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

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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.