>> Background information

Development

Phosphorus load into inland waters from point sources - mainly from industry and municipalities - has decreased considerably during the past decades. 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. The loading from municipalities has also decreased steeply since the 1970s as more waste waters are directed to sewage treatment plants.

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 anthopogenic phosphorus load rises up to 69%. 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.

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.