The purpose of this paper is to investigate the influence of horizontal grid resolution in a physical model on an embedded primary production model. The area for the experiment was along the west coast of Norway, from 60◦N to 70◦N, an area of high mesoscale activity. The HYbrid Coordinate Ocean Model was coupled with the NORWegian ECOlogical Model system, and run in a nested system, consisting of three model grids with horizontal resolution of 50 km, 16 km and 4.5 km (hereafter: COARSE, MEDIUM and FINE) in the focus area. Two main results were obtained, first, the composition of the phytoplankton functional groups changed with increasing model grid resolution. In FINE, the diatoms produced a larger part (60%) of the total annual primary production than the flagellates, whereas in COARSE and MEDIUM, the primary production from the two phytoplankton groups were equal. This was explained by a higher transport of silicate into the euphotic layer in FINE compared to the other two. Second, the differences in the primary production first became large when the resolution of the model grid reached the Rossby radius of deformation. Whereas the total net primary production in MEDIUM only was 5% larger than in COARSE, the total net primary production in FINE was 20% higher than in COARSE. This was explained by the models ability to resolve mesoscale activity.