We show a simple mechanism by which changes in the rate of horizontal stirring by mesoscale ocean eddies can trigger or suppress plankton blooms and lead to an abrupt change in the average plankton density. We consider a single species phytoplankton model with logistic growth and grazing and spatially non-uniform carrying capacity. The local dynamics has multiple steady states for some values of the carrying capacity that can lead to localized blooms as fluid moves across regions with different properties. We show that for this model even small changes in the ratio of biological timescales relative to the flow timescales can greatly enhance or reduce the global phytoplankton productivity. Thus this may be a possible mechanism by which changes in horizontal mixing can trigger plankton blooms or cause regime shifts in some oceanic regions. Comparison between the spatially distributed model and Lagrangian simulations considering temporal fluctuations along fluid trajectories, demonstrates that small scale transport processes also play an important role in the development of plankton blooms having significant influence on global biomass.