Although the concentrations of macro-nutrients are generally high in the Southern Ocean the distribution of phytoplankton biomass is highly variable:In the region of the Polar Front phytoplankton blooms are often recorded whereas the southern ACC is characterized by low phytoplankton biomass and is therefore part of the large HNLC area in the Southern Ocean. As is typical for frontal systems, the hydrography in the region of the Antarctic Polar Front is characterized by meanders and eddies as well as up- and downwelling cells which redistribute nutrients and influence the depth of the euphotic zone. To study the processes leading to the high variability in phyto- und zooplankton biomass a Biological Model for the Antarctic Polar Front (BIMAP) was developped. The model is based on nitrogen fluxes but considers possible silicate limitation. The 1D-model version comprises two compartments of each phyto- and zooplankton, nitrate, ammonium, silicate and three compartments of detritus. The model is forced by annual cycles of the mixed layer depth at different positions which are derived from a 3D-ocean-model with $(1/3)^\circ$ resolution. Model results indicate that at least part of the ecosystem's regional variability is covered by the impact of different annual cycles of the mixed layer depth. However, mesoscale features of the hydrography and biological processes which are not covered in the ``traditional'' model concept appear to play an important role as well.