Phytoplankton bloom development and mesoscale hydrography was studied in detail during the SO-JGOFS cruise of RV POLARSTERN in Dec/Jan 1995/96 at the Polar Front (PF) in the South Atlantic. Increased surface chlorophyll concentrations (up to 2 mg/m$^{3}$) coincided with the meandering position of the PF and were associated with a strong gradient in dissolved Si concentrations from 0 to greater than 26 $\mu$Mol. Nitrate levels were alwaysgreater 20 $\mu$Mol. The phytoplankton was characterized by elongated, spiny forms of highly silicified species (e.g. Thallassiothrix spp., Chaetoceros spp., Pseudonitzschia spp.).Areas with high phytoplankton biomass were characterized by high ratios of biogenic silicate (BSi) to particulate organic nitrogen (PON) with BSi : PON being greater than 1 but mostly above 2. Accordingly, uptake rates of BSi : PON were greater than 1.In contrast to high phytoplankton biomass and increased uptake of dissolved silicate due to plankton growth, no vertical flux was measured. We thus conclude that during the austral summer 95/96, a slowly growing, not sinking phytoplankton assemblage was associated with the northern part of the PF. In contrast to previous years, this bloom did not alter the CO$_{2}$ balance (NIOZ-results) between the atmosphere and upper ocean significantly.