Influence Of Wintertime Hydrographic Conditions On Cyanobacteria Blooms In The Baltic

No abstracts are to be cited without prior reference to the author.

The influence of wintertime hydrographic conditions on the inter-annual variability of late summer cyanobacteria (cb) blooms in the Baltic Sea is investigated. A simulation with the 3D ecosystem model ERGOM was carried out with realistic forcing. 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 excess phosphorus (eDIP) concentration in the surface layer has a determining influence on late summer cb blooms. The amount of available eDIP in the surface layer is coupled to the wintertime hydrographic conditions by the mixed layer depth. Finally, the simulation results suggest an impact of the large scale wintertime atmospheric conditions, namely the North Atlantic Oscillation (NAO), on the occurrence of cb blooms in the Baltic Sea. For the first time consequences of NAO-related changes in the nutrient availability on the phytoplankton dynamics are outlined. 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 → high surface layer eDIP concentration →(potentially) strong cb bloom and vice versa.