to 7 x 10(3) cells/cm(2) gravel surface, respectively. The DGGE profiles of epilithon biomass samples revealed major 16S rDNA bands that matched bands of pure oil-utilizing bacterial isolates. The microbial communities showed a degree of consistency in all sites and seasons. The microbial consortia coating gravel particles are potentially suitable tools for self-cleaning of oily Gulf coasts. They are rich in oil-utilizing bacteria whose activities are probably enhanced by oxygen produced by the phototrophic partners in the consortia. The combination of conventional microbiological analysis with molecular approaches gives an enhanced idea about natural microbial communities especially those with environmental application potential.

Longitudinal changes in bacterial community composition in river epilithic biofilms: influence of nutrients and organic matter

Epilithic bacterial communities play a major role in biogeochemical cycles of rivers; however, distributional patterns and controls of epilithic communities remain unclear. The objective of the present study was to examine possible environmental factors that affect longitudinal distri- butional patterns of epilithic bacterial communities in 2 rivers (Yasu and Ado Rivers) draining the Lake Biwa basin, Japan. Phylogenetic analyses were conducted using denaturing gradient gel electrophoresis (DGGE) of 16S rRNA genes and fluorescence in situ hybridization (FISH) with oligonucleotide probes targeted to small subunit rRNA. Non-metric multidimensional scaling and canonical correspondence analysis of the DGGE profile indicated that a clear shift in community composition occurred at the middle reach of the Yasu River. This shift was most closely related to variation in the nitrogen stable isotope ratio (δ 15 N) of epilithic materials (BIOENV analysis, ρw = 0.512), which is an indicator of the extent of anthropogenic nitrogen loading. The concentration of dissolved organic carbon (DOC) also affected community composition (ρw = 0.510 for the combina- tion of δ 15 N and DOC). The longitudinal pattern of the DGGE profile was less evident in the Ado River. In both rivers, FISH results indicated that Alphaproteobacteria and Betaproteobacteria gener- ally accounted for a large fraction (37 to 71%) of total bacterial abundance, indicating that species and clones affiliated with a limited number of the major phylogenetic groups occupy diverse epilithic habitats. Our results are consistent with the hypothesis that nutrients and organic matter of anthropo- genic origin are major determinants of epilithic bacterial community structure in rivers.

Epilithic diatoms as indicators of water quality and ecological status of streams of Sudety Mountains (South-Western Poland)

Epilithic diatom communities in high-altitude streams in the Polish part of Sudety Mountains were studied in the years 2004-2006 to assess water quality and ecological status. In the Polish river typology, these water bodies have been separated as the type 3: the Sudety stream characterized by a small catchment area (< 100 km 2 ), V- or U-shaped valley, high velocity of flow, significant channel slope and a bed composed of boulders, stones and cobbles. The stream sections under study are located in Karkonosze Mountains and in Snieznik Massif and have been chosen following the European Union's recommendations for reference sites. The Karkonosze streams possess slightly acidic or neutral waters, with very poor electrolyte content, low total hardness and low total P content. Streams in Snieznik Massif have alkaline waters, with poor or medium electrolyte content, higher total hardness and higher total P content. Altogether 184 diatom taxa were determined. Species diversity ranged from poor (8-9 taxa) to moderately rich (82 taxa). The most abundant were Diatoma mesodon, Eunotia exigua, Achnanthidium minutissimum, Fragilaria capucina var. rumpens, Encvonema minutum, Fragilariforma virescens, Gomphonema gracile and Tabellaria flocculosa. Three diatom indices were applied in order to assess water quality: the Trophic Index (TI), the Saprobic Index (SI) and the Index of Pollution Sensibility (IPS). The TI values varied from 0.53 to 1.83 and in majority of the streams indicated ultraoligotrophic or oligotrophic waters. The SI values changed from 1.09 to 1.55 and indicated none (oligosaprobic) or slight (oligo-β-mesosaprobic) organic pollution. The IPS values were usually not lower than 17.7 and indicated waters of low trophy and unpolluted or imperceptibly polluted. Most of the streams studied may serve as reference sites for the Polish type 3 of running waters, i.e. the stream of Sudety Mts.

High rates of nitrogen fixation (acetylene reduction) on coral skeletons following bleaching mortality

Nubbins of the coral Acropora aspera were artificially bleached and nitrogen fixation ( acetylene reduction) rates were measured on the developing epilithic communities. Seasonal comparisons were made between corals that died in summer of heat stress and corals that died in winter from natural cold stress. Rates of acetylene reduction from artificially bleached corals peaked at 26.66 nmol cm(-2) h(-1) 2 weeks after summer mortality, while rates from natural winter mortality peaked at 18.07 nmol cm(-2) h(-1) 12 days after coral death. Comparative rates of acetylene reduction taken from live corals and coral rubble ranged between 0.56 and 1.16 nmol cm(-2) h(-1), and 0.15 and 12.77 nmol cm(-2) h(-1), respectively. N-2-fixation rates from dead corals were up to 30 times greater than those measured on live corals. The observed increase in N-2-fixation from dead corals may increase the availability of nitrogen for use in trophic processes within the reef for an extended period following the initial mortality event. If the spatial scale over which coral mortality has occurred in past thermal bleaching events is considered the ramifications of such an increase may be substantial.

Indirect effects of brook trout (Salvelinus fontinalis) on the structure of epilithic algal communities in an oligotrophic boreal forest stream

The impact of fish on epilithic algae community structure was studied in a replicated series of artificial outdoor channels fed by a boreal forest stream (Canada) to test the hypotheses that brook trout (Salvelinus fontinalis) induce a three-trophic chain top-down change in epilithic community structure, at both taxonomic and physiognomic levels. Fish were introduced into five of ten channels and the epilithic algae growing on tiles were sampled four times during summer to determine biomass, algal physiognomy and species composition in relation to presence of fish and invertebrate community structure. Diatom biomass showed distinctive development patterns, increasing at the beginning of fish introduction and decreasing towards the end of the experiment. The responses were the reverse in the absence of fish. Chrysophyceae increased in the absence of fish, particularly at the end of the experiment. From the seven algal physiognomic types considered, epilithon overstory, attached erected algae and loosely attached algae were generally more abundant in the presence of fish. Species' biomass was markedly different in both treatments and followed contrasted temporal patterns. In the presence of fish, attached erected diatoms species rapidly reached a complex spatial overstory structure, while in the absence of fish the epilithic community remained at a younger successional stage. Partial redundancy analysis showed that those changes were mediated by large grazing invertebrates, which had lower abundance over the bottom surfaces in the presence of fish. Variance partitioning analysis reinforced the hypothesis that most of the variability in epilithic community structure was explained by fish-invertebrate interactions and that the effect of fish presence alone was not significant. The results show that reduction in total epilithic algal biomass was a transient top-down effect, but the three-trophic level control on major taxa, species composition and community physiognomy remained along the summer experiment.

Epilithic communities in a lake littoral zone: the role of water-column transport and habitat development for dispersal and colonization of meiofauna

Abstract During the past 30 y, periphyton has been recognized as a key component in the benthos of lake littoral zones. However, the role of meiofauna-sized organisms living in the periphyton and mechanisms regulating and influencing those communities largely have been neglected, and the process of community development and colonization pathways of periphytic meiofauna are unknown in lakes. We studied these processes on littoral hard substrates in an oligotrophic lake. We manipulated the access of meiofauna to artificial hard substrates for a period of 57 d in a field experiment. In one treatment, the direct colonization pathway via active crawling was prohibited by elevating substrates into the water column. Development of the meiofaunal community was compared between elevated and nonelevated (control) treatments. In addition, relationships among the meiofaunal communities on the substrates in the field experiment and those in sediment traps and on natural hard substrates were examined. Periphyton biomas...

BRANDINIA MOSIMANNIAE GEN. NOV. ET SP. NOV., A NEW MARINE EPILITHIC DIATOM FROM THE ANTARCTIC COASTS

The new solitary species Brandinia mosimanniae L. F. Fernandes &L. K. Procopiak is described for Antarctic Peninsula, living on rocks from intertidal to 14m depth sublittoral. Its main features are lanceolate valves with rostrate to capitate apices, narrow sternum but expanded at the centre, two conspicuous rimoportulae, one per apex, uniseriate striae composed of areolae bearing peg-like voláte occlusions and cingulum with open bands. Comparisons with allied species such as the colonial Fragilaria striatula Lyngbye and the related genera Fossula, Synedropsis and Ulnaria are made. B. mosimanniae was a common to abundant diatom within epilithic communities in all of the samples examined.

Origin of phytoplankton and the environmental factors governing the structure of microalgal communities in lowland streams

Information on the structure of microalgal assemblages in the epiphyton and epilithon is necessary to understand the origin of phytoplankton in lowland rivers. To this end, we carried out concurrent investigations on phytoplankton, epiphyton and epilithon in 18 reaches of three Estonian rivers during the midsummers of 2002 and 2003. A total of 251 taxa was recorded, of which 192 were epiphyton species, 158 were epilithon species and 150 were phytoplankton species. Canonical correspondence analysis (CCA), based on the 31 most abundant taxa, indicated differences in the structure of the algal assemblages between the different biotopes (phytoplankton, epiphyton and epilithon) as well as between the studied rivers. The composition of the phytoplankton clearly differed from that of the other biotopes, with prevailing small flagellates, a chrysophyte (Synura uvella) and cryptophytes (Rhodomonas lacustris and Cryptomonas erosa). The epiphyton was characterized by a large number of diatoms, while the epilithic community contained filamentous cyanobacteria (Phormidium tergestinum and Planktolyngya sp.) and a green alga (Stigeoclonium tenue) in addition to diatoms. Based on redundancy analysis (RDA), phosphorous was the most relevant parameter determining the distribution of species in the phytoplankton assemblages. Shading by trees on the river bank, dissolved oxygen concentration and water temperature as well as river width determined the distribution of species in the epiphyton. The data set on the epilithon did not reveal any significant relationships between species distribution and the measured environmental parameters.