Caught Between Invasions: Community Structure and Environmental Drivers of Cyanobacteria in a Vulnerable Post-glacial Lake Region.

Invasive cyanobacterial species can profoundly disrupt aquatic ecosystems by outcompeting native taxa, forming blooms and often producing harmful toxins, ultimately threatening biodiversity and water quality. This study investigated the composition and spatial distribution of cyanobacterial communities in post-glacial temperate lakes located in between central and eastern Poland. Particular emphasis was placed on the occurrence of invasive cyanobacteria and their associations with environmental variables. To achieve these objectives, water samples were collected from multiple lakes during the summers of 2023 and 2024. Key environmental parameters were analyzed in relation to cyanobacterial diversity, community structure, and occurrence of invasive taxa. A total of 170 cyanobacterial taxa were recorded in 2023 and 120 in 2024, including invasive species such as Raphidiopsis raciborskii, Raphidiopsis mediterranea, Chrysosporum bergii, and Sphaerospermopsis aphanizomenoides. Although these species exhibited relatively low overall abundance, their prevalence increased noticeably in 2024. Redundancy analysis could not identify specific environmental variables affecting invasive species. Next-generation sequencing was used to identify invasive species from environmental DNA and to compare these results with microscopic analyses, indicating that invasive species were more diverse than found microscopically. According to the complete dataset, the species investigated as “invasive” in north-eastern Poland are more appropriately regarded as alien taxa, though the slow and steady spread of these species is visible.