  Plankton constitutes the productive base of aquatic ecosystems and plays an
essential role in the global carbon cycle. The impact of hydrodynamic
conditions on the biological activity of plankton species can manifest in a
variety of different ways and the understanding of the basic effects due to
multiscale complex flows still appears incomplete. Here, we consider a
predator-prey model of plankton dynamics in the presence of a turbulent flow
past an idealized island, to investigate the conditions under which an algal
bloom is observed, and the resulting patchiness of plankton distributions. By
means of direct numerical simulations, we explore the role of the turbulent
intensity and of the island shape. In particular, we focus on population
variance spectra, and on their relation with the statistical features of the
turbulent flow, as well as on the correlation between the spatial distribution
of the planktonic species and velocity field persistent structures. We find
that both the average biomass and local plankton dynamics critically depend on
the relation between advective and biological time scales, confirming and
extending previous results obtained in simpler flow settings. We further
provide evidence that, in the present system, due to local flow action plankton
accumulates in localized filamentary regions. Small turbulent scales are found
to impact the statistics of plankton density fields at very fine scales, but
the main global features of the population dynamics only weakly depend on the
Reynolds number and are also found to be remarkably independent of the
geometrical details of the obstacle.
