Nutrient inputs to the Baltic Sea

Since the early 1900s, the Baltic Sea has become increasingly eutrophied as a result of increasing inputs of the nutrients nitrogen and phosphorus from anthropogenic activities in the catchment area and at sea. ​

Where do they come from?

Over 85 million (2006) people live within the Baltic Sea catchment area. Human populations as well as anthropogenic activities such as agriculture and industry contribute the majority of nutrient input to the Baltic Sea. For more information see the HELCOM Fifth Baltic Sea Pollution Load Compilation.

Nutrient inputs enter the Baltic Sea are either via air or water (Figure 1).

 

Figure 1. Different sources of nutrients to the sea and examples of nitrogen and phosphorus cycles. The flow related to ammonia volatilization shown in the figure applies only to nitrogen. In this report, also combustion and atmospheric deposition deal only with nitrogen. Emissions of phosphorus to the atmosphere by dust from soils are not shown in the figure. (Source: Ærtebjerg et al. 2003).

 

A part of the nutrient input to the Baltic Sea originates from outside the HELCOM area. Distant sources contribute with a significant portion of atmospheric inputs of nitrogen. As indicated in Figure 1 nutrients enter inland waters by different pathways and are thereafter affected by a variety of processes in rivers and lakes. The amount of rainfall and the resulting water flow in rivers, as well as groundwater inflow to inland surface waters, are important con­trolling factors de­termining the actual amounts of nutrients entering the Baltic Sea. Biological, physical, morphological and chemical factors also retain and/or transform nutrients within river systems and surrounding river valleys before they enter the sea.

Another cause for increased nutrient levels in the sea, especially in the case of phosphorus, is 'internal loading' which is caused by phosphorus pools accumulated in the sediments of the sea bed being released back to the water under anoxic conditions.

References:

Ærtebjerg, G., Andersen, J.H. & Hansen, O.S. (eds.) (2003) Nutrients and Eutrophication in Danish Marine Waters. A Challenge for Science and Management. National Environmental Research Institute. 126 pp.