Pathways and sources of nutrient inputs
The land-based nutrient inputs entering the Baltic Sea are either airborne or waterborne. The main pathways of nutrient, i.e. nitrogen and phosphorus, input to the Baltic Sea are:
direct atmospheric deposition on the Baltic Sea water surface;
riverine inputs of nutrients to the sea. Rivers transport nutrients that have been discharged or lost to inland surface waters within the Baltic Sea catchment area;
point sources discharging directly to the sea.
The different sources for the inputs of nitrogen and phosphorus are from:
atmospheric emissions of airborne nitrogen compounds emitted from traffic or combustion of fossil fuels (heat and power generation), and from animal manure and husbandry, etc.;
point sources including inputs from municipalities, industries and fish-farms both discharging into inland surface waters and directly into the Baltic sea;
diffuse sources, which mainly originate from agriculture, but also include nutrient losses from e.g. managed forestry and urban areas;
and natural background sources, mainly referring to natural erosion and leakage from unmanaged areas and the corresponding nutrient losses from e.g. agricultural and managed forested land that would occur irrespective of human activities.
Large proportions of nutrient loads originate far away from the sea, and even from outside the HELCOM area. Many processes occur after nutrient input into the catchment area which affect their final input into the Baltic Sea. Rainfall and subsequent river run-off, as well as groundwater inflow to inland surface waters, are controlling factors that determine the final amounts of nutrients entering the Baltic Sea. Biological, physical, morphological, and chemical factors also retain and/or transform nutrients within river systems.
Another cause for increased nutrient levels in the sea, especially in the case of phosphorus, is the "internal load"; phosphorus reserves accumulated in the sediments of the sea bed are released back to the water under anoxic conditions.
In 2000, the total inputs amounted to 1009700 tonnes of nitrogen and 34500 tonnes of phosphorus.
About 75 % of the nitrogen entered the Baltic Sea as waterborne input and 25 % as airborne input. Agriculture and managed forestry contributed almost 60 % of the waterborne nitrogen inputs to the sea, 28 % entered from natural background sources and 13 % came from point sources. The airborne nitrogen input has been calculated as direct atmospheric deposition on the Baltic Sea. It originated from emissions to the air from inside as well as outside the Baltic Sea catchment area and from ship traffic.
It has been estimated that airborne phosphorus input is only 1-5 % of total phosphorus input. Agriculture together with managed forestry contributed nearly 50 % of the waterborne phosphorus inputs to the sea and point sources and natural background sources each contributed approximately 25 % of the phosphorus input to the Baltic Sea.
HELCOM Recommendations, EU Regulations and national legislation have contributed to stricter controls on industry and municipal wastewater treatment plants. As a result nutrient pollution from point sources has reduced significantly between 1985 and 2000. However, the reduction targets for diffuse sources such as agriculture have not yet been fulfilled. Although a lot of measures have already been taken by many countries, further measures in many regions seem to be necessary to reduce pollution from agriculture. In open sea areas, airborne inputs, which also originate from outside the HELCOM countries, are a main source of nitrogen input. These additional sources are ship traffic on the Baltic Sea as well as non-HELCOM countries outside the Baltic Sea catchment area.
For further information on nutrient inputs, see: