The Swedish Agency for Marine and Water Management has commissioned SMED to evaluate nitrogen and phosphorus loads from different sources, on lakes, watercourses and the sea across Sweden for the year 2017. The aim is to provide the basis for Sweden's national reporting to the HELCOM "Pollution Load Compilation 7 - PLC7" and to support water management work in Sweden. This report, together with its background reports, presents results, source data, and calculation methods with a level of detail intended to achieve full transparency and traceability as well as to permit further applicability in Swedish water management.
The calculations followed, as far as possible, the methodology developed in the previous project (PLC6). Large amounts of data have been processed and calculated to provide comprehensive information for the whole of Sweden divided into approximately 24 500 water bodies. The results are freely available via the web tool ”Technical Calculation System: Water” (TBV, tbv.smhi.se).
The results are presented as gross and net nutrient loads, for the total, anthropogenic and background load. Gross loads are the amount of nutrients released at the source of a watercourse or lake. Net loads are the proportion of the gross loads that reaches the sea.
The two largest sources of the total nutrient loads to the sea for both nitrogen and phosphorus (net load) constitute of agricultural and forest land, with 33 400 and 31 670 tonnes/year of nitrogen, respectively, and 1 010 and 870 tonnes/year of phosphorus in 2017. Together, these sources account for about 60 % of the total load.
For anthropogenic loads, agriculture is the largest source (19 470 tonnes/year and 710 tonnes/year of phosphorus), followed by emissions from sewage treatment plants (14 050 tonnes of nitrogen and 230 tonnes/year of phosphorus). Nutrient loads from growing forests are only included in the background loads, while clear cuts, which are classified as an anthropogenic load, only contributes 1 540 tonnes/year of nitrogen and 20/year tonnes of phosphorus.
The Bothnian Sea, the Baltic Proper and the Kattegat are the sea basins that receive the most nitrogen from Sweden's total loads (28 560 tonnes/year, 26 150 tonnes/year and 27 700 tonnes/year, respectively, or approximately 25% each). In the Bothnian Sea, however, the greater part of the loads are natural background loads. The Baltic Proper and the Kattegat receive most of Sweden's anthropogenic nitrogen load, 30% and 31% respectively.
For phosphorus, the Bothnian Sea is the basin that receives the largest load (1 040 tonnes/year or 32% of the total load). Just under a quarter of Sweden's total phosphorus load reaches the Baltic Proper (790 tonnes/year) and about one fifth reaches the Bothnian Bay and the Kattegat (640 and 620 tonnes/year respectively).
The Baltic Sea Action Plan (BSAP) provides emission targets, with the aim of achieving good environmental status in the Baltic Sea and the Kattegat. For phosphorus, the target has been achieved in all sea basins except the Baltic Proper, where the target is challenging, and it will be difficult to reduce the phosphorus load below the target (308 tonnes/year). This requires extensive measures for the anthropogenic sources, but further challenging is that the background loads are a significant proportion of the total loads. The total net loads of phosphorus to the Baltic Proper are 760 tonnes/year of which 230 tonnes/year are calculated as background loads. For the Baltic Proper to be able to achieve good environmental status with regard to eutrophication, measures will also be needed in the other sub-basins of the Baltic Sea.
Due to differences in methods and input data, it is not possible to directly compare how the loads from diffuse sources have changed since PLC6 (2014). One difference in methodology that should be noted is the calculation of the background load of phosphorus from agricultural land. The background calculation method has been developed between different PLC calculations, which has also led to strongly varying results for the background load. In PLC6 a high background load was calculated because an update of the model turned out to give a presumably too high loss of particulate phosphorus. This has been corrected in the latest model version (PLC6.5 and PLC7) which is one of the reasons why the background load in PLC7 is lower than I PLC6. It is however important to note that the background calculation always will be uncertain because the calculations, to a large extent, are based on assumptions and because data to compare the calculation results with are lacking.
To be able to compare diffuse loads between the years there is a need for recalculation of old PLC data, either recalculation of the old data based on the new methodology or based on new data and the old methodology. This kind of recalculations may clarify the effect of implemented measures to reduce the load, or if the changes are mainly due to refined input and improved calculation methods. Nutrient loads from point sources are calculated using the same methodology as in previous calculations. Emissions to the sea in PLC 7 (2017) from sewage treatment plants are approximately of the same magnitude as in PLC 6 (in 2014) 230 and 240 tonnes of phosphorus and 14 000 tonnes of nitrogen (net). Industries have reduced their nutrient loads to the sea and are responsible for 210 tonnes of phosphorus in 2017 and 3 320 tonnes of nitrogen, compared with 250 phosphorus and 3 800 tonnes of nitrogen in 2014.