Since large, rapidly-sinking particles account for most of the vertical flux in the ocean, mechanisms responsible for particle aggregation largely control the transport of carbon to depth. The particle flux resulting from a variety of different phytoplankton bloom conditions was simulated with a numerical model in which phytoplankton growth dynamics were combined with physical aggregation, particle size-dependent sedimentation and degradation. Model results demonstrated that particle flux to the deep ocean be generated by solely invoking physical aggregation during phytoplankton blooms. Sensitivity of the model in response to variations of both physico-chemical and biological paramters was tested. The model outcome, described as the fraction of export production leaving the upper ocean carbon pool, proved to be most sensitive to biological variables such as phytoplankton cell size, stickness, and growth characteristics (i.e. solitary vs chain-forming). Changes in these factors strongly affect the efficiency of the "biological pump" and could be explain interannual and geographic variance in deep-ocean flux. © 1992.