The nitrogen load to the Baltic sea — Present situation, acceptable future load and suggested source reduction

Abstract

The Baltic Sea is one of the most polluted sea areas in the world. Except for different local pollution sources, the overall environmental problem is eutrophication. The eutrophic situation is caused by increased external load of nutrients during decades. Since nitrogen (N) is generally considered the main limiting nutrient for primary production in the Baltic Sea, the countries within the Baltic Sea drainage basin have agreed to reduce the anthropogenic waterborne input of N by 50% between 1987 and 1995. Furthermore, N emissions to the atmosphere are to be frozen at the level that existed at the end of the 1980s. A number of countries have also declared their intention to reduce N emissions to the atmosphere by 30% between 1980 and 1998. Presently, the annual N load to the Baltic Sea totals 1,409,000 tonnes, of which 980,000 tonnes (69%) are waterborne discharges (rivers, point and non-point sources), and 134,000 tonnes (10%) are caused by N₂ fixation. The rest, 296,000 tonnes (21%) is atmospheric deposition on the sea surface. Some 60% of the atmospheric deposition is caused by emissions from sources in the drainage area and the rest is emissions from areas outside the drainage area. The highest acceptable future N load to the Baltic Sea is estimated at 600,000 tonnes/year. This means a necessary reduction of 57%, compared to the present load. Therefore, comprehensive load reductions must be conducted from anthropogenic sources, e.g. agriculture, forest and forestry, industry, and municipal sewage treatment plants and rural living. The atmospheric N deposition must be reduced by almost 40%, and the measures must focus on emission reductions in traffic, combustion, and agriculture.