Cyanobacteria blooms in the Baltic Sea

In the Baltic Sea, cyanobacteria regularly form massive blooms in late summer. These blooms can produce toxins and have the potential to counteract management efforts to limit eutrophication because they add bioavailable nitrogen (fixed from the atmosphere) to an already over-fertilized system. Despite their critical role and substantial research progress, the controls on cyanobacteria are still not comprehensively understood. This can limit the accuracy of model-based projections. Our study adds to the ongoing discussion by providing a comparison of existing contemporary model formulations. This comparison is supplemented by a unique combination of satellite estimates of cyanobacteria blooms, in-situ nutrient observations and output of a high-resolution general ocean circulation model (MOMBA). We retrace bloom origins and conditions by calculating the trajectories of respective water parcels backwards in time. In an attempt to identify drivers of bloom development, we find that blooms originate and manifest themselves predominantly offshore. Potential implications for contemporary modelling approaches are discussed.