Water quality and the eutrophication of the Baltic Sea depend on the nutrient load in rivers. Therefore, nitrogen and phosphorus management in river basins is at the same time management of the Baltic Sea. We compare the impact of two 50 % nutrient reduction strategies on the Baltic Sea using a 3Dcoupled physical-bio-chemical model of the Baltic Sea (Neumann 2000). The first strategy assumes a proportional 50 % nutrient reduction in every riparian country, as suggest by HELCOM. The second approach is based on existing socio-economic calculations by Gren (2000) suggesting an optimal costeffective 50 %-nutrient reduction. In this cost-effective approach the nutrient reduction measures in the river basin are realised in countries like Poland, Russia, Lithuania, Latvia and Estonia, where the costs per reduced ton of N and P are lowest. The consequence is a reduction of the nutrient loads by more than 50 % in the southern and south-eastern Baltic countries and less reduction in Scandinavia. In previous studies the general impact of a 50 % nutrient reduction (Neumann et al. 2002) and the short term effects on the Baltic coastal waters (Neumann & Schernewski 2001) were analysed. In this study we focus on the different effects of the two approaches in different regions of the Baltic Sea in a longer perspective and refer to consequences for river basin management.
The model takes into account the real meteorological conditions of the 1980’s and we assumed an immediate nutrient load reduction by 50 %, starting in winter 1980. The reduction has immediate effects on all coastal waters. A reduced nutrient availability and shifts between the utilisation of nitrogen and phosphorus are visible in the Arkona, Bornholm and Gotland Sea already in the first summer. After 3 years the mean annual phosphorus and more intensive, the nitrogen concentrations are reduced in the south-eastern Baltic Sea. The reduction in chlorophyll a, an indicator of algal biomass, shows only a decline of about 10 %. The model suggests that the 50 % reduction of the nitrogen as well as the phosphorous load, favours blue-green algae blooms in the south-eastern Baltic Sea. In the cost-effective approach, blooms of the potentially toxic blue-green algae become even more pronounced.
Referring to our simulation results, the scheduled measures to abate eutrophication in the Baltic Sea will partly fail and generate undesirable side effects. A more pronounced reduction of the phosphorous loads might prevent an increase in blue-green algae blooms. The cost-effective approach shows, compared to the proportional load reduction approach, clear positive effects in the western, German part of the Baltic Sea and no serious differences along the Swedish coast. Due to an increase of bluegreen algae blooms, there are negative implications for the south-eastern Baltic regions. Altogether the cost-effective approach reaches the same objective with one fourth of the costs and is strongly recommendable. It makes a reallocation of water quality investments between the countries in the Baltic necessary.