Abstract. Four taxa belonging to the complex of species around Achnanthidium minutissimum were
found during the ongoing taxonomic revision of the Antarctic freshwater and limno-terrestrial diatom
flora. Two taxa were previously described as Achnanthidium lailae and A. sieminskae. Two others were
formerly identified as A. minutissimum but detailed light and scanning electron microscopical observations
revealed sufficient morphological differences compared to the type of A. minutissimum, to justify their
separation and description as new taxa: Achnanthidium indistinctum and A. maritimo-antarcticum. The
morphology and ecology of all four taxa are discussed comparing the species with morphologically
similar taxa. The biogeographical consequences of the splitting of the former A. minutissimum complex
in the Antarctic Region are discussed.

Abstract
The presentation of three distinct Neidium taxa from the sub-Antarctic and Maritime Antarctic region after examining more than 1500 samples from a broad variety of aquatic and terrestrial (micro-)habitats suggests that taxa representing the genus Neidium Pfitzer are not only quite rare in these regions, but that the three species also represent well-defined biogeographical distributions. N. nyvltii sp. nov., N. antarcticum sp. nov. and N. aubertii Manguin are characterized by linear to linear-lanceolate valves with rostrate to capitate apices, a single prominent longitudinal canal along the valve margins and lacinia covering the distal raphe endings. Neidium nyvltii is a benthic alkalophile growing in the Maritime Antarctic region under high total phosphorus (TP) and moderately high chloride concentrations, while the closest taxa for comparison (N. kozlowii Mereschkowsky and its varieties) are also alkalophilic, growing under lower TP concentrations in northern temperate-Arctic regions. Neidium aubertii is an acidophile with a sub-Antarctic distribution, whereas the most comparable taxa are N. bisulcatum (Lagestedt) Cleve, and N. bergii (A. Cleve-Euler) Krammer & Lange-Bertalot both are alkalophiles with northern hemisphere distribution and commonly occurring in the Arctic. There is some evidence of parallel similarities in valve forms between the polar northern and southern hemispheres. The significance of this parallelism is yet to be determined.

Abstract
The type material of Achnanthidium lanceolatum (transferred in 1999 to the genus Planothidium) is investigated to reveal the identity of several Planothidium populations from the Antarctic Region. The morphology and the ultrastructure was analysed using light and scanning electron microscopy. The results of this analysis revealed the presence of two so far undescribed Planothidium species in the investigated Antarctic material. Both species are described as new to science: P rost-rolanceolatum VAN DE VIJVER, KOPALOVA et ZIDAROVA Sp. nov. and P subantarcticum VAN DE VIJVER et C.E.WETZEL sp. nov. The new species can be differentiated based on differences in valve outline and the shape and size of the central area. Planothidium lanceolatum is formally lectotypified. Notes on the ecology and distribution of the new Antarctic species are added.

Abstract
The diversity, ecology and biogeography of diatoms in lakes, seepage areas and streams on the Ulu Peninsula, a large ice-free area in the northern part of James Ross Island (Weddell Sea), were studied. A diverse diatom flora of 123 taxa was observed, dominated by several Nitzschia taxa, Psammothidium papilio, Eolimna jamesrossensis, Fragilaria capucina and Fistulifera saprophila. The results from the similarity and diversity analysis suggest James Ross Island to be biogeographically positioned within the Maritime Antarctic region, yet with some affinities with the flora of Continental Antarctica, as shown by the presence of Luticola gaussii and Achnanthes taylorensis. Based on our data, James Ross Island can thus be located close to the boundary of the two main Antarctic biogeographical regions. Diatom communities present in streams and seepage areas could be clearly distinguished from those in lakes, the latter being much more species rich. Based on the multivariate analysis, conductivity and nutrients were selected as the two main environmental factors determining the diatom composition in the Ulu Peninsula lakes. The revised taxonomy of the Antarctic diatom flora induced the construction of a transfer function for water conductivity in the studied lakes that can be applied in further palaeoecological studies.

Abstract
Diatom and water chemistry samples were collected from 49 lakes, pools and rivers on Byers Peninsula (Livingston Island, South Shetland Islands) during the summer of 2009. A diverse diatom flora of 143 taxa was found. Fragilaria capucina s.l., Psammothidium papilio, Navicula dobrinatemniskovae and several Nitzschia taxa dominated the flora. The biogeographical analysis showed that more than 55% of all observed taxa presented a restricted Antarctic biogeographic distribution and only 30% had a cosmopolitan distribution, contrary to previously published data. Cluster analysis and Principal Components Analysis were used to classify the samples based on their chemical characteristics, revealing that nutrients and specific conductance were the main factors dividing the samples into four groups: young lakes, coastal lakes, larger lakes on the central plateau and smaller, temporary pools. Diatom communities corresponded well to this division and were strongly influenced by salinity and nutrients.

Abstract
Four new small-celled naviculoid diatom species belonging to the genera Adlafia, Chamaepinnularia, Mayamaea, and Microcostatus were found during a survey of the non-marine diatom flora of the South Shetland Islands and James Ross Island (Maritime Antarctic Region). Following both light and scanning electron microscopy, the following species were described as new: Adlafia submuscora sp. nov., Mayamaea josefelsteri Kopalova, Nedbalova & Van de Vijver sp. nov., Microcostatus australoshetlandicus sp. nov. and Chamaepinnularia antarctica sp. nov. The new species are compared to other morphologically similar species from Europe, South America and the (sub-)Antarctic region. Data on their ecology and biogeography are included. In addition, the morphology, biogeography and taxonomic position of two other small-celled naviculoid diatoms, Craticula glaberrima (W. & G.S.West) nov. comb. and Sellaphora nana (Hustedt) Lange-Bertalot et al. are discussed.

Časopis zařazen do databáze WOS, ale IF zatím nebyl přidělen.