Modelled source apportionment of nutrient loads to Baltic Sea basins under current and future conditions

René Capell, Alena Bartosova, Johan Strömqvist and Berit Arheimer


Swedish Meteorological and Hydrological Institute, Norrköping, Sweden


Riverine nitrogen and phosphorus loads to the Baltic Sea originate from multiple sources such as diffuse leakage from agricultural areas and point emissions from industrial and waste water treatment plants. Besides variation in overall load amounts from different land areas around the Baltic Sea, there is also spatial variation in relative load contributions from individual source types.

A source apportionment of nitrogen and phosphorus loads at sea outlets allows us to differentiate between net contributions of different sources and to compare their relative importance for riverine nutrient emissions into the Baltic Sea. Because nutrient emissions undergo different retention-removal processes during transport from source to sea outlets, the importance of load contributions from individual source types can differ between gross emissions into surface waters and net contributions at sea outlets. We can estimate surface water retention and removal by comparing gross nutrient emission into surface water and net transports at sea outlets. Such an analysis of nutrient load source apportionment can be used to support the design of effective programs of measures to target load reductions into the Baltic.

Here, we conduct a comparative analysis of nutrient load source apportionment under current and future conditions. We use E-HYPE v.3.1.4, a pan-European integrated rainfall-runoff and nutrient transport model developed by SMHI, to conduct a source apportionment analysis for nutrient loads into the Baltic Sea as a whole and into individual Baltic Sea sub-basins as defined by HELCOM based on available data for hydrological and nutrient model calibration, gross emission estimates, and reduction-removal processes. In particular, we include the following aspects in our analysis of E-HYPE modelled nutrient loads:

  • Long-term source apportionment into Baltic Sea sub-basins under current (1981 to 2010) and future (2036 to 2065) conditions using climate forcing data from four climate models under IPCC Representative Concentration Pathway (RCP) 8.5
  • Inter-annual variation of source apportionment
  • Comparison of future source apportionment under three IPCC Shared Socioeconomic Pathways (SSPs): SSP1 (Sustainability), SSP2 (Middle of the road), and SSP5 (Fossil-fueled development)

Long-term source apportionment under current and future climate conditions give a holistic overview over the relative importance of different nutrient sources around the Baltic today, and provide insight into expectable changes due to climate change impacts. By comparing model results from four different climate models, we can complement projected changes with climate model uncertainty estimates, while inter-annual variation in loads provide an insight into the impact of hydrological conditions on load amounts. Finally, a comparison of loads under different SSPs allows comparing the impact of socioeconomic developments on nutrient loads from different sources.