In this work the rapid expansion of a small-celled (<15 µm) monoraphid aquatic invasive species (AIS) along the Central European lotic systems is reported, using the integrated dataset of two large-scale monitoring programs, as supplemented by additional records. Achnanthidium delmontii Pérès, Le Cohu & Barthès 2012 (ADMO) was discovered in 2007 and formally described in 2012, on the basis of specimens from a French river. ADMO was first detected in the upper sections of River Danube in 2013, and has been detectable since 2013, and from 2015 onwards in Hungary. The abundance and the number of occupied habitat types by the species have gradually increased. In 2019, ADMO was found to be among the most abundant and the most frequent species in the River Danube, with a mean relative abundance of at least 5%, and a frequency of at least 10% in samples. To extend the Danubian dataset, the relative abundances of ADMO from 79 freshwater lotic samples were studied to assess their potential of the species as an indicator organism. Weighted average regression was employed to determine the species’ optima and tolerances for 18 environmental variables. ADMO has a wide ecological range, which serves to confirm its potential invasive behaviour. In the case of the following variables, the values were found to be consistent with previously published data on the requirements of the taxon. Despite ADMO prefers high temperatures (estimated optimum = 21.8 °C), its spread shows a downstream pattern of the main watercourses of Europe. The species was first found abundantly from the upper section of River Danube, then with increasing abundance in the middle section, while remaining rare in the lower section. This distribution demonstrates that the River Danube serves as an important linear route for the species' invasion.
The results of the study will help improve the ecological classification of water bodies and inform water protection actions undertaken under the auspices of the Water Framework Directive (WFD).