Sources of nutrient loading in the Baltic Sea

Finland has more than ten times more forestry than agricultural land. However, most of the nitrogen and phosphorus entering the Baltic Sea comes from cultivated fields.

Arable land is more nutrient-rich than woodland and is tilled annually. Also, a large portion of cultivated fields is located along rivers and in low-lying coastal catchment areas. This increases their loading impact on the sea.

By contrast, catchment areas with a high density of lakes are in fact very efficient at retaining nutrients. In particular, lakes in which the water exchange rate is slow, effectively reduce the levels of nutrients ending up in the sea.

It is good to remember that not all land-to-sea loading is introduced via rivers. Nutrient loads can also come directly from point sources along the coast, such as urban centres and industry.

 Agricultural field close to a river bank.

Agriculture is the largest source of nutrient loading

Agriculture is Finland's most important source of nutrient loads to the Baltic Sea. In the period from 2008 to 2012, it accounted for 47-78% of the phosphorus loads of human origin, depending on the sea area.

A significant part of the agricultural nutrient load is discharged into marine areas which are sensitive to eutrophication. These sea areas are made vulnerable because of their shallowness, as well as by poor water exchange rates caused by archipelagos and shoals.

Agriculture has been loading the Baltic Sea since the 1950s

In the 1950s, agricultural land use and production began to become more efficient. Fertilisation increased the nutrient content of fields, especially between the 1960s and 1980s. Since then, the use of fertilisers has been reduced, and the nutrient balance of the fields has decreased. Nevertheless, the effects of former fertilisation are still visible in the soil.

Nowadays, farm sizes have increased, with some specialising regionally and different production types being concentrated in different parts of the country. Cereal production is concentrated in the southern and southwestern coastal zones. Livestock farming, in turn, is located in Inner Finland and Northern Ostrobothnia, while pork and poultry production is concentrated in southwestern Finland and southern Ostrobothnia. This sharp separation between livestock farming and cereal production has made it difficult to utilise manure in cereal production.

Agricultural field prepared with a tractor.

The nutrient loading of arable fields was brought under control in the 2000s

The abundant use of artificial fertilisers resulted in long-term growth in nutrient levels on arable land. However, since the late 1980s, the amount of fertilisation has fallen, particularly for phosphorus levels but also for nitrogen.

The phosphorus load from agriculture continued to increase until the 1990s. After this, the load started to decline. Today, the phosphorus load has decreased by about 20%, compared to the levels in the late 1990s. This decrease is highest (28%) in the catchment areas of the Kvarken and the Bay of Bothnia (28%).

By contrast, nitrogen discharges into the sea continued to increase into the 21st century, particularly from rivers discharging into the Bay of Bothnia. The increase is assumed to be due to the growth in arable land and livestock production, as well as an increase in the application of manure fertilisers. The nitrogen load only showed a slight decrease in the period from 2007 to 2012.

Nutrient loading from agriculture can be further reduced

The nutrient load from agriculture can be reduced by intensifying nutrient recycling, fertilising according to the needs of the plants, utilising livestock manure as fertiliser, and fixing the nutrients in the soil with gypsum.

The agri-environmental compensation scheme, formerly known as the agri-environmental support scheme, aims to reduce nutrient loading from agriculture by guiding the activities of farmers. In general, it is easier to limit the fertilisation of fields than to try to deal with the nutrients that escape from them.

It should be noted that soil properties change slowly. A reduction in the quantity of fertiliser does not immediately translate into a reduction in nutrient load. Besides, the characteristics of the water area may delay its improvement, even if the external load is reduced.

Urban wastewater pollution has decreased but still requires improvement

The second-largest source of nitrogen loading in Finnish marine areas is from human wastewater. However, its loading contribution has been reduced due to the enhanced removal of nitrogen from municipal wastewater.

The wastewater from almost all Finnish urban areas has been treated at treatment plants since the mid-1980s. Nevertheless, reducing the nutrient load of municipal wastewater is still important.

The loading from wastewater treatment plants has decreased markedly, particularly for organic matter and phosphorus. Typically, over 90% of the phosphorus is removed, with removal rates greater than 95% achieved at the largest treatment plants. By contrast, on average only half of the nitrogen entering wastewater plants is removed. Nitrogen removal is particularly concentrated in purification plants located near the coast.

In particular, the preparedness of small wastewater treatment plants for disruption is often still inadequate. In the event of operational disturbances, emergency procedures often resort to spill-over measures. This is especially problematic in poorly maintained and leaking sewerage networks.

The treatment of wastewater from dispersed settlement areas is inadequate in places

The second-largest phosphorus load comes from dispersed settlement areas. In Finland, about 800,000 inhabitants still live in properties that are not connected to the water supply and sewerage network. These properties have their own wastewater system instead.

Significant improvements are being made in the treatment of wastewater from dispersed and recreational settlements. In recent years, considerable investments have been made to reduce the nutrient loading caused by sparsely populated areas.

Nutrient loading from forestry is still visible after a long period

Forestry is the third-largest source of phosphorus loading in water bodies. While it only accounts for a small proportion of the total discharge into the sea, it can be a significant local source of phosphorous, particularly in the high-peat and drained catchment area of the Bay of Bothnia.

Over the past decades, the nutrient loading caused by forestry has been reduced, in particular by the reduction of forest drainage and the introduction of more effective water protection measures. The reduction in drainage will further reduce the nitrogen load slightly. However, phosphorus loads are likely to increase slightly as the activity of phosphate fertilisation of peatlands has increased in recent years.

Contrary to previous estimates, the nutrient loading caused by forestry measures appears to continue for decades after the drainage has been completed.

Industrial plant.

In Finland, the pulp and paper industry cause nutrient loading in waterways

The industrial nutrient load entering watercourses was at its peak in the early 1970s. Since then, the load has dropped significantly. The heaviest burden on waterways in Finland has been from the pulp and paper industry.

The phosphorus and nitrogen loads from industry began a clear decline in the second half of the 1980s. This was due to both improvements in industrial processes and a clear increase in the efficiency of wastewater treatment.

Between 1985 and 1995, industrial phosphorus and nitrogen loading decreased by 57% and 34%, respectively. During the next decade, i.e. 1995-2005, phosphorus and nitrogen were further reduced by 46% and 22%, respectively. However, there was only a small decrease in industrial nutrient loading in the 2000s.

Nitrogen also enters the sea from the atmosphere

The third-largest source of nitrogen loading to the seas comes from nitrogen deposited from the air, i.e. by the atmospheric deposition of nitrogen into lakes. If the atmospheric deposition of nitrogen entering the sea from rivers and lakes is added to that deposited directly to the sea, it would make the total nitrogen deposition the second-largest source of nitrogen loading in Finnish coastal waters.

Fish farming causes localised nutrient loading

In Finland, fish farming is concentrated in the sea areas off the southwest and southern coasts. In the Archipelago Sea, fish farming accounts for 3% and 2%, respectively, of the total phosphorus and nitrogen loads (excluding fish farms in Åland). However, within the areas where fish farming is concentrated, it is the largest source of localised nutrient loading.

The nutrient load caused by aquaculture was at its highest in the late 1980s. Since then, the load has dropped to less than one-third of what it was at that time. This decrease in nutrient loading was mainly driven by a decrease in fish farming production and a reduction in the specific load.

 An infographic about nutrient loading in the Gulf of Finland.
Water protection measures taken in the Gulf of Finland have reduced nutrient loads coming from the mainland. Although the phosphorus load entering the Gulf of Finland has decreased significantly, the management of nitrogen loading is more difficult. A lot of nutrients are introduced via rivers. This diffuse load is difficult to reduce. Source: SYKE. Illustration: Kaskas Media Oy.

Natural leaching is a type of nutrient loading that is not dependent on humans

Not all of the nutrient flow from the catchment area is due to human activity. The part of the nutrient stream that comes from the catchment area but which is independent of human activity is known as natural leaching, i.e. loading that occurs naturally in nature. Together, the nutrient loading caused by human activity and natural leaching constitutes the total nutrient load.

Between 2010 and 2016, Finnish rivers transported an average of 3,500 tonnes of phosphorus and 77,000 tonnes of nitrogen annually. Natural leaching accounted for 28% and 38% of the phosphorus and nitrogen loads, respectively. The remaining nutrient loading was caused by human activities.