BS1 Phosphorus

>> Background information

Development

Most of the anthropogenic phosphorus load from Finland flows into the Baltic Sea in river water. On average 3 600 tonnes of phosphorus was discharged annually in the 2000s. More than 90% of this came dissolved in river water. Finland's contribution to the total phosphorus load of the Baltic Sea is approximately 10%.

Since 1990 there has been a moderate declining trend in phosphorus load from rivers. During this time annual phosphorus loading has decreased by some 20% although annual fluctuations complicate interpreration.

The loading of nutrients correlates closely with weather conditions and the quantity of water flowing in rivers. In years with abundant rain more nutrients are leached from soil than in dry years. In the 2000s warm winters have shortened the period when soils are frozen and covered with snow. Therefore more nutrients are leached during winter than before.

The phosphorus concentration in surface water differs between areas of the Baltic Sea. In the Bothnian Bay the concentration has stabilized on the level of the mid-1980s. In the Gulf of Finland and Archipelago Sea, where the concentrations are higher, there has been more fluctuation. These changes are propably linked to the trends of the Baltic Proper.

Impact on biodiversity

Primary production in the Finnish part of the Baltic Sea is mostly limited by the amount of available phosphorus. This is most apparent in the northern sea areas. Phosphorus limitation means that any additional amount of the nutrient in sea water increases primary production which, in turn, often leads to reduced water transparency, changes in the composition of phytoplankton and benthic communities as well as decreasing oxygen levels, for example.

While phosphorus may often be considered the most important nutrient causing eutrophication (nutrient enrichment) in the northern Baltic Sea the role of nitrogen is often equally important.

Eutrophication causes changes in species composition and abundance. Many submerged vascular plants and algae suffer from increasing water turbidity. Well-known examples of such clear water species include the Bladderwrack and Eelgrass. While their abundance decreases other plants, especially those living in the surface water layer, become more common. A visible example in the Baltic Sea is the Blanket Weed, which grows on rocks in the shoreline.