From lakes to the sea: testing the survival of freshwater bloom-forming cyanobacteria and cyanotoxin-related risk in Baltic coastal waters under a changing climate

Brackish coastal waters are increasingly susceptible to harmful cyanobacterial blooms and toxin contamination, and climate change may enhance the persistence of bloom-forming species across salinity gradients. The Baltic Sea, one of the world’s largest brackish basins is characterised by distinct salinity gradients, high productivity, and pronounced sensitivity to cyanobacterial blooms. Cyanobacteria are highly adaptable and tolerat of diverse environmental conditions. However, the response of freshwater strains to brackish water from various Baltic coastal sites, and the extent to which such conditions limit their persistence, remains uncertain. This study addressed three main objectives: (1) evaluating the ability of common freshwater bloom-forming filamentous cyanobacteria to grow in water from different Baltic coastal sites; (2) examining the combined effects of warming and CO₂ enrichment on their performance under simulated brackish conditions; and (3) testing whether selected freshwater strain can persist in pairwise co-culture with the resident Baltic cyanobacterium Nodularia spumigena. Results showed that several freshwater strains grew in water from both the fresher northern and more saline southern Baltic coastal sites. However, their responses varied by strain and were influenced by site-specific water properties, climate conditions, and biotic interactions. Notably, a cylindrospermopsin-producing strain of Aphanizomenon gracile from an eutrophic inland lake showed the highest performance and also grew in co-culture with Nodularia. Simulations indicate that certain freshwater cyanobacteria can tolerate brackish water from different Baltic coastal sites under controlled short-term conditions. Moreover, findings suggest that freshwater strains capable of persisting under brackish conditions may contribute to cyanotoxin presence risk. This risk may affect water from both the fresher northern and saltier southern Baltic coasts, highlighting an emerging ecological and public health concern. The simplified, nature-safe experimental approach provides a foundation for more complex, field-based studies assessing the ecological relevance of freshwater cyanobacteria in transitional brackish coastal systems.