A solid knowledge of the mechanisms that influence successional patterns and species dominance in phytoplankton blooms and the subsequent fate of biomass after bloom termination is a prerequisite to predict the response of plankton communities to environmental changes and the associated past and future changes in geochemical cycles and pCO2. Process studies like iron fertilization experiments are a new and powerful approach to elucidate the in-situ mechanisms that determine dominance and succession patterns in phytoplankton blooms.All iron fertilization studies conducted so far have shown that iron is a limiting factor for phytoplankton growth in HNLC areas. Model studies and a recent comparison of all iron fertilization experiments have suggested that the deep mixed layers occurring in the Southern Ocean, might constrain the built-up of phytoplankton biomass and iron utilization below the levels found in other oceanic areas with stratified regimes. Here we report growth rates of phytoplankton species and biomass (in chlorophyll a) accumulation from two fertilization experiments carried out in the Southern Ocean (EISENEX and EIFEX). We show that despite deep mixed layers (> 40 to about 100 m depth) dominant species showed high growth rates that argue against the importance of light limitation in controlling biomass accumulation in the Southern Ocean.