Invasive microalge in the Danube River and floodplain waters (Croatia)

This report provides a summary of the current knowledge about the invasive microalgal species in the Kopački Rit floodplain, one of the largest preserved river-floodplain systems in the Middle Danube (Croatia). Benefiting from their small size microalgae have a potential to spread very quickly between the main river bed and various floodplain biotopes, such as permanent and temporary shallow lakes, wetlands and channels. During the almost half century of algal research, several alien or potentially invasive species were observed. Among toxic cyanobacterial species, Raphidiopsis mediterranea blooming was registered only in the past. Nowadays (from 2003 onward) ubiquitous cyanobacteria of tropical origin Cylindrospermopsis raciborskii periodically formed massive blooms in the floodplain biotopes during the long isolation period from the income of floodwater in summer. Other potentially invasive cyanobacteria (Cuspidothrix issatschenkoi and Dolichospermum compactum in phytoplankton, Gloeotrichia echinulata in periphyton) and chlorophytes (Monactinus simplex and Staurastrum planctonicum in phytoplankton, Hydrodictyon reticulatum in periphyton) occur sporadically in the floodplain biotopes. Alien diatoms are found to be prominent in the river phytoplankton. Regular blooms of Skeletonema potamos and abundant populations of Actinocyclus normanii during the conditions of high water temperatures and low water levels in summer-autumn periods represent nowadays the characteristic feature of the Danube phytoplankton. Due to ongoing spread and severe bloom formation, diatom Didymosphenia geminata can be potentially invasive, but until now it was found in low abundance in phytoplankton and phytobenthos of the main river bed. It seems that introduction and spreading of these invasive algal species can be linked to the Southern Invasive Corridor going from the Black Sea along the Danube Canal–Main–Rhine. Further international and multidisciplinary investigations focused on the alien and invasive species in the Danube River Basin are needed to ensure a long-term conservation of this unique floodplain and the preservation of its biodiversity.