IW1 Phosphorus

 >> Background information

Development

Phosphorus load into inland waters from point sources - mainly from industry and municipalities - has decreased considerably during the past decades. In 2011 it was 470 tons, 67 % less than in 1990. In the 2000s the share of phosphorus load from point sources was about 13 %.

The amounts of loading from point sources started to decrease markedly in the 1980s when effective treatment facilities were installed in pulp factories, in particular. In industry sector the phosphorus load?s decreasing trend has continued till the 2000s.

The loading from municipalities has also decreased steeply since the 1970s as more waste waters are directed to sewage treatment plants. At the moment the treatment plants can pure 95 % of the phosphorus in the sewage water.

Other point sources are fish farming, fur farming and peat production. The phosphorus load from fish farms has halved since 1990. Also the load from peat production has decreased, but in fur farming the loading has remained at the same level.

The prevention of loading from diffuse sources has not been as effective. Besides scattered dwellings, the most important sources of diffuse loading are agriculture and forestry. Agriculture's share of all anthropogenic phosphorus loading has been about 64 % in the 2000s. There have been significant investments in water protection in agriculture. As a result, the use of fertilizers has decreased and the amount of border strips covered by vegetation has increased. Yet, the load has not significantly decreased, since phosphorus has accumulated in the soil of fields that have been fertilized for decades.

The nutrient load from forestry has been reduced by limiting first-time forest drainage, but the load will probably still increase since ditch clearing remains common. In addition, the fertilization of peatlands has increased. The share of phosphorus loading from forestry has been about 7 % in the 2000s.

About a million people still live outside the sewage system in Finland. Scattered dwellings are the single largest phosphorus source after agriculture and industry. The phosphorus load from scattered dwellings has decreased slightly since 1990.

Impact on biodiversity

Phosphorus is generally the most important nutrient limiting primary production in Finnish lakes. As the amount of phosphorus in the water increases, lakes become eutrophic and their vegetation increases. At first the amount of different habitats and species usually increases. The increase is however a local phenomenon. If the share of eutrophic lakes in a catchment area is great, the regional species richness is actually reduced, because the species of clear water lakes are lost. In addition, as the nutrient concentrations become too high the number of species decreases even locally. For example perch and epiphytic algae are species which benefit from mild increase of nutrients but begin to decline as eutrophication proceeds.

Eutrophication affects the whole ecosystem and changes species composition. Fishes become generally more abundant, but fish communities become dominated by roach and other cyprinid species. At the same time salmonids become rearer. The most visible products of eutrophication are algal blooms (IW6) which significantly weaken the habitat quality for submerged vegetation and benthic communities.

The long-term effects of phosphorus arise from its tendency to sediment on the bottom of a water body. This sedimented phosphorus may be released back into the water and to the use of organisms. Phosphorus is released in anoxic conditions, usually during winter or in the middle of summer when the water is stratified and the oxygen concentration near the bottom is decreased. This internal load may harm the ecosystem processes of a lake for years, even after the external load has decreased.