Abstract: A very unique phytoplankton bloom appears every year during the austral spring/summer in the Northern Kerguelen Plateau region. The Kerguelen Ocean and Plateau compared Study (KEOPS) showed that an increase in subsurface iron coming up from the seafloor through vertical mixing was responsible for the observed increase in chlorophyll-a above the plateau. We demonstrate that the bloom pattern is not a simple increase of biomass over shallow water: it is strongly influenced by the bathymetry and its spatial extent controlled by strong currents around the plateau. Here we focus on the lateral mixing process to find explanations for the particular shape of the bloom. We use the Smagorinsky (1963) formula to estimate and map fields of lateral mixing time scales ( ) due to barotropic tidal currents, barotropic atmospheric forced currents, Ekman and geostrophic velocities. Results show that short time scale mixing is strongly influenced by the tidal process while the other processes have minor influences. Comparisons of lateral mixing coefficient and satellite chlorophyll-a images show that the spatial pattern of the bloom seems to be delimited by a barrier of high lateral mixing that is essentially due to tides. This emphasises the role played by the tides over the Kerguelen Plateau, in supplying iron to the phytoplankton and by containing the horizontal shape of the bloom. This is one of the first times such a link has been demonstrated, which has implications for the study of iron advection in the ocean.