Retracing cyanobacteria blooms in the Baltic Sea

In late summer, massive blooms and surface scums of cyanobacteria emerge regularly in the Baltic Sea. The bacteria can produce toxins and add bioavailable nitrogen fixed from atmospheric nitrogen to an already over-fertilized system. This counteracts management efforts targeted at improving water quality. Despite their critical role, the controls on cyanobacteria blooms are not comprehensively understood yet. This limits the usability of models-based bloom forecasts and projections into our warming future. Here we add to the discussion by combining, for the first time, satellite estimates of cyanobacteria blooms with output of a high-resolution general ocean circulation model and in-situ nutrient observations. 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 where conditions are more nutrient-depleted compared to more coastal environments.

Automated summary

Massive blooms of cyanobacteria regularly occur in the Baltic Sea during late summer, which can negatively impact water quality. Understanding the controls on cyanobacteria blooms is crucial for better management efforts. By combining satellite data, ocean circulation models, and nutrient observations, this study identifies offshore, nutrient-depleted areas as the primary origins of blooms.