A 3D-biochemical model of the Baltic Sea was deployed to investigate the effects of two different 50 % nitrogen and phosphorus load reductions scenarios on the Baltic Sea. The first scenario, according to the Helsinki Commission, assumed a proportional 50 % load reduction in all riparian countries. The second was based on a cost-effective optimal approach by Gren [1], with significant regional differences in load reduction. The comparative simulations covered a period of 16 month (January 1980 - May 1981). In the central Baltic Sea a 50 % load reduction caused a decrease in inorganic nitrogen concentration by 39 % and inorganic phosphorus declined by 8.3 % in the upper water layer (30 m). Chlorohyll-a, as an indicator for algae biomass, was reduced by 9 %. The comparison between the two 50 % reduction scenarios revealed pronounced differenced mainly in coastal waters. Near large rivers in the southern Baltic, like the Oder and the Vistula, the cost-effective scenario showed a further decrease of chlorophyll-a concentrations by more than 5 %. Altogether the water quality in southern Baltic Sea, especially in Germany, Poland and the Baltic states clearly benefits from a cost-effective approach. Along the south coast of the Baltic Sea, bathing and summer tourism is most intensive, a major economic factor and high coastal water quality is essential. This is especially true for the intended further growth of tourism. Especially the German coast has a high net economic benefit from improved water quality of Polish rivers. Therefore, from an ecological and economical viewpoint it is reasonable that Germany invests money in improved Polish water treatment measures. |
