  In the marine environment biological processes are strongly affected by
oceanic currents, particularly by eddies (vortices) formed by the hydrodynamic
flow field. Employing a kinematic flow field coupled to a population dynamical
model for plankton growth, we study the impact of an intermittent upwelling of
nutrients on triggering harmful algal blooms (HABs). Though it is widely
believed that additional nutrients boost the formation of HABs or algal blooms
in general, we show that the response of the plankton to nutrient plumes
depends crucially on the mesoscale hydrodynamic flow structure. In general
nutrients can either be quickly washed out from the observation area, or can be
captured by the vortices in the flow. The occurrence of either scenario depends
on the relation between the time scales of the vortex formation and nutrient
upwelling as well as the time instants at which upwelling pulse occurs and how
long do they last. We show that these two scenarios result in very different
responses in plankton dynamics which makes it very difficult to predict,
whether nutrient upwelling will lead to a HAB or not. This explains, why
observational data are sometimes inconclusive establishing a correlation
between upwelling events and plankton blooms.
