Rarely reported Planothidium Round et Bukhtiyarova (Bacillariophyceae) from flowing waters of southern Poland

The aim of this study was to document new records for Poland of Planothidium species occurring in running waters of southern Poland and to characterise their morphology, ecology and distribution. The study material was collected during the spring (April–June) and autumn (September–October) seasons in years 2013, 2017, 2023 and 2024. Species identification was conducted using light microscopy and scanning electron microscopy. Environmental parameters were measured in situ and complemented by laboratory analyses of water chemistry. Ecological status was assessed using diatom indices (Specific Pollution Sensitivity Index, Generic Diatom Index). Five Planothidium species not previously reported from Poland were identified: Planothidium cavilanceolatum, Planothidium daui, Planothidium gallicum, Planothidium hinzianum and Planothidium potapovae. Most taxa occurred rarely and predominantly as individual valves, which limits the precise interpretation of their ecological preferences. In our study, P. daui exhibited a broad ecological amplitude, occurring under oligo- to eutrophic conditions, whereas P. cavilanceolatum, P. gallicum and P. hinzianum were restricted to specific habitats and showed low abundances. In contrast, P. potapovae reached dominant status in the upper course of the Szum River, where it was associated with oligotrophic conditions, sandy substrates and high calcium concentrations.