>> Background information

Development

The decrease of nitrogen loading from all sources has been much less significant than that of phosphorus. Relative to phosphorus, the effectiveness of nitrogen removals from waste water has remained much lower. Loading from industry has approximately halved since the mid 1980s, but loading from municipalities did not begin to decrease until mid 1990s. By 2011 it has decreased by some 30%.

22% of total nitrogen load comes from point sources, 65% from scattered loading, i. e. agriculture, forestry and scattered dwellings. Agriculture’s share of total nitrogen load is 57%. The volume of diffuse nitrogen loading has not decreased. On the contrary, loading from agriculture into inland waters has probably even increased.

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 recent years the nitrogen load from forestry has slightly decreased. The nitrogen load from scattered dwellings has remained steady since mid 1990s.

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.