PLANKTON SPECIES SELECTION DURING THE IRON-FERTILIZATION EXPERIMENT EISENEX IN THE SOUTHERN OCEANV. Smetacek (1), P. Assmy (1), J. Henjes (1), C. Klaas (2)(1) Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany, (2) Max-Planck-Institute for Biogeochemistry, Winzerlaer Str. 10, 07745 Jena, Germanyvsmetacek@awi-bremerhaven.deAmple evidence has now accumulated showing that phytoplankton growth rates in the modern Southern Ocean are limited by iron availability. A major source of iron to the land-remote ocean is aeolian dust and it is very likely that higher dust input during glacials will have resulted in higher productivity and much greater CO2 drawdown. The phytoplankton blooms characteristic of iron-replete ecosystems tend to be dominated by a few key species (generally diatoms) whose biomass fuels the biological carbon pump. In situ iron fertilization experiments provide a powerful tool to study the ecology and biological characteristics of these key species which determine magnitude and composition of flux. During EisenEx, carried out with RV Polarstern in the Polar Frontal Zone during November 2000, the ecology of dominant phytoplankton species in the iron-fertilized bloom was studied in detail. We show that selective grazing pressure of a large range of protozoo- and metazooplankton was a major factor determining species selection. Although the fate of the bloom could not be followed due to lack of shiptime the data provide insights on factors governing the dynamics of plankton blooms that will help in interpreting the sedimentary record.