The influence of drought on diatom assemblages in a temperate climate zone: A case study from the Carpathian Mountains, Poland

The aim of our research was to trace changes in the composition and structure of diatom communities and diatom ecological traits under the influence of drought. We also examined the impact of drought on the results of the Polish Diatom Index (IO) and the Indice de Polluo-Sensibilité Spécifique (IPS), which is the most commonly used diatom index in European countries, as a preliminary assessment of how changes in watercourses during climate-change related drought affect environmental quality assessments. The studies were conducted on watercourses in an area devoid of significant anthropogenic pressures, i.e., the protected area of Magura National Park in the Carpathian Mountains, Poland. The chemical parameters and ecological condition based on diatom analysis permitted classifying the watercourses as having good and very good status. Drought periods had a significant impact on some water parameters such as electrolytic conductivity, pH, nitrogen compounds, chlorides, and sulfates. Changes in the composition and structure of diatom communities were observed; however, in larger watercourses they were much smaller than those in smaller watercourses of a higher order. During drought periods, the share of diatoms from the motile guild and terrestrial species increased significantly in the streams. Drought had positive effects on the increased biodiversity of benthic diatom communities. Diatom index values showed statistically significant differences between drought-free and drought periods. In the present study, the differences observed were small enough to not affect the final ecological status classification. However, given the significant increase in the abundance of motile species, which are often classified as indicators of pollution, we believe that in watercourses with lower ecological statuses the occurrence of motile taxa related to drought symptoms (e.g., low water current, formation of depository habitats) could have had an impact on lowering statuses during ecological water assessments.