Phytoplankton blooms play a major role in global elemental cycles and marine food chains but different species impact these in different ways. Hence a mechanistic understanding of the factors regulating species dominance is a prerequisite to predictive modelling. Results from EisenEx revealed 6 distinct response behaviours to iron fertilization. Five species of the over 50 recorded, contributed 48% of total diatom biomass. Of these, two species (a pennate and a centric) reacted immediately with accumulation rates about double those of other species and built up population size exponentially. In contrast species with initially highest abundances responded after a week with a brief period of rapid accumulation which levelled off later. The ratio of live cells to intact empty and broken frustules (LC:EBF), used as a measure of mortality, increased steadily inside the fertilized patch indicating that the grazer assemblage could not keep pace with growth of the dominant species. Species-specific differences in the LC:EBF ratio and the ratio of intact empty to broken frustules indicated that selective grazing by proto- and metazooplankton played an important role in structuring the diatom assemblage.