Future increase of filamentous cyanobacteria in coastal Baltic Sea predicted by multiple realm models of marine, terrestrial, and climate change scenarios

Blooms of filamentous cyanobacteria regularly occur in the Baltic Sea during warm summer months. These blooms can be toxic and interfere with recreational activities. However, the underlying drivers of these events and how their distribution might change in response to future climate conditions remain poorly understood. To investigate this, we applied a multi-realm modeling approach that integrates environmental data from both marine and terrestrial systems, combined with climate projections, to predict future filamentous and nitrogen-fixing cyanobacterial distribution along Sweden's Baltic Sea coast. Our models identified several key factors significantly influencing bloom distribution: terrestrial temperature, precipitation during the wettest quarter, sea surface temperature, nitrate levels, and interactions between land-based and marine environmental variables. Our projections suggest an expansion of filamentous cyanobacteria in the northern Baltic Proper, Bothnian Sea, Bothnian Bay, and Arkona Basin driven by rising land temperatures by 2070, increasing sea surface temperatures by 2100, and declining salinity in specific basins. Overall, the results demonstrate that incorporating environmental data from both land and sea improves predictions of cyanobacterial distribution in coastal Baltic Sea regions. This multi-realm modeling strategy may also prove valuable for forecasting and managing harmful cyanobacterial blooms in other coastal areas experiencing similar environmental challenges.