Articles | Volume 19, issue 2
https://doi.org/10.5194/hess-19-981-2015
https://doi.org/10.5194/hess-19-981-2015
Research article
 | 
23 Feb 2015
Research article |  | 23 Feb 2015

Nitrogen surface water retention in the Baltic Sea drainage basin

P. Stålnacke, A. Pengerud, A. Vassiljev, E. Smedberg, C.-M. Mörth, H. E. Hägg, C. Humborg, and H. E. Andersen

Abstract. In this paper, we estimate the surface water retention of nitrogen (N) in all the 117 drainage basins to the Baltic Sea with the use of a statistical model (MESAW) for source apportionment of riverine loads of pollutants. Our results show that the MESAW model was able to estimate the N load at the river mouth of 88 Baltic Sea rivers, for which we had observed data, with a sufficient degree of precision and accuracy. The estimated retention parameters were also statistically significant. Our results show that around 380 000 t of N are annually retained in surface waters draining to the Baltic Sea. The total annual riverine load from the 117 basins to the Baltic Sea was estimated at 570 000 t of N, giving a total surface water N retention of around 40%. In terms of absolute retention values, three major river basins account for 50% of the total retention in the 117 basins; i.e. around 104 000 t of N are retained in Neva, 55 000 t in Vistula and 32 000 t in Oder. The largest retention was found in river basins with a high percentage of lakes as indicated by a strong relationship between N retention (%) and share of lake area in the river drainage areas. For example in Göta älv, we estimated a total N retention of 72%, whereof 67% of the retention occurred in the lakes of that drainage area (Lake Vänern primarily). The obtained results will hopefully enable the Helsinki Commission (HELCOM) to refine the nutrient load targets in the Baltic Sea Action Plan (BSAP), as well as to better identify cost-efficient measures to reduce nutrient loadings to the Baltic Sea.

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Short summary
In this study, we used the MESAW statistical model to estimate the surface water N retention. Such large-scale estimates are lacking for the Baltic Sea and there are only a few studies of this globally. Our results show that around 380 000t of N are annually retained in surface waters draining to the Baltic Sea. The total annual riverine load from the 117 basins to the Baltic Sea was estimated at 570 000t of N, yielding a total surface water N retention of around 40%.