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The small, chain-forming centric diatom Skeletonema potamos (Weber) Hasle is one of the most important species of phytoplankton in the River Danube in Hungary. In low water periods from May to October, its abundance is high, contributing 10–20% of total biomass (its fresh weight is 3–6 mg 1−1). The first data about its occurrence in Hungary, in the southern Danube stretch, were published by Schmidt & Voros (1981). The presence of S. potamos was noted in Danube samples taken at the end of fifties near Budapest (Kiss, 1986). This species became abundant in the Hungarian stretch of River Danube at the end of sixties simultaneously with eutrophication.

The population of an unknown naviculoid diatom from Lake Vrana in Croatia was identified as Navicula hedinii Hustedt, a species that was described in 1922 from a small lake in eastern Turkestan. This species has some similarities with Navicula pseudocrassirostris Hustedt, a marine species found in European coastal waters. Based on the ultrastructure of the two species, they can no longer be included within the taxonomical concept of Navicula s.s. Bory. Following a morphological comparison analysis of both species with genera bearing similar characters (Adlafia, Veigaludwigia, Kobayasiella, Cavinula, Stenoneis, Sellaphora, Cosmioneis), a new genus, Envekadea Van de Vijver, Gligora, Kralj & Cocquyt gen. nov. is proposed for the three species. The new genus is characterized by a sigmoid raphe course with golfclub-like terminal fissures deflected into opposite directions, by a typical areolae structure, the presence of an external velum covering the areolae and the presence of 1 chloroplast, H-shaped in valve view.

Seasonal dynamics of all major protozoan groups were investigated in the plankton of the River Danube, upstream of Budapest (Hungary), by bi-weekly sampling over a 1-year long period. Sixty-one heterotrophic flagellate, 14 naked amoeba, 50 testate amoeba, 4 heliozoan and 83 ciliate morphospecies were identified. The estimated abundance ranges of major groups throughout the year were as follows: heterotrophic flagellates, 0.27-7.8 x 10(6)ind.l(-1); naked amoebae, max. 3300ind.l(-1); testaceans, max. 1600ind.l(-1); heliozoans, max. 8500ind.l(-1); ciliates, 132-34,000ind.l(-1). In terms of biovolume, heterotrophic flagellates dominated throughout the year (max. 0.58mm(3)l(-1)), and ciliates only exceeded their biovolume in summer (max. 0.76mm(3)l(-1)). Naked amoeba and heliozoan biovolume was about one, and testacean biovolume 1-3, orders of magnitude lower than that of ciliates. In winter, flagellates, mainly chrysomonads, had the highest biomass, whilst ciliates were dominated by peritrichs. In 2005 from April to July a long spring/summer peak occurred for all protozoan groups. Beside chrysomonads typical flagellates were choanoflagellates, bicosoecids and abundant microflagellates (large chrysomonads and Collodictyon). Most abundant ciliates were oligotrichs, while Phascolodon, Urotricha, Vorticella, haptorids, Suctoria, Climacostomum and Stokesia also contributed significantly to biovolume during rapid succession processes. In October and November a second high protozoan peak occurred, with flagellate dominance, and slightly different taxonomic composition.

The centric diatom Cyclotella choctawhatcheeana Prasad has been found for the first time in the Adriatic Sea. It develops dense populations in the highly stratified, oligotrophic, karstic estuary of the Zrmanja River (eastern Adriatic Sea, Croatia). Cyclotella choctawhacheeana contributed to the phytoplankton community in the estuary with high abundance of up to 2 x 10(6) cells L-1. Maximum abundance was found in the marine layer, below the halocline, at a salinity range between 13-28, but it was able to develop over a very wide salinity range. The maximum development appeared in the temperature range of 8-18 degrees C. It was present in 52% of samples throughout the year. The significant negative correlation with silica, nitrates and total inorganic nitrogen showed its preference for a marine, oligotrophic environment. We consider C. choctawhacheeana to be a significant constituent of the phytoplankton of the east Adriatic coast. It can be assumed that its succes is based on species wide tolerance to fluctuations of environmental factors.

The decay of the influence of reeds on the succession of algae coating them was stuided. In the same lake, and at very close places, periphyton colonization was found to be significantly different. At a ’degraded’ point, the quantity of algae fluctuated with respect to both biomass and the number of individuals. The number of taxa was low in the first three weeks and began to grow only in the fourth week. The dominant species were also different.

Cyclotella meneghiniana Kutzing is one of the most commonly found and intensively studied freshwater diatom species. However, it is considered taxonomically problematic because of its unusually wide ecological range and large frustule ultrastructural variation. As part of a study of morphological and genetic variation in this morphospecies, we surveyed nucleotide variation in the hypervariable D1/D2 regions of the 28S rDNA, in the ribosomal internal transcribed spacer region (containing ITS1, the 5.8S rDNA and ITS2) and in the 18S rDNA in a collection of 20 sympatric strains. High genetic variability and strong indications of genetic structure among the Cyclotella meneghiniana strains were found. Representatives of four genetically distinct — apparently reproductively isolated — groups were revealed among them. The random distribution of ITS variation within these four groups indicated that the genetic structure in Cyclotella meneghiniana can probably be explained by the presence of cryptic sexual species rather than by the lack of allogamous sexual reproduction. The morphological features traditionally used for species identification in this group cannot distinguish these putative cryptic species.

Conventional and landmark-based geometric morphometric approaches were used to clarify the taxonomic identity of a centric diatom morph. This morph, found in cultures isolated from the Geeste estuary (northern Germany), showed an intermediate valve morphology between that of typical specimens of Cyclotella meneghiniana Kützing and of Cyclotella scaldensis Muylaert & Sabbe. Its internal valve ultrastructure was compared to that of cells from (1) cultures of C. meneghiniana isolated from the same field samples and grown in the same conditions, (2) field material from the samples from which the cultures were isolated, and (3) a sample from the type locality of C. scaldensis. The morphometric analyses were used to determine (1) whether morphological variation of these morphs was continuous, or whether there were distinct morphological groups, and (2) how effective the alternative morphometric approaches were in answering this question. Both approaches proved informative and their results complemented each other, supporting the conclusion that three distinct size-reduction series were present in the samples investigated. Since the different morphs occurred sympatrically, we suggest that they probably belong to three reproductively isolated species. © 2005 Taylor & Francis Group, LLC.

18S rDNA sequences of six Naviculaceae species [Amphora montana, Gomphonema parvulum, Eolimna minima (syn. Navicula minima), Eolimna subminuscula (syn. Navicula subminuscula), Navicula veneta and Phaeodactylum tricornutum] were determined in order to assess the monophyly of this important group of diatoms, to date not included in 18S rDNA databases, and also that of the recently described genus Eolimna. Phylogenetic trees were constructed using other known diatom 18S rDNA sequences, and best tree topologies obtained were tested against alternative trees for their reliability. The analyses do not reject the monophyly of Naviculaceae and strongly support the separation of the genus Eolimna from Navicula sensu lato. The two species of Eolimna, however, do not appear to be each other's closest relatives among the species investigated: rather, E. subminuscula shows affinities to A. montana, and E. minima to P. tricornutum. A. montana, a species which it has been proposed should be transferred into a separate taxon from the other five species, was found to have grouped well within them in all analyses.

Periphyton was collected from sand-blasted slides exposed in the main channel of River Danube at God (1669 rkm). Samples were taken from 21 May to 23 November in 1984. Three to four days after placing the substrates into the river the first traces of an observable coating appeared. The formation of the algal coating in the Danube presented certain periodical features. From time to time, an essential decrease in the number of individuals could be observed, which generally coincided with changes in the structure of the community. This could be traced well, by grouping the algae according to morphological categories and following the temporal changes affecting these categories. The early phase of the colonization process is dominated by relatively large araphid and biraphid diatoms. In the second phase small bi-and monoraphid species are dominant Finally, the medium size mono-and biraphid algae are dominating.

The diversity of microeukaryote communities inhabiting rivers is still poorly known. Here, we have analyzed the periphytic and planktonic microeukaryote communities present in one section of the River Danube by two different methods: 18S rRNA-based terminal restriction fragment length polymorphism with fragment sequencing and microscopical analysis of the phytoplankton and periphyton. Both data sets were then related to environmental variables. Molecular fingerprinting revealed diverse communities with fluctuating composition, with the majority of sequences affiliated to the groups Bacillariophyta, Synurophyceae and Chlorophyceae. This was in accordance with microscopical data. The total number of detected T-RFs during the study period was 145, with more than half of the T-RFs being restricted to either plankton or periphyton. This suggests that the likely different natural selection regimes experienced by microeukaryotes in these two environments may promote the presence of different lineages in each of them. Significant correlations were found between phytoplankton chlorophyll-a content, phosphorus content, temperature, and the T-RFLP pattern of the planktonic microeukaryotic community, suggesting that the former environmental factors are especially important in structuring the planktonic microeukaryote communities in the River Danube. These data, together with earlier studies suggest that molecular methods are an invaluable addition in pursuit of the better understanding of the diversity and fluctuation of freshwater microeukaryotic communities.

P. 257-274 The type material of Gomphonema rosenstockianum LANGE–BERT. et E. REICHARDT from La Gomera (Canary Islands, Spain) and epilithic material of rivers from several European countries were examined using light and scanning electron microscopy in order to improve the knowledge on the taxonomical status of the G. rosenstockianum and G. tergestinum (GRUNOW) M. SCHMIDT species complex. Two other Gomphonema species, G. supertergestinum E. REICHARDT and G. angustius E. REICHARDT, recently described and belonging to the same group, are also presented in detail. After the analysis of several populations of G. rosenstockianum and G. tergestinum it was possible to define several reliable criteria to allow morphological differentiation of both species under LM and SEM as well. A geometric morphometric analysis clearly demonstrated the separation of these taxa considering valve outline, size and shape of the central area and position of the stigma. Apart from the morphological analysis, compilation of the results of this study has also revealed differences in ecological preferences of the two species. World and European distribution maps of the four Gomphonema taxa are presented. SI

During seven year period (1999-2005) phytoplankton assemblages of reservoir systems in Croatian part of Drava River (input/output waters and inner waters) was investigated. According to Regulation on water qualification in Republic of Croatia the one of the relevant water quality factors are abundance (cells per liter) of net-phytoplankton and Pantle-Buck saprobity index. Samples were collected seasonally four times a year at eight different sites of three reservoirs. Phytoplankton samples were taken by filtrating 10L of water through 25 μ m phytoplankton net. Index of saprobity was calculated for each sample. To explore the value of such monitoring procedure, on phytoplankton data set several more indexes were calculated: Trophic Diatom Index, Generic Diatom Index, Shannon-Wienner index and Pielou's index. All five indexes showed increase in eutrophication and pollution. During whole investigation period the most abundant group was Bacillariophyceae. Changes in species dominance, same as accompanying species in assemblages, were compared to physical, chemical and biological characteristics of the River. Species with high abundance and low frequency were also discussed. Differences in species dominance between input and output river waters were observed. During seven years of research, quality of Drava River changed from oligotrophic (I category) to fairly eutrophic (II-III category), characterized by oligo to  (beta)- mesosapobic and α (alpha)-mesosapobic species, respectively. In general, during investigation primary dominance of Astrionella formosa Hass./Fragilaria crotonensis Kitt. later was replaced by Melosira varians Ag. The study can be base of species occurring in Croatian part of Drava River basin and in future should be extended to nanophytoplankton and used as a testing tool for developing monitoring system according to Water Framework Directive.

Even though the hydrological and hydromorphological characteristics of the River Tisza could qualify it as a standard for international water quality assessment systems, it is among the less known streams due to the lack of systematic middle-and long-term examination programmes. The natural self-purification abilities of the river are often put to test by pollution conveyed by its tributaries. If we want to maintain, or even improve, the present ecological conditions of the Tisza, we have to take its whole drainage basin as an integrated ecological system, with all the advantages and disadvantages this means. According to algological and physico-chemical variables, the main channel of the River Tisza can be divided into three parts in Hungary. The first part, the Tiszabecs section, receives special attention, as this is the only “ upstream-type” large river in Hungary with all featurespertaining to such streams. The following section of the mean channel extends to Ároktő at the mean water stages, and to Tiszadob at low water stages. This is the most problematic section in terms of water quality. As a result of the slowed-down flow of the river, “ nutrients” arriving from both abroad and the Hungarian areas yield a high potentional trophic level. Out arriving from both abroad and the Hungarian areas yield a high potentional trophic level. Out of the Hungarian streams, the River Szamos and the Lónyai-channel have the most significant effect on the plankton composition of the Tisza, as both convey considerable amounts of pollutants into the Tisza. The tributaries have such a strong influence on the main channel – especially at low water periods – that they can entirely transform its structure of organisms. Owing to this, it is practically impossible to define a planktonic population pertaining to this section. Therefore, it is essential to properly detect the pollution load on the Tisza coming from its tributaries. The quite balanced lower section reaches from Ároktő at mean water periods and from Tiszadob at low water periods to Tiszasziget. The section below the Kisköre-reservoir to the entry of the River Hármas-Körös is characterised by a fairly rich plankton population. However, this abundance falls back between the entry of the Hármas-Körös and the collection point at Tiszasziget.