Background and aim – Mountain streams represent sensitive ecosystems of prime importance for biodiversity conservation. However, the knowledge of the impact of sewage pollution related to intensive recreational exploitation is still limited in these habitats. Our aim was to assess the response of stream diatom assemblages to sewage pollution in the Giant Mountains and the Bohemian Forest, and to find the main factors influencing the community structure. Methods – Epilithic samples from sixteen streams were collected, both in unaffected stream sections and at sites downstream of sewage outflows from mountain cottages or small villages. Basic physical and chemical parameters were measured and relative abundances of diatoms were determined using light microscopy. Key results – A total number of 153 diatom taxa belonging to 44 genera were identified. Based on species data, samples were divided into two groups: the first belonged mainly to sites influenced by pollution, whereas the second consisted of not or slightly affected sites. Both groups were further divided into several subgroups characterised by specific ecological conditions and assemblage composition. Canonical correspondence analysis (CCA) revealed five environmental parameters with significant influence on diatom species composition, some of which were related to sewage pollution (pH, water temperature, discharge volume and concentrations of nitrate nitrogen and organic nitrogen). Conclusions – As a consequence of sewage pollution, oligo- to mesotraphentic taxa were outcompeted by pollution tolerant taxa. However, pollution had no significant influence on diatom diversity, which was similar at sites both upstream and downstream of the outflows. In this study, diatom assemblages performed well as indicators of sewage pollution, and biomonitoring proved to be a useful tool in the detection of environmental stress in mountain streams. Overall, the response of diatom assemblages showed that an increase of recreational activities might significantly alter the ecological status of these ecosystems.

ASSEMBLAGE COMPOSITION; BOHEMIAN FOREST; EPILITHIC DIATOMS; GIANT MOUNTAINS; MOUNTAIN STREAM; NITROGEN CONCENTRATION; RECREATION; SEWAGE POLLUTION