Assessing the short-term response of stream diatoms to acidity using inter-basin transplantations and chemical diffusing substrates

1. Acid‐base status has major effects on diatoms, but there is little information on their short‐term response to changing acidity. This is despite the use of diatoms as bioindicators in streams where acid episodes are important during rainstorms (hours to days) or snowmelt (days to weeks). In the Llyn Brianne experimental catchments (Wales, UK), we attempted to mimic the effects of short‐term acidification by (i) reciprocally transplanting diatoms between two streams of contrasting acidity and (ii) using acid‐diffusing substrates. 2. Diatom diversity decreased rapidly on substrata transplanted from the circumneutral into the acidic stream, and increased in the reciprocal transplantation. Changes in dominant taxa occurred within three days in the acidic stream because of the rapid growth of Eunotia exigua, and by nine days in the circumneutral stream because of the proliferation of Achnanthidium minutissimum. Transplants were near indistinguishable from ambient assemblages by day 12. 3. There were no effects of enclosures on assemblage composition, but diatoms responded more rapidly to altered chemistry in enclosures with coarse mesh (26 × 50 mm) than finer mesh (320 μm). 4. Chemical diffusing substrates comprised terracotta tiles attached to dosing reservoirs that created locally acid (using H₂SO₄) or metal‐rich conditions (using MnSO₄) in the circumneutral stream over 26 days. Diatom responses were compared with reference substrates dosed with deionised or circumneutral stream water, and we also assessed whether effects were moderated by macroinvertebrate grazers. 5. Surface pH was lower by 1–2 pH units on acid‐dosed substrates than on reference tiles or in surrounding streamwater. Grazed assemblages on acid‐dosed substrates differed significantly from ungrazed reference assemblages, acquiring significantly greater relative abundance of Eunotia spp. However, the magnitude of response was less than in the between‐stream transplantations either because (i) metal exposure and base cation concentrations differed between the transplants and dosing substrates or (ii) diatom response to reduced pH on the diffusing substrates was restricted by the scarcity of acidobiontic diatoms in the circumneutral stream. Similar filter, founder or dominance effects might also affect diatom responses to real acid episodes. 6. These data show that diatom assemblages can respond rapidly and directly to changes in acid‐base status, but short‐term acidification might affect diatoms more rapidly than subsequent recovery. Because the experimental methods used were imperfect representations of episodic effects, diatom response to real acid events requires further field evaluation.     

Biology of epiphytic Chironomidae (Diptera:Nematocera) in chalk streams

Submerged macrophytes are a conspicuous feature of chalk stream ecosystems, supporting large populations of diatoms and invertebrates, including larvae of the nematocerous family Chironomidae. Close temporal and spatial association is evident between chironomid larvae and diatoms, and diatoms are frequently a major component of the food of midge larvae. Larvae provided with food that is rich in diatoms survive better and grow and develop more quickly than larvae that are supplied with food that is predominantly fine organic detritus. First instar larvae selectively feed on diatoms but it is likely that selection is on the basis of particle size, rather than for diatoms per se. Three species of epiphytic larvae (2 Cricotopus spp. and Eukiefferiella ilkleyensis) transfer to a diet that is predominantly of fresh plant material (Ranunculus calcareus leaves) in the third and fourth instar. The life cycle strategies of many species of Orthocladiinae equip them to exploit temporarily favourable environmental conditions very rapidly and effectively. This fact helps to explain the coexistence of species with similar strategies of resource utilization, as part of the normal epiphytic fauna of chalk streams.     

Life cycle and nymphal feeding of Rhabdiopteryx christinae Theischinger 1975 (Plecoptera: Taeniopterygidae)

In the present paper some aspects of the nymphal biology of Rhabdiopteryx christinae, an endemic species from the Iberian Peninsula, are studied in a temporary stream from Southern Spain. R. christinae has an univoltine life cycle, with very short, fast and constant nymphal development, only having four months (December-March), related with the seasonality of the stream. In order to resist the period in which the stream has no water, an egg diapause probably occurs. The study of nymphal gut contents showed that R. christinae acts mainly as collector (detritus) and scraper (epiphytic and epilitic diatoms), as it is generally accepted for the family Taeniopterygidae.     

ENVIRONMENTAL FACTORS INFLUENCING DIATOM CELL MOTILITY1

This article describes the effect of various environmental conditions on the speed of freshwater diatoms. The diatoms display a wide distribution of cell speeds, from 0 to > 20 μm·s^?l, as indicated by both the distribution of speeds within a cell population as well as the distribution of 1-s interval speeds of individual cells. Cell length has little or no effect on cell speed, as the speed of large postconjugal populations of Craticula and smaller preconjugal cell populations were not significantly different. The diatoms showed a broad pH tolerance, with Craticula spp. and Nitzschia spp. displaying active motility between pH 3 and 12 and pH 4 and 10, respectively, with velocity maxima for both species at approximately pH 7. In contrast to previous reports on marine diatoms, these freshwater diatoms do not require millimolar levels of external calcium for motility, as both Craticula spp. and Nitzschia spp. showed significant motility in distilled water (<0.5 μM calcium) for up to 5 h. Although addition of ≥500 μM of EGTA inhibited motility, this inhibition did not appear to be due to calcium chelation, as EGTA solutions preincubated with up to 20-fold excess calcium, magnesium, or both still inhibited motility with the same dose-dependent response as EGTA alone. Moreover, significant motility was restored by rinsing EGTA-treated cells in distilled water, Ca-free medium, or regular diatom medium. The calcium channel inhibitors lanthanum and ruthenium red also inhibited motility in a dose-dependent manner, suggesting that regulation of internal calcium stores may be important in motility. Craticula motility also declines rapidly in medium ≥50 mOsM while the isotonic concentration appears to be 100–120 mOsM, suggesting that movement may require the plasma membrane to be exerting a force ≥50–70 mOsM osmotic pressure (1–1.5 atm) against the cell wall.     

Benthic Diatoms as Indicators of Stream Sediment Concentrationin Hong Kong

Diatoms are photosynthetic unicellular, eukaryotic, microorganisms (algae) that are distinguished by their silicified (SiO₂‐nH₂O) cell walls. They have often been employed to assess salinity, pH, and nutrient conditions. Our data suggest that, they may also be used to assess suspended solid levels in streams. The ratio of motile to attached benthic diatoms from five different stream sites in Hong Kong was correlated with the level of suspended solids for each of these sites. Sites with high concentrations of suspended solids had a high percentage of motile diatoms on their rocks while sites with low concentrations of suspended solids had a high percentage of attached (non‐motile) diatoms on their rocks. When water carried by a stream has a high concentration of suspended solids, benthic diatoms in the stream are often covered in a layer of silt. Those diatoms that are motile are able to get on top of this silt layer while attached diatoms cannot. If the silt layer is not washed away, the attached diatoms perish and the percentage of motile diatoms increases. Thus, streams with high concentrations of suspended solids display a higher proportion of motile diatoms than do clear water streams with low concentrations of suspended solids. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Artificial surfaces as a method of studying populations of benthic micro-algae in fresh water

Most work on the ecology of communities of attached algae has involved the investigation of populations which develop on artificial surfaces exposed in the water for a known period of time. The nature and position of the surface and length of time of exposure have been shown to influence the population which develops. No studies have related the structure of these populations to populations on natural surfaces.

Microscope slides which had been exposed in a pond and a stream for two and four weeks were sampled over a period of more than one year, and natural vegetation was sampled at the same time. The populations of attached diatoms on each substrate were analysed.

The diatoms in the pond grew mainly in summer on the slides and in winter on the natural host. In the stream the main growth of the population was over the summer period in all cases. There were twice as many species of diatoms growing on natural hosts as on the slides, and the percentage importance of the species was different. The time of year at which some species grew was different on the slides and the natural vegetation. It was concluded that under these conditions the use of artificial surfaces is not a valid ecological method.     

THE ROLE OF DIATOMS IN STROMATOLITE GROWTH: TWO EXAMPLES FORM MODERN FRESHWATER SETTINGS1

Some modern laminated find calcified stromatolitic structures are partially or completely formed by eukaryotes. Diatom populations in freshwater environments with elevated ionic concentrations contribute to calcite precipitation, and the formation of distinctive mineral-rich stromatolitic laminae. Two types of stromatolite-forming diatom populations were observed. In the first example, in stromatolites growing on a quarry ledge near Laegerdorf, North Germany, calcite crystals with biogenic imprints form around polysaccharide stalks of the diatom Gomphonema olivaceum var. calcarea (Cleve) Cleve-Euler. These individually precipitated crystals eventually become cemented together in layers, forming rigid, laminated stromatolitic deposits which drape over the quarry ledge. In the second example, in stromatolites forming in a shallow stream near Cuatro Ciénegas, Coahuila, Mexico, diatomaceous laminae also form by the accumulation of carbonate particles in a matrix of diatoms and their extracellular polysaccharide products. These laminae become thick enough to drape over individual stromatolite heads. The diatoms responsible for these deposits are Amphora aff. A. katii Selva, Nitzschia denticula Grun., and six other species. At Cuatro Ciénegas, in addition to the diatomaceous laminae, carbonate-rich cyanobacterial layers, dominated by two cyanobacterial species with different fabrics and porosities, are also present and contribute substantially to the growth of the stromatolites. In both the Laegerdorf and Cuatro Ciénegas examples, entire stromatolites or thick laminations on stromatolites are built by a small number of diatom species which produce copious amounts of extracellular stalk, gel, and sheath material, a propertuy they share with cyanobacterial stromatolite builders.     

THE ECOLOGY OF EPIPELIC ALGAE OF FIVE WELSH LAKES,WITH SPECIAL REFERENCE TO VOLVOCALEAN GREEN FLAGELLATES (CHLOROPHYCEAE)1

The ecology of epipelic algae on the marginal sediments of five Welsh lakes was studied over an annual cycle. The lakes, Llydaw, Cwellyn, Padarn, Maelog and Coron ranged from very oligotrophic to nutrient-rich. Attention was focussed on chlamydomonad flagellates, diatoms, blue-green algae and euglenoids and the different proportions of these in algae in the epipelon of lakes of contrasting water quality. A total of 75 algal taxa was found in the five lakes, 25 were species of volvocalean flagellates. Mean annual population density of these flagellates differed by an order of magnitude between the lakes. The greatest population density was recorded for Chlamydomonas anticontata Schiller in nutrient-rich Llyn Maelog. Twenty species of pennate diatoms were recorded frequently in the epipelon. In the nutrient-rich lakes, Maelog and Coron, pennate diatoms were dominant on the sediments, where they exhibited population maxima in spring and autumn. Increase in numbers of epipelic diatoms was recorded when silica concentrations were minimum in the overlying lake waters. Navicula hungarica Grun. achieved the maximum population density, 260 000 cells · cm^?2. Euglenoids formed large epipelic populations during late-summer and autumn in these nutrient-rich lakes. Blue-green algae were more important, proportionally, in the nutrient-poor mountain lakes, which had sediments of higher organic content. Chlamydomonads were the major algal component of the epipelon in the mountain lakes, Llydaw and Cwellyn, where the sediments were characterized by larger particle size, and higher organic content. In the nutrient-rich lakes, where the sediments had higher calcium content, chlamydomonads formed significant populations only during spring and summer, when nutrient levels were minimal in the overlying lakewaters.     

Effect of pH on growth of acid stream algae

Five species of algae (Chlamydomonas applanata var. acidophila, Euglena mutabilis, Gloeochrysis turfosa, Hormidium rivulare, Stichococcus bacillaris) were isolated from a stream at pH 2·6–3·1, and their laboratory growth studied. Growth of all species could occur at pH values lower than those from which they were isolated, the lowermost limits being quite similar to those recorded for the particular species growing anywhere in England. Morphological differences were apparent with all five species at the lowermost pH values. These took place with Stichococcus bacillaris at pH values at which there was little reduction in growth rate, but with the other species obvious differences in morphology were correlated with a marked reduction in growth rate below the optimum rate. At the uppermost pH value tested, however, no obvious morphological differences were apparent. The effect of including 10% stream water in the medium was rather similar for all five species. No influence on growth rate was detectable at the lower pH values, but higher pH values led to a decrease in growth rate as compared with that found in medium lacking stream water.     

Salt pollution in a Japanese stream and its effects on water chemistry and epilithic algal chlorophyll-a

Concentrations of major ions, total phosphorus, dissolved organic carbon, biological oxygen demand and chlorophyll-a of epilithic algae were determined weekly at nine sites in a Japanese stream receiving effluent from a groundwater treatment plant. The concentrations of four major cations (Na, K, Ca and Mg) and chloride ion increased significantly immediately at downstream sites of the effluent outfall. The ionic concentrations decreased with increasing dilution from merging tributaries but never reached the original concentrations and relative composition of stream water within a 10.7 km stream distance from the outfall. The changes in total ionic concentration and relative ionic proportion also changed the chlorophyll-a content of epilithic algae. The results also showed significantly higher chlorophyll-a content in epilithic algae under moderate salinity.     

Long-term changes of diatoms and chemistry in headwater streams polluted by atmospheric deposition of sulphur and nitrogen compounds

1. Samples for the analysis of attached diatoms and surface water chemistry were taken at thirty-five stations in sixteen soft-water streams in the Netherlands during autumn 1990. Most were located in pine forest and heathland catchments, fed by deep groundwater (median residence time 87 years), with no direct influence of agricultural or urban drainage water. 2. The chemical data from sixteen stations were compared with data collected in 1974 and 1981. There were no significant increases in pH (6.3 in 1974, 6–4 in 1990) or in sulphate (200 mmol m^?3 in 1974, 229 mmol m^?3 in 1990). Over the same period nitrate increased significantly from 17 to 158 mmol m^?3, while alkalinity decreased significantly from 355 to 251 meq m^?3. 3. The most important correlates with the distribution of diatoms, as revealed by canonical correspondence analysis, were pH, nitrate, cross-sectional area of the stream and the Ca/(Ca + Cl) ratio. 4. Diatom assemblages of upstream stations indicated more acid conditions than those of downstream stations. Also there was considerable variation in the diatom assemblages of different habitats, 5. A diatom-pH transfer function was developed using the weighted-averaging method. The function was applied to comparable pairs of samples which were collected in 1974 and 1990 at ten stations. There was a significant decrease in the median diatom-inferred pH from 6.78 in 1974 to 6.55 in 1990. 6. The diatom assemblages are diverse and contain a high proportion of filamentous diatoms, particularly Aulacoseira crenulata and A. alpigena, which are rare in Western Europe. This is a response to the low current velocity (median 5 cm^?1) and the relatively constant deep groundwater discharge.     

Using change-point analysis and weighted averaging approaches to explore the relationships between common benthic diatoms and in-stream environmental variables in mid-atlantic highlands streams,USA

Diatoms are increasingly being used in the bioassessment of aquatic systems. However, autecological information for many common taxa is incomplete. We explored the potential of classification (CT) and regression tree (RT) approaches to identify the hierarchical interaction among water quality variables in predicting the relative abundance of ten common stream diatom taxa in the Mid-Atlantic Highlands ecoregion. RT analysis was also used to identify environmental change points corresponding to major shifts in species abundances. We also used traditional weighted-averaging approaches (WA) to model taxon pH and total phosphorus (TP) optima. RT and WA approaches provided different, yet complementary, information on the complex relationships between common stream diatoms and environmental variables. Both RT and CT highlighted the interaction of stream acidity (pH, acid neutralizing capacity (ANC)), and TP in structuring the stream diatom assemblage. For the RT of taxa, where pH was an important predictor, higher pH predicted higher relative abundances. In contrast, higher TP predicted lower relative abundances for some diatom taxa (e.g., Achnanthidium minutissimum (Kütz.) Czarnecki), while predicting higher relative abundances for other taxa (e.g., Planothidium lanceolatum (Bréb.) Round & Bukht., Gomphonema parvulum (Kütz.) Kütz.). The environmental change point for pH derived from RT analysis was lower than WA optima for all species. We suggest that RT change point analysis can be used to complement traditional WA optima approaches, especially when diatom taxa’s abundances are affected by interactive environmental factors, to provide more refined information on stream diatom environmental preferences. Handling editor: L. Naselli-Flores     

Changes in diatom-dominated biofilms during simulated improvements in water quality: implications for diatom-based monitoring in rivers

Although benthic diatoms are used to assess river water quality, there are few data on the rate at which diatom assemblages react to changes in water quality. The aim of this study was to assess the reaction time of diatoms and to discuss the changes occurring during water quality improvement on the basis of their autecological characteristics. In order to simulate this improvement, diatom-dominated biofilms grown on artificial sandstone substrata were transferred from several polluted rivers to an unpolluted river. They were sampled three times: before transfer and 1 and 2 months after transfer. The ecology and growth-forms of the taxa explained most of the changes in species composition observed during the experiment. Adnate diatoms gradually replaced motile and stalked taxa. Gomphonema parvulum, a stalked diatom positioned vertically in the biofilm, is adapted for light and space competition in high-density algal biofilms. When transferred to an unpolluted site, this growth-form is less competitive and does not tolerate the high grazing pressure. Fistulifera saprophila is a single celled motile diatom, living in organic matrices. When the artificial substrata were transferred to the unpolluted site, this particular ecological niche disappeared quickly. On the other hand, Achnanthidium minutissimum, which is considered to be cosmopolitan and an early colonizer, increased during the first month of transfer and then decreased. It was gradually replaced by A. biasolettianum, which was the taxon best suited to this pristine stream. The changes observed differed between treatments depending on the species composition and architecture of the biofilms. In particular, biofilms dominated by stalked and motile diatoms were more quickly modified than those dominated by small motile diatoms. The diatom index reflects these changes, and its values showed that about 60 days following a water quality improvement were necessary for transferred diatom assemblages to reach diatom index values similar as those at the unpolluted river.     

The ecology of the diatoms of the klip river,Southern Transvaal

Diatom associations formed by 2% or more of species, found by the Thomasson analysis method, are discussed in relation to the physical and chemical vicissitudes of a stream in which the head waters become cut off from those of the middle reaches by a barren zone caused by mineral and acid pollution from dumps resulting from the gold-mining industry on the Witwatersrand. Associations are found indicative of the originating waters, of regions of instability due to the pollution, whether it be its onset or recovery therefrom and of the lower recovered middle reaches. In each reach there are found to be associations indicative of pH, nitrogen content, dissolved oxygen and alkalinity. Thus the associations enable one to "read" the conditions of the river. An additional association occurs at the very turbid mouth of the river which is indicative of low light requirement. The work supports the findings of CHOLNOKY (1958 et seq.) in his researches into the diatom ecology of South African streams.     

A model for nutrient and water flow and their uptake by plants grown in a soilless culture

The objective of this study was to develop a sensitive means of control to optimize nutrient concentrations in the root zone of a soilless system, considering plant water and nutrient uptake, and solution circulation rates. A model is proposed to simulate ornamental plants growth in a channel with a non-interacting soilless substrate, irrigated by point sources with constant discharge rates, spaced uniformly along the channel. The model accounts for compensation for transpiration water losses and consequent salinity buildup, and its interactions with plant growth and nutrient uptake. The added water may contain given concentrations of nutrients and/or toxic (saline) compounds, which would cause salinity buildup. Uptake of each solute is specific, according to a Michaelis–Menten kinetics mechanism, but passive uptake by the transpiration stream is also accounted for. Plant growth is affected by time/age and ionic balance in the solution. The model was calibrated with lettuce (Lactuca sativa L.) plants grown in volcanic ash. Simulation of potassium concentration change as a result of discharge rate and emitter spacing revealed that the two parameters could compensate one for the other, once a target lower limit is set. Potassium appeared to be most sensitive to sodium accumulation in the growth medium; this accumulation changed ionic concentration balance, which affected pH and bicarbonate concentration. Passive uptake of calcium by the transpiration stream is highly affected by the root fraction involved, but its calculated contribution is below published values is highly affected by the root fraction involved, but its calculated contribution is below published values.     

A coded checklist and ecological indicator values of freshwater diatoms from The Netherlands

This first comprehensive checklist of the diatoms from fresh and weakly brackish water in The Netherlands comprises 948 taxa, belonging to 776 species in 56 genera. The generaNavicula, which has a very wide ecological amplitude, andNitzschia, which has many pollution tolerant species, are most numerous. Each taxon is identified with a unique eight-letter code, to facilitate computer processing of data. Ecological indicator values for pH, salinity, nitrogen uptake metabolism, oxygen, saprobity, trophic state and moisture are presented.     

Diatom communities of acidic mountain streams in Poland

A comparison has been made of the species composition of diatom communities developing in acidic Polish mountain streams which flow over calcium-poor substrates: sandstones in the Silesian Beskid (section of the Western Carpathians), the witokrzyskie Mts, and over granite in the Karkonosze range (in the Sudetic Mts). The number of taxa and diversity of the diatom assemblages decreased along a decreasing pH gradient. The correlation between pH and the number of taxa was positive and significant (r ² = 0.69, p < 0.005). A small number of species (< 20) and low diversity were found in the communities developing in strongly acidic streams such as in the witokrzyskie Mts with pH 4.1–5.2, and in the Silesian Beskid with pH 3.5–4.0. In the stream of the Karkonosze Mts, with pH 5.2–6.0, the communities were characterized by their greater number of species and higher diversity.Acidobiontic and acidophilous diatoms were generally dominant. The pH-indiferent forms were less abundant, and their proportion increased above pH 5.0. Eunotia exigua, E. paludosa var. trinacria, E. tenella and Pinnularia subcapitata dominated in streams with the lowest pH, while E. exigua, E. sudetica and Achnanthes kryophila predominated in a stream with water pH above 5.2. Eunotia exigua, a common acidobiontic species was present in all the examined communities, and was a strong dominant in waters of pH 5.0. A corresponding decrease in abundance of E. exigua was observed with an increase in pH.     

Some physical and chemical characteristics of an arctic beaded stream

Imnavait Creek is a tundra stream in the Arctic Foothills of Alaska. The stream is beaded, i.e. consists of pools (up to ∼ 2 m deep) connected by narrow channels. Peat dominates pool and channel substrate materials with occasional rock and moss substrates. The watershed is underlain by ice-bonded till and is hydrologically watertight. Because of low rates of weathering, bedrock and till do not contribute significantly to ionic composition of the stream water. Breakup occurs in late May to early June with surface flow until September. During periods of low rainfall, channel flow is reduced and pools become hydrologically isolated and thermally stratified (with very high surface water temperatures). Streamwater is acidic (pH values 5.3–6.1) with very low alkalinity (≤3 mg l⁻¹). The major transport of ions occurs in early flow derived from snow melt. Organic carbon concentrations are high with very high ratios of dissolved to particulate organic carbon. Dissolved inorganic nitrogen concentrations appear to be very low. High concentrations of dissolved organic material may indicate a central role for DOM in trophic dynamics.     

Changes in gill histology of fathead minnows and yellow perch transferred to soft water or acidified soft water with particular reference to chloride cells

Summary Fathead minnows, Pimephales promelas, and yellow perch, Perca flavescens, were transferred from moderately soft Lake Superior water (hardness 45mg/l as CaCO₃) to very soft diluted Lake Superior water (hardness 4.5mg/l). Sulfuric acid was added in some treatments by means of a multichannel diluter. In very soft water, chloride cells proliferated in the gills, especially in the epithelium of the secondary lamellae. When exposed to acid, chloride cells were damaged and less abundant in the secondary lamellae, and blood osmolality was reduced at pH 5.0 (x = 188 mOsm/kg, 9 days exposure; normal 280 mOsm/kg) for the minnows and pH 4.1 (x = 218 mOsm/kg, 58 days exposure; normal 329 mOsm/kg) for the perch. Certain chloride cells which form gland-like clusters in the primary lamellae of perch gills showed little damage even at pH 4.1. The present study supports the view that chloride cells proliferate in very soft fresh water to help maintain ionic balances, and that damage to these cells in acidified soft water may be related to diminished ionoregulatory capacity. The greater acid tolerance of chloride cells of, and the higher blood osmolality maintained by, perch could help to explain the greater tolerance of this species to low pH. In some cases, a species' ability to acclimate to very soft water and acidified soft water may depend upon the number, distribution, and physiology of its chloride cells.     

Diatoms from Alto Adige/Südtirol (Northern Italy): characterization of assemblages and their application for biological quality assessment in the context of the Water Framework Directive

The Water Framework Directive 2000/60/EC (WFD) requires the analysis of biological elements of aquatic ecosystems to assess water quality. Diatoms are the component of the periphyton most commonly used to classify lotic environments. Within the context of the WFD the concept of ‘reference conditions’ was introduced and biological quality of watercourses is expressed as Ecological Quality Ratio (EQR). This study was carried out in Alto Adige/Südtirol (Province of Bolzano-Bozen, northern Italy), belonging to the Alpine eco-region, and to the hydro-ecoregion Inner Alps. During 2006–2009, epilithic diatoms were sampled from monitoring and reference sites of seven stream types. Diatom assemblages were analysed with TWINSPAN and CCA analyses to investigate species association and distribution in relation to stream characteristics. Altitude and geology resulted to be the most important factors influencing diatom assemblage composition, and were used to describe new stream types. Indicator species analysis was used to characterize reference assemblages. The biological quality of watercourses was assessed using different diatom indices: Specific Pollution sensitivity Index (IPS), Eutrophication and Pollution Index with Diatoms (EPI-D), Trophic Index (TI). We tested also the Intercalibration Common Metric index (ICM).