Modelling regional and large-scale response of Baltic Sea ecosystems to nutrient load reductions

The entire Baltic Sea, as well as many of its different sub-regions, are su bject to eutrophication due to high nutrient inputs. To plan expensive wate r management measures one needs a tool to quantify effects of different wat er management policy decisions. The tools implemented here are simulation m odels based on similar descriptions of biochemical interactions in the wate r and sediments but coupled to different hydrodynamical models. For the Bal tic Proper a 1D physical model with high vertical resolution but horizontal ly integrated was used. Simulations for 20 years made with 50% load reducti on each 5 year show that for this domain and at these scales the recovery w ould take decades. The most effective is reduction of phosphorus, while red uction of only nitrogen leads to a dramatic increase in cyanobacteria bloom s. For the Gulf of Finland a high-resolution 3D hydrodynamic model was coup led to a more crude 3D-box biogeochemical model describing concrete conditi ons during August and November 1991. In the Eastern Gulf of Finland the eff ects of a 50% load reduction from the St. Petersburg region are pronounced even after two weeks. Here, nitrogen reduction would be more beneficial tha n that of phosphorus, both locally and at a larger scale. The conclusion fr om these simulations is that the difference in effects of nitrogen versus p hosphorus reduction is dependent on scales and locations of management.