An ecological model evaluation of two nutrient abatement strategies for the Baltic Sea

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.