The composition of the diatom assemblages was analysed in four rivers of Upper Silesia, Poland in 2017. The diatom assemblages studied were found to reflect anthropogenic salinization caused by mining activities. The assemblages in those rivers characterised by the highest salinity (Bolina and Mleczna) showed a relatively low taxonomic richness. The diatom assemblages were dominated by species typical of brackish or marine waters. The rivers with a minimal or weak anthropogenic impact (Centuria and Mitręga) supported taxonomically richer diatom assemblages typical of mid-altitude siliceous or calcareous streams (respectively), that have a fine particulate substratum. The presence of a new species, Planothidium nanum sp. nov., was revealed. The new species shows a unique set of morphological characters, including small size; its elliptical outline as well as very widely-spaced central striae on the sternum valve (sinus) and widely-spaced central striae on the raphe valve allow to separate it from other similar Planothidium.

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Madagascar is an isolated island characterized by a high degree of endemism at all taxonomic levels. Diatom assemblages of the region are still poorly known and sporadic sampling events in various habitats (e.g. lagoons, mangroves) have revealed a large number of taxa that could not be identified. This study presents detailed descriptions of two new species of Luticola: L. nosybeana and L. madagascarensis, collected from mangrove roots on Nosy Be Island. Comparisons with the described congeners showed that the density of striae in Luticola nosybeana is higher than that in L. belawanensis and proximal raphe endings terminate as irregular, shallow grooves. Luticola madagascarensis differs from L. similis in the shape of proximal raphe endings, which are short and expanded in the latter, while continue with irregular, shallow, elongated L-shaped grooves in L. madagascarensis. Luticola nosybeana and L. madagascarensis can be distinguished under a light microscope by the shape of the central area (bow-tie shaped in L. madagascarensis and deltoid in L. nosybeana) and isolated pores (robust and well visible in L. madagascarensis, poorly discernible in L. nosybeana). The two new species are unique in their habitat preferences: while all known congeners are freshwater, the new species inhabit estuarine mangroves.

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