Threat of eutrophication to the Baltic Sea
A widespread and persistent problem
This excess of nutrients leads to a severe disruption of the Baltic Sea ecosystem, with effects including increasing bottom areas with no oxygen and extensive algal blooms in summertime. Two main nutrients are to blame: nitrogen (N) and phosphorus (P). Today, the Baltic Sea contains 5 times as much nitrogen and 8 times as much phosphorus as it did 100 years ago. About 80% of the inputs come from land-based activities, including sewage, industrial and municipal waste and agricultural run-off. The rest is mainly from nitrous gasses, emitted when burning fossil fuels, from traffic, industry, power generation and heating.
The good news is that many of the worst point sources of pollution have been addressed and significant gains have been made, including by improving wastewater treatment facilities and addressing industry runoff. The share of the total nutrient load contributed by these sectors has decreased substantially.
Agricultural runoff contributes to the nutrient load
Despite these improvements however, two sources continue to exacerbate the situation. Firstly, agricultural runoff from around the Baltic watershed continues its substantial contribution to the nutrient load, primarily through five main rivers - the Neva, Nemunas, Daugava,Vistula, and Oder. The expected development of agriculture around the region will worsen conditions if reductions in nutrient input to the Baltic Sea are not taken.
Secondly, much of the phosphorus already released to the Baltic Sea is now stored as an environmental liability in the sediments of the deeper parts of the basin. Anoxic (oxygen-free) zones enhance the release of phosphorus from the sediment – so called “internal loading” – and make it available for enhanced algal blooms. The problem of the internal load in the Baltic Sea has been known for many years.