The reduction of nutrient loads is the overriding strategy in order to abate eutrophication and to improve the ecological state of the Baltic Sea. A 3D-ecosystem model of the Baltic Sea was used to analyze the effects of two different 50% nitrogen and phosphorus load reduction scenarios. The first scenario assumed a proportional 50% load reduction in all riparian countries. The second was based on a cost-effective approach by Gren [I.-M. Gren, 2000. Managing a sea. Cost-Effective Nutrient Reduction To The Baltic Sea. Earthscan Publ., London. Ch. 43–56.] with significant regional differences in load reduction.The simulations suggest that a 50% reduction of nitrogen and phosphorus loads affect predominantly the coastal waters and favours cyanobacteria blooms in the central Baltic Sea. In the cost-effective approach, blooms of the potentially toxic cyanobacteria become even more pronounced in the northern part of the Baltic Proper. The comparison between the two 50% reduction scenarios reveals differences mainly in coastal waters. Near large rivers in the southern Baltic, like the Oder and the Vistula, the cost-effective scenario shows a greater decrease of nutrients and chlorophyll-a concentrations. Altogether the water quality in southern Baltic Sea, especially in Germany, Poland and the Baltic states benefits from a cost-effective approach. However, differences in nutrient and chlorophyll-a concentrations between the scenarios are small.
Referring to our simulation results, in a medium-term perspective the measures to abate eutrophication in the Baltic Sea will be not very efficient concerning the central Baltic Sea and might generate undesirable summer blooms of nitrogen-fixing cyanobacteria. These are a result of nutrient imbalances in the system and may persist for several decades. A more pronounced and early reduction of the phosphorus loads might shorten the period of bloom persistence.
Keywords: Baltic Sea; Biogeochemical cycle; Modelling; Nutrient loads