Eutrophication is one of the greatest problems of the Baltic Sea. Due to excessive loading of nitrogen and phosphorus, filamentous algae have grown more common and in several areas outcompeted perennial red and brown algae. Offshore, the production and abundance of phytoplankton has increased creating surface accumulations and decreasing visibility. Biodegradation of sedimented algae consumes oxygen and is a factor contributing to the creation of anoxic bottoms. Many species of fish and benthic fauna in general suffer from the lack of oxygen which in addition to this dissolves sediment-bound nutrients for resuspension (internal loading) elevating pelagial nutrient levels even further. Total fish biomass generally increases with eutrophication but species richness decrease; cyprinid species thrive but species requiring clear and well-oxygenated waters become scarce.
Nutrient loading from Finland to the Baltic is ca. 4100 metric tons of phosphorus and 74000 metric tons of nitrogen. The proportion of natural runoff, independent of human activity, is ca. 28% for phosphorus and 38% for nitrogen. Agriculture is the most important anthropogenic nutrient source in Finland, responsible for approximately 31% of phosphorus and 27% of the nitrogen ending up in the Baltic. Phosphorus is efficiently removed from finnish wastewater and only ca. 3% of the total phosphorus input enters the Baltic in this way. Nitrogen is more difficult to remove and thus ca. 15% of the total nitrogen loading orginates from wastewater. Other important nutrient sources are industry, rainbow trout aquaculture, forestry as well as combustion engine traffic in the case of nitrogen. In addition to terrestrial runoff, large amounts of nutrients are transported through air but quantifying this pathway is difficult.
However, the quantity of loading alone does not tell the relative impact of the different nutrient sources to the Baltic ecosystem. When analysing nutrient input it is important to take into account the temporal and spatial scales of the loading. Rainbow trout aquaculture can be used as an example: even if it contributes only 1% of total nitrogen and ca.2% total phosphorus output from Finland it can be a significant cause of eutrophication locally. Same logic applies to many other loading sources.Information of nutrient input from different countries surrounding the Baltic can be found from the www pages below.