Eutrophication is the enrichment of nutrients in an ecosystem. Excessive amounts of nutrients encourage the growth of algae and other aquatic plants, which in turns leads to multitude of negative effects such as extensive growth of algae (algae blooms) and oxygen depletion in the sea.
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.
However, agricultural runoff from around the Baltic Sea continues to be a major source of nutrient loading to the Baltic Sea. The main pathways for nutrients are the five main rives - the Neva, Nemunas, Daugava,Vistula and Oder. The expected development of agriculture around the region will worsen conditions if reductions in nutrient inputs to the Baltic Sea are not taken.
Another problem is that 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 that in turn encourages algal blooms. The problem of the internal load in the Baltic Sea has been known for many years
Excessive amounts of nitrogen and phosphorus
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.