The importance of local versus external nutrient loads for Chl a and primary production in the Western Baltic Sea

•Primary production was reformulated allowing carbon fixation during N or P limitation.•Model scenarios testing effect of local and external nutrients loads were conducted.•Nutrient reductions caused a shift from surface to deep primary production.•Effects of local nutrient loading showed a steep gradient from the coast and outwards.•Local and external nutrient reductions were both important for the ecological status. The Western Baltic Sea is affected by eutrophication and receives nutrients from local land-based sources, atmospheric deposition and by advection from the neighbouring North Sea and Baltic Sea. In the present study, we evaluated the importance of local (Danish) versus external (Baltic Sea–North Sea) nutrient loads for surface Chl a-concentrations, total areal primary production and the vertical distributions of primary production in the W. Baltic Sea using the 3D coupled HBM-ERGOM model. This was assessed by improving descriptions of nutrient loads and primary production in the model and by conducting scenarios of different nutrient loads according to (i) the eutrophication level in 1990, (ii) the new Danish Water Plan (DWP) and iii) DWP+ the Baltic Sea Action Plan (BSAP2). The model results showed that local nutrient reductions have a profound effect on Chl a concentrations and primary production in the coastal areas of the Western Kattegat and the Belt Sea with the highest impact