Inter-annual variability in cyanobacteria blooms in the Baltic Sea controlled by wintertime hydrographic conditions

We investigated inter-annual variability of late summer cyanobacteria (cb) blooms in the Baltic Sea using the 3D ecosystem model ERGOM. A long-term simulation for the period 1982-1993 was carried out with realistic forcing. The simulated variability in cb blooms was in good agreement with satellite observations. Further simulations were performed in order to separate the influence of wintertime nutrient conditions from the impact of weather conditions during the growth period. There is strong evidence that the late-winter phosphate concentration in the surface layer has a determinant influence on late summer cb blooms. A large fraction of the inter-annual variability in cb blooms can be attributed to the variability in excess dissolved inorganic phosphorus (eDIP) in the surface layer. The amount of available eDIP in the surface layer depends on the wintertime mixedlayer depth and on the magnitude and persistency of coastal upwelling. The simulation results suggest an impact of the large-scale atmospheric conditions in winter, namely the North Atlantic Oscillation (NAO), on the occurrence of cb blooms in the Baltic Sea. It is hypothesised that the impact of the NAO is transferred by a cause-and-effect chain starting in winter and developing until late summer: high NAO index --> high wind stress, low ice cover --> high mixed-layer depth, strong upwelling --> high surface-layer eDIP concentration --> (potentially) strong cb bloom.