The main strait (Great Belt) connecting the North Sea and the Baltic Sea constitutes a quasi-stationary front and exposes phytoplankton to various degrees of water column mixing. Here, we examine phytoplankton community distributions (using the cell abundance of 4 readily identifiable diatoms) and estimate new production along the strait during early spring. Vertical turbulent mixing was ~10 times greater at stations in the strait compared to stations outside the strait. New production in the strait was on average ~50 mg C m-2 d-1, i.e. 8% of the average total primary production and could explain the increase in chlorophyll observed along the strait. An NMDS-analysis of phytoplankton community composition showed significant spatial groupings. However, variation of species abundances could not be explained by the general transport, where the abundance of the largest species decreased during the passage of the strait. A relatively small species (Guinardia delicatula) showed an increasing dominance in and above the subsurface chlorophyll maximum along the strait, and the bottom layer was also correspondingly dominated by a relatively small species (Skeletonema marinoi). This phytoplankton composition could be explained by photosynthetic traits associated with more efficient light usage of small cells together with increased nutrient supply in the strait.