Observations on the morphology and vegetative cell division of the diatomDonkinia recta

Wild and cultured material ofDonkinia recta (Donk.) Grun. has been examined using light microscopy, and both the structure of the living cell and its vegetative cell division are described. Unlike most naviculoid diatoms,D. recta has four chloroplasts per cell, each with four oblong pyrenoids, and division of the chloroplasts follows rather than precedes mitosis and cell division. These two unusual features are probably linked. The species is briefly discussed in relation to other naviculoid taxa in the light of these findings.     

Morphology reinforces proposed molecular phylogenetic affinities: a revised classification for Gelechioidea (Lepidoptera)

Gelechioidea are one of the most species rich and least studied superfamilies of Lepidoptera. We examine the interrelationships within the superfamily using the densest taxon sampling to date, combined with the most extensive ever morphological and molecular character data. We perform partitioned and combined analyses using maximum likelihood, Bayesian and parsimony approaches. The combined dataset consists of 155 exemplar species of Gelechioidea, representing nearly all subfamilies recognized in recent classifications. Parsimony analyses are performed with a dataset including 28 additional terminal taxa with only morphological data available. We use eight genes with a total of 6127 bp, and morphological data with 253 characters derived from larval, pupal, and adult morphology. The analyses of combined data yield more resolved trees and significantly better‐supported groupings than either dataset when analysed alone. The recurrent monophyletic groupings in all our model‐based analyses support a revision of the family classification. Deeper relationships vary between analyses and data partitions, leaving them ambiguous. The place of the root remains a challenge for future research. We propose a revised classification and suggest the division of Gelechioidea into 16 families. We redefine Depressariidae Meyrick, 1883 for a monophylum that includes Acriinae, Aeolanthinae, Cryptolechiinae, Depressariinae, Ethmiinae, Hypercalliinae, Hypertrophinae, Peleopodinae, Oditinae, Stenomatinae, Carcina, and a diversity of predominantly New World taxa previously excluded from Lypusidae (Amphisbatidae s. authors) but left without family position. A monophyletic Oecophoridae s. s., including Deuterogoniinae and Pleurotinae, is obtained for the first time with significant support. Elachistidae s. l. is found to be polyphyletic, and Elachistidae is restricted to comprise Agonoxeninae, Elachistinae, and Parametriotinae. Batrachedridae are polyphyletic, with several genera pending further study. Apart from the core Batrachedra, the taxa previously included in this family are grouped in an expanded Pterolonchidae, together with Coelopoetinae and Syringopainae. Lypusidae s. s. and Chimabachidae form a monophylum; Chimabachinae is united with Lypusidae as a subfamily, stat. n. Our results contradict the subfamily classifications of several families, notably Lecithoceridae and Autostichidae, but due to insufficient sampling of taxa we refrain from comprehensive taxonomic conclusions on the subfamily level, and encourage focused studies to resolve these groups.     

Eukaryotic pathogens (Chytridiomycota and Oomycota) infecting marine microphytobenthic diatoms – a methodological comparison

Using sediment samples from the Solthörn tidal flat (southern North Sea, Germany), collected in bi‐weekly intervals from June to July 2012, a range of qualitative and quantitative screening methods for oomycete and chytrid pathogens infecting benthic diatoms were evaluated. Pre‐treatment of sediment samples using short ultrasound pulses and gradient centrifugation, in combination with CalcoFluor White, showed the best results in the visualization of both pathogen groups. The highest number of infected benthic diatoms was observed in mid July (5.8% of the total benthic diatom community). Most infections were caused by chytrids and, in a few cases, oomycetes (Lagenisma Drebes (host: Coscinodiscus radiatus Ehrenberg) and Ectrogella Zopf (hosts: Dimeregramma minor in Pritchard and Gyrosigma peisonis). Among the chytrids, sporangium morphology indicated the presence of five different morphotypes, infecting mainly epipelic taxa of the orders Naviculales (e.g., Navicula digitoradiata) and Achnanthales (e.g., Achnanthes brevipes Agardh). The presence of multiple pathogens in several epipelic diatom taxa suggests a significant role for fungal parasitism in affecting microphytobenthic diatom succession.     

PHYLOGENETIC ANALYSES OF THE BRYOPSIDALES (ULVOPHYCEAE,CHLOROPHYTA) BASED ON RUBISCO LARGE SUBUNIT GENE SEQUENCES1

Current taxonomy of the Bryopsidales recognizes eight families; most of which are further categorized into two suborders, the Bryopsidineae and Halimedineae. This concept was supported by early molecular phylogenetic analyses based on rRNA sequence data, but subsequent cladistic analyses of morphological characters inferred monophyly in only the Halimedineae. These conflicting results prompted the current analysis of 32 taxa from this diverse group of green algae based on plastid‐encoded RUBISCO large subunit (rbcL) gene sequences. Results of these analyses suggested that the Halimedineae and Bryopsidineae are distinct monophyletic lineages. The families Bryopsidaceae, Caulerpaceae, Codiaceae, Derbesiaceae, and Halimediaceae were inferred as monophyletic, however the Udoteaceae was inferred as non‐monophyletic. The phylogenetic position of two taxa with uncertain subordinal affinity, Dichotomosiphon tuberosus Lawson and Pseudocodium floridanum Dawes & Mathieson, were also inferred. Pseudocodium was consistently placed within the halimedinean clade suggesting its inclusion into this suborder, however familial affinity was not resolved. D. tuberosus was the inferred sister taxon of the Halimedineae based on analyses of rbcL sequence data and thus a possible member of this suborder.     

A phylogenetic analysis of Pisum based on morphological characters, and allozyme and RAPD markers

Cladistic analyses of 17 wild and cultivated pea taxa were performed using morphological characters, and allozyme and RAPD (random amplified polymorphic DNA) markers. Both branch-and-bound and bootstrap searches produced cladograms that confirmed the close relationships among the wild species and cultivars of Pisum proposed by a variety of systematic studies. Intraspecific rankings were supported for northern P. humile, southern P. humile, P. elatius and P. sativum, which together comprise a single-species complex. P. fulvum, while clearly the most divergent of the pea taxa, could also be assigned to the same species complex without violating the hierarchial logic of the cladogram. Its inclusion or exclusion depends on whether the level of interfertility it displays with other pea taxa or its overall morphological and chromosomal distinction are emphasized. As suggested by previous studies, northern P. humile was the most likely sister taxon to cultivated P. sativum; although, rigorous phylogenetic evaluation revealed a close genealogical affinity among P. elatius, northern P. humile and P. sativum. Despite their limited number, the 16 morphological characters and allozyme markers used precisely organized the pea taxa into established taxonomic groupings, perhaps in part reflecting the role morphology has played historically in pea classification. The RAPD data also generally supported these same groupings and provided additional information regarding the relationships among the taxa. Given that RAPDs are relatively quick and easy to use, are refractory to many environmental influences, can be generated in large numbers, and can complement traditional characters that may be limited in availability, they provide a valuable new resource for phylogenetic studies.     

A Total Evidence Phylogeny of the Arctoidea (Carnivora: Mammalia): Relationships Among Basal Taxa

A total evidence phylogenetic analysis was performed for 14 extant and 18 fossil caniform genera using a data matrix of 5.6 kbp of concatenated sequence data from six independent loci and 80 morphological characters from the cranium and dentition. Maximum parsimony analysis recovered a single most parsimonious cladogram (MPC). The topology of the extant taxa in the MPC agreed with previous molecular phylogenies. Phylogenetic positions for fossil taxa indicate that several taxa previously described as early members of extant families (e.g., Bathygale and Plesictis) are likely stem taxa at the base of the Arctoidea. Taxa in the “Paleomustelidae” were found to be paraphyletic, but a monophyletic Oligobuninae was recovered within this set of taxa. This clade was closely related to the extant genera Gulo and Martes, therefore, nested within the extant radiation of the family Mustelidae. This analysis provides a resolution to several discrepancies between phylogenies considering either fossil taxa or extant taxa separately, and provides a framework for incorporating fossil and extant taxa into comprehensive combined evidence analyses.     

Attached diatom communities from the lower Truckee River,summer and fall, 1986

The algal flora of the Truckee River below Reno, Washoe County, Nevada was examined during the summer and fall months of 1986. This reach of the lower Truckee River exhibited a substantial gradient in dissolved inorganic nitrogen associated with sources which included treated wastewater from the Reno metropolitan area. The algal communities were similar to those of other river systems of the Great Basin. Cyanophyta and Chlorophyta formed encrusting mats on the substrate, with the nitrogen fixer Calothrix atricha relatively abundant upstream from the nitrogen source. Diatoms were abundant within and upon this mat. A total of 139 diatom taxa and 11 taxa other than diatoms was identified from this flora. Several of the diatom taxa, including Achnanthes minutissima, Diatoma vulgare, Nitzschia dissipata, and Nitzschia palea, demonstrated distinct downriver patterns in relative abundance.     

CLADISTICS OF THE BRYOPSIDALES: A PRELIMINARY ANALYSIS

The Bryopsidales contains some of the most species rich and ecologically dominant algae in tropical ecosystems. However, the evolutionary relationships among the 29 genera and several hundred species of this order remain poorly resolved. Because of a lack of known reproductive characters for many taxa, evolutionary hypotheses grouped genera by similarities in morphological characters. To apply standard cladistical analyses to further our understanding of this group, this study presents the first comprehensive compilation of reported morphological, reproductive, and subcellular characteristics for genera in the Bryopsidales. Computer-assisted cladistical analyses ultimately identified phylogenetically informative and uninformative characters. Although the topology of the trees generated in this study is expected to change as additional data are added to this matrix, many traditional groupings and recent groupings based on molecular data were supported.     

Diatom assemblages of the Cuyahoga river,N. E. Ohio (USA)

The Cuyahoga River is a heavily polluted tributary of Lake Erie located in N. E. Ohio (USA).One hundred seventy taxa of diatoms were identified from seven locations along the Cuyahoga River. The most frequently collected diatoms from each station were Cyclotella meneghiniana, Gomphonema parvulum, Navicula cryptocephala, N. cryptocephala var. veneta, N. lanceolata, N. menisculus, N. minima, N. minuscula, N. pelliculosa, Nitzschia amphibia, N. ovalis, N. palea, and Rhoicosphenia curvata.In the most heavily damaged region of the river these widely-distributed taxa were the only frequently collected diatoms.The relatively undamaged section of the river contained 94% more taxa than the most heavily damaged section. Chief among these additional taxa were Achnanthes lanceolata, A. lanceolata var. dubia, Cocconeisplacentula, Fragilaria vaucheriae, Melosira varians, Meridion circulare, Navicula mutica var. tropica, N. symetrica, Nitzschia dissipata, Stephanodiscus astraea, and Synedra rumpens.The degree of similarity between diatom assemblages along the Cuyahoga River as measured by a modification of Sørensen's index of similarity in species-frequency of occurrence composition was related to the degree of chemical-physical water quality similarities and to the extent of similarity between areas of geologic and biologic substrates. The most dissimilar diatom assemblages usually occurred between locations that were the most dissimilar with respect to chemical-physical water quality conditions rather than to dissimilarities with respect to sediment-substrate composition.     

Phylogenetic analysis of higher-level relationships of Odonata

Abstract. This is the most comprehensive analysis of higher‐level relationships in Odonata conducted thus far. The analysis was based on a detailed study of the skeletal morphology and wing venation of adults, complemented with a few larval characters, resulting in 122 phylogenetically informative characters. Eighty‐five genera from forty‐five currently recognized families and subfamilies were examined. In most cases, several species were chosen to serve as exemplars for a given genus. The seven fossil outgroup taxa included were exemplar genera from five successively more distant odonatoid orders and suborders: Tarsophlebiidae (the closest sister group of Odonata, previously placed as a family within ‘Anisozygoptera’), Archizygoptera, Protanisoptera, Protodonata and Geroptera. Parsimony analysis of the data, in which characters were treated both under equal weights and implied weighting, produced cladograms that were highly congruent, and in spite of considerable homoplasy in the odonate data, many groupings in the most parsimonious cladograms were well supported in all analyses, as indicated by Bremer support. The analyses supported the monophyly of both Anisoptera and Zygoptera, contrary to the well known hypothesis of zygopteran paraphyly. Within Zygoptera, two large sister clades were indicated, one comprised of the classical (Selysian) Calopterygoidea, except that Amphipterygidae, which have traditionally been placed as a calopterygoid family, nested within the other large zygopteran clade comprised of Fraser's ‘Lestinoidea’ plus ‘Coenagrionoidea’ (both of which were shown to be paraphyletic as currently defined). Philoganga alone appeared as the sister group to the rest of the Zygoptera in unweighted cladograms, whereas Philoganga + Diphlebia comprised the sister group to the remaining Zygoptera in all weighted cladograms. ‘Anisozygoptera’ was confirmed as a paraphyletic assemblage that forms a ‘grade’ towards the true Anisoptera, with Epiophlebia as the most basal taxon. Within Anisoptera, Petaluridae appeared as the sister group to other dragonflies.     

Expanded phylogenies of canonical and non-canonical types of methionine adenosyltransferase reveal a complex history of these gene families in eukaryotes

Most eukaryotes possess the highly-conserved enzyme methionine adenosyltransferase (MAT) that produces S-adenosyl-l-methionine, a molecule essential to a variety of cellular processes. However, a recent study revealed that genomes of a very few eukaryote lineages encode a highly divergent type of MAT (called MATX), instead of the canonical MAT enzyme. Since MATX-containing eukaryotes are phylogenetically interspersed with MAT-containing organisms, it is likely that the MATX gene was spread into the MAT-containing groups via multiple eukaryote-to-eukaryote lateral gene transfer events. Here, we further investigate the evolutionary history of these gene families by vastly increasing the sampling of species containing MAT (22 new taxa) and MATX (8 new taxa). Our expanded analyses reveal the first example of lateral transfer of a MAT gene between the pelagophycean alga Aureococcus anophagefferens and a cryptomonad. The increased MATX sampling also provided new insights into the evolution of MATX. Specifically, our MATX phylogeny robustly grouped the haptophyte homologues with the Aureococcus homologue to the exclusion of the diatom homologues, suggesting a transfer of the MATX gene between haptophytes and pelagophytes. Various scenarios of MAT and MATX gene family evolution in diatoms are re-evaluated in light of the new data.     

Epilithic diatoms of mountain lakes of the Tatra Mountains (Slovakia)

In the framework of the international project EMERGE, species composition of epilithic diatoms of 34 selected high mountain lakes of Slovak part of the Tatra Mountains were investigated. In all, 127 taxa of diatoms belonging to 26 genera were recorded. Comparison of the epilithic assemblages of the investigated lakes showed differences both in relative abundance and taxa present in the individual lakes. On the basis of species composition and relative abundance of epilithic diatoms using hierarchical cluster analysis, two main groups of lakes were created. The first group includes 4 lakes which can be classified as strongly acidified and 9 lakes which are acidified or threatened by acidification. In this group, acidophilous species such as Achnanthes helvetica, A. marginulata, Aulacoseira distans and Tabellaria flocculosa prevailed. The second group includes 19 non-acidified lakes. The most common diatoms in these lakes were Achnanthes minutissima, A. helvetica, Cymbella minuta and Denticula tenuis which occur practically in all these lakes but with different abundances. The division of lakes due to their diatom species composition mostly confirmed the status of these lakes as judged from their water chemistry.     

Phylogeny of the sea spiders (Arthropoda, Pycnogonida) based on direct optimization of six loci and morphology

Higher‐level phylogenetics of Pycnogonida has been discussed for many decades but scarcely studied from a cladistic perspective. Traditional taxonomic classifications are yet to be tested and affinities among families and genera are not well understood. Pycnogonida includes more than 1300 species described, but no systematic revisions at any level are available. Previous attempts to propose a phylogeny of the sea spiders were limited in characters and taxon sampling, therefore not allowing a robust test of relationships among lineages. Herein, we present the first comprehensive phylogenetic analysis of the Pycnogonida based on a total evidence approach and Direct Optimization. Sixty‐three pycnogonid species representing all families including fossil taxa were included. For most of the extant taxa more than 6 kb of nuclear and mitochondrial DNA and 78 morphological characters were scored. The most parsimonious hypotheses obtained in equally weighted total evidence analyses show the two most diverse families Ammotheidae and Callipallenidae to be non‐monophyletic. Austrodecidae + Colossendeidae + Pycnogonidae are in the basal most clade, these are morphologically diverse groups of species mostly found in cold waters. The raising of the family Pallenopsidae is supported, while Eurycyde and Ascorhynchus are definitely separated from Ammotheidae. The four fossil taxa are grouped within living Pycnogonida, instead of being an early derived clade. This phylogeny represents a solid framework to work towards the understanding of pycnogonid systematics, providing a data set and a testable hypothesis that indicate those clades that need severe testing, especially some of the deep nodes of the pycnogonid tree and the relationships of ammotheid and callipallenid forms. The inclusion of more rare taxa and additional sources of evidence are necessary for a phylogenetic classification of the Pycnogonida. © The Willi Hennig Society 2006.     

The familial and subfamilial relationships ofApocynaceae andAsclepiadaceae evaluated withrbcL data

Sequence data for therbcL gene from twenty-four taxa of the familiesApocynaceae andAsclepiadaceae were cladistically analysed in order to evaluate the existing familial and subfamilial classification. The taxa sampled represent all described subfamilies and a majority of the described tribes. The cladistic analysis shows that theAsclepiadaceae are nested within theApocynaceae. An amalgamation of the two families is therefore recommended. The subfamilial classification is also in need of revision: the subfamiliesPlumerioideae andApocynoideae of the current classifications are paraphyletic, as are many of the tribes. Potential subfamily candidates and characters traditionally used in the classification are discussed.     

PHYLOGENY OF THE RUBIACEAE AND THE LOGANIACEAE: CONGRUENCE OR CONFLICT BETWEEN MORPHOLOGICAL AND MOLECULAR DATA?

Phylogenetic analyses of 33 genera of Rubiaceae were performed using morphological and a few chemical characters. Parsimony analysis based on 29 characters resulted in eight equally parsimonious trees, with a consistency index of 0.40 and a retention index of 0.69. These results were compared to a phylogenetic analysis of the same genera based on chloroplast DNA restriction site data. There are discrepancies between the two analyses, but if we consider groupings reflected in the present classification there is much congruency. With the exception of four genera, all the genera are positioned in the same group of taxa in the two analyses. Clades of taxa representing three of the four subfamilies (~the Antirheoideae, ~the Rubioideae, and the ~Ixoroideae) are monophyletic, while the fourth subfamily Cinchonoideae is shown to be paraphyletic. Both analyses support a widened tribe Chiococceae, including the former subtribe Portlandiinae (Condamineeae). Furthermore, in both analyses the tribe Hamelieae is placed outside the subfamily Rubioideae where it is now housed. In search for the most plausible sister group to the Rubiaceae, the genus Cinchona (Rubiaceae) was analyzed together with 13 genera of the Loganiaceae, Nerium (Apocynaceae), and Exacum (Gentianaceae). Cornus (Comaceae), Olea (Oleaceae), and these two genera together were used as outgroups. The analysis, including 25 characters, 16 taxa, and with Cornus and Olea together as an outgroup, resulted in four equally parsimonious trees, with a consistency index of 0.53 and a retention index of 0.62. The non-Loganiaceae taxa Cinchona (Rubiaceae), Nerium (Apocynaceae), and Exacum (Gentianaceae) were all found to have their closest relatives within the Loganiaceae indicating that the Loganiaceae are paraphyletic and ought to be reclassified. As a result of the morphological data the most plausible sister group to the Rubiaceae is the tribe Gelsemieae of the Loganiaceae.     

Aufwuchs Protozoa and Bacteria on the Gills of the Rock Crab,Cancer irroratus Say: A Survey by Light and Electron Microscopy1

Histological examination of gills of the rock crab (Cancer irroratus Say, 1917) taken from specimens collected in polluted and nonpolluted areas indicated that gill discoloration was not directly related to the degree of microbial fouling. Light microscope studies demonstrated that bacteria, peritrich and suctorian ciliates, and some naviculoid diatoms were the principal members of the fouling community. Examination of the bacteria by electron microscopy showed that several distinct types were present, and that some of these were attached to the gill by holdfast structures while others were enmeshed in a filamentous slime layer. An unusual small marine flagellate and an unidentified amoeba were also examined ultrastructurally.     

Kulikovskiyia gen. nov. (Bacillariophyceae) from the lateritic rock pools of the Western Ghats,India and from Hainan Province,China

A new triundulate naviculoid diatom genus is described from the Western Ghats of Peninsular India and Hainan Province, China. The new taxon, Kulikovskiyia gen. nov. has robust conical spines along its margin and at the apices and the external valve face has longitudinally‐oriented siliceous slat system extending the length of the valve. The external distal raphe ends bifurcate and terminates on the valve face. There appear to be superficial similarities between this Asian genus and species and Playaensis, a genus comprised of two species found only in the western USA. The systematic position of Kulikovskiyia is discussed, and other than noting its similarities to other biraphid naviculoid diatoms due to symmetry features and the position of the raphe, we are uncertain about its systematic placement at finer levels of classification.     

Comparing the responses of diatoms and macro- invertebrates to metals in upland streams of Wales and Cornwall

SUMMARY 1. Surprisingly few data compare the apparent responses of diatoms and macroinvertebrates to metals in streams. We examined variation in metals, diatoms and macroinvertebrates between 51 streams in metal‐mining areas of Wales and Cornwall, U.K., using a survey design with multiple reference and polluted sites. 2. To quantify variations in metals between sites, we calculated cumulative criterion unit (CCU) scores, a recently defined measure of total stream metal concentration and toxicity, to account for additive effects of each metal relative to putative toxic thresholds. We compared assemblage responses among epilithic diatoms and macroinvertebrates to CCU scores or individual metal concentrations using correlation and detrended correspondence analysis (DCA). 3. Macroinvertebrate diversity, richness and total abundance declined and evenness increased with increasing copper concentrations. Trends with CCU scores were significant but less pronounced. Some individual macroinvertebrate taxa varied significantly in abundance with CCU scores, copper or zinc, but overall assemblage composition correlated only with manganese, pH and nitrate. 4. Among diatoms, pH and conductivity explained the major variations in assemblage composition, and neither diversity, richness nor evenness varied with metal concentration. Nevertheless, the single strongest predictor of diatom assemblages on ordination axis 2 was the CCU score. The abundances of some macroinvertebrate taxa, particularly grazers, also explained significant variations in diatom assemblages that were linked to both metals and acid–base status. 5. Diatom species apparently tolerant of high metal concentrations included Psammothidium helveticum, Eunotia subarcuatoides, Pinnularia subcapitata and Sellaphora seminulum. Of these, P. helveticum, E. subarcuatoides and P. subcapitata were abundant at lower pH than S.seminulum and might indicate metal enrichment over different pH ranges. Sensitive species included Fragilaria capucina var. rumpens, Achnanthes oblongella and Tabellaria flocculosa. 6. We conclude that macroinvertebrates at these sites reflected metal pollution most strongly through variations in diversity while effects on diatoms were best reflected by changes in assemblage composition. We suggest that, with further refinement, CCU scores might be useful in evaluating the possible effects of metal pollution on benthic organisms in European rivers.     

The phylogenetic interrelationships of the higher taxa of apterygote hexapods

The phylogeny of the basal hexapods, the so-called apterygote insects, was studied using parsimony analysis procedures. Most analyses took into account 47 characters mainly based on external morphology, and 19 taxa including 14 apterygote representatives, 3 pterygotes and also 2 distantly related myriapods were used as outgroups. The binary and multistate characters are discussed in detail and treated as unordered and equally weighted. Other analyses were performed using a second data set in which 28 characters, based on internal anatomy and already used in a previous work ( Bitsch & Bitsch 1998 ), were added to the first data set. This second matrix was restricted to 12 terminal taxa, the same as those of our previous work. The results of the different analyses are generally congruent. They strongly support the monophyly of several orders (Protura, Collembola, Archaeognatha) and of two groupings (Ectognatha, Dicondylia). Three other assemblages (Ellipura, Diplura, Entognatha) appear as parsimonious phylogenetic hypotheses, but they are never supported by the cladistical analyses and are based on a very small number of autapomorphies; so, the monophyly of each of them is not firmly established. Archaeognatha appears as the sister group of the Dicondylia. The three unresolved representatives of the Zygentoma are found as the sister group of the Pterygota. The results are discussed in the light of current concepts in hexapod phylogeny.     

Distribution patterns of benthic diatoms in a Pampean river exposed to seasonal floods: the Cuarto River (Argentina)

The diatom community growing on cobbles and sand substrata along the Cuarto River (Córdoba, Argentina) was studied during 2000 and 2001. Multivariate analyses of the data (PCA and CCA) showed distinct differences in water chemistry and substrata types between the upstream sites (sites 1–10) and downstream sites (sites 11–19). Sites 1–10 supported an epilithic diatom community associated with low water conductivity and gravel substrata. This consisted of adnate Achnanthes (A. biasolettiana, A. minutissima), as well as stalked (Gomphonema aff. angustum, Reimeria uniseriata, Fragilaria capucina var. rumpens) or prostrate (Nitzschia lacuum) taxa. Downstream sites were associated with high conductivity, fast flowing waters and finer substrata (sand, silt), and were colonised by prostrate diatoms, including several species of Navicula (N. pupula, N. mutica, N. veneta, N. insociabilis) and Nitzschia (N. umbonata, N. palea). Variations in water flow caused significant changes in the diatom communities of the river. During periods of low flow (winter and autumn), chain forming (Diatoma vulgaris, D. moniliformis) or stalked (Synedra ulna) taxa partially replaced the former community of Navicula and Nitzschia in the downstream sites. High flow (in summer) led to diatoms of large size being replaced by smaller size diatoms, such as Achnanthes lanceolata, Navicula mutica, Hantzschia amphioxys and Amphora montana. The severe effect of floods in the lower part of the Cuarto led to these taxa having a much higher proportion in the lower stretch of the river, taking advantage of the subaerial conditions created by the floods. Local episodes of water pollution were associated with a transient shift towards the dominance of Navicula pupula, Synedra ulna, Nitzschia lacuum and Reimeria uniseriata during winter, when inputs were least diluted because of the low flow.