Oceanic phytoplankton assemblages composed predominantly of picophytoplankton respond to the onset of favorable growth conditions with diatom-dominated blooms, the formation of which involves characteristic growth and accumulation responses by both diatoms and the ambient nondiatom community. Contrary to conventional wisdom, both groups of phytoplankton increase in growth rates and absolute abundance, but the biomass increase of the ambient nondiatom assemblage is modest, especially compared to the order of magnitude or more increase of diatom biomass. This enormous proportional increase in diatom biomass has fostered the misconception that diatoms replace the nondiatom taxa by succession as the bloom matures. However, while the relative abundance of the nondiatom taxa decreases dramatically, their absolute biomass increases modestly and the specific growth rate of picophytoplankton in the bloom increases; at the same time, protistan grazing rate also increases, holding the picophytoplankton assemblage in the bloom to a new steady state biomass concentration. Recent evidence for the ubiquity of the additive response pattern in pelagic diatom blooms comes from observations in many oceanic regions where equatorial upwelling, eddy dynamics, tropical instability waves, and oceanic iron-addition experiments have allowed documentation of the biological response to rapid onset of favorable nutrient, micronutrient or light conditions. The response of diatoms to these favorable conditions is well known; this report offers a more accurate description of the response of the ambient nondiatom taxa to rapid onset of favorable conditions. Realistic representation of the growth dynamics of both the diatoms and nondiatoms in blooms is required to improve forecasting of how future conditions will affect processes that control carbon recycling and export.