The mechanisms by which pelagic communities are regulated in physically dynamic environments have only recently begun to be elucidated. Phytoplankton assemblages in upwelling regions are only influenced by chemical and biological interactions, but are dramatically impacted by a three-dimensional advective regime. The "success" of a phytoplankton assemblage, as defined by its biomass, growth and persistence through time, is often primarily controlled by the combined effects of particle sinking and its interaction with the physical field. It is shown that the pattern of circulation deduced from surface drogues is usually not coherent throughout the euphotic zone. Vertical movement of an upwelled water parcel has the potential for rapidly changing the light field which phytoplankton experience, thereby substantially altering primary production within an upwelling system.