By using an experimental approach to study how upwelling deep water modulates chemistry and biology response of surface plankton communities the authors address a highly relevant scientific topic as expected climate change will significantly alter ocean currents and upwelling patterns. This is particularly important as stimulation of highly productive phytoplankton blooms by upwelling water are generally attributed to the inputs of water rich in inorganic nutrients into the sunlit surface ocean. By using an experimental approach the authors could disentangle the different drivers of phytoplankton bloom dynamics in the highly productive northern Humboldt Upwelling System. Their results indicate that upwelling modifies phytoplankton succession after upwelling by additional factors and not just the physical supply of inorganic nutrients with the deep water. Accordingly, the most influential drivers were the stoichiometry of the inorganic nutrients added and deep water microbial communities, which can cause heterogeneity in phytoplankton bloom succession and control stoichiometry of residual inorganic nutrients during phytoplankton bloom brake down. Thus, deep water community composition are a major control of phytoplankton bloom dynamics in surface waters following upwelling events which have profound implications for ocean biodiversity, productivity and biochemistry which need to be taken into account in ocean models.