IW2 Nitrogen

>> Background information

Development

The decrease of nitrogen loading from all sources has been much less significant than that of phosphorus. In the 2000s the average annual nitrogen loading was about 76 000 tons. The share of nitrogen loading from point sources was 22 %, from scattered loading 61 % and from airborne deposit 17 %.

The nitrogen loading from point sources has decreased during recent decades. Loading from industry sector has nearly halved since 1980. The loading from municipalities has also decreased somewhat but is still nonetheless quite high. Relative to phosphorus, the effectiveness of nitrogen removals from waste water has remained much lower. Other point sources are fish farming, fur farming and peat production. Nitrogen load from fish farming and peat production have decreased since 1995. The load from fur farming has remained at the same level.

The nitrogen loading from scattered sources has grown slightly since the mid-1990s. Agriculture is the main source of anthropogenic nitrogen load, in the 2000s its share has been over 50 %. In forestry the nitrogen load is mainly due to draining forest land. The acreage of first-time draining has decreased, but ditch clearing still causes a great nitrogen load in inland waters. In the 2000s the share of nitrogen load from forestry has been about 5 %. The loading from scattered dwellings has remained pretty much at the same level during the recent decade, it?s share has been about 3 %.

Impact on biodiversity

The extra nitrogen in inland waters causes eutrophication. The effects on biodiversity are quite similar with nitrogen and phosphorous loading, although nitrogen is not quite so strong a nutrient.

Eutrophication causes changes on many food chain levels. In eutrophic inland waters the amount of water vegetation and the number of species increase. At the same time the species composition changes when the species of low nutrient waters disappear.

The total amount of fish increases. The cyprinid fish species benefit from eutrophication while the salmonoids suffer. The amount of animal plankton decreases because of larger fish stock. This further increases the amount of phytoplankton and hence the turbidity of water. The bottom flora tends to diminish when the amount of light reaching bottom decreases.

In severly eutrophic lakes the decomposition and the oxygen consumption accelerates. Lack of oxygen affects many species harmfully. Benthos and fish species composition become poorer and oxygen depletion during winter months might cause high fish mortality.

At first eutrophication improves the circumstances for many species but after eutrophic conditions progress the species' richness starts to fall again.