  Turbulence has been recognized as a factor of paramount importance for the
survival or extinction of sinking phytoplankton species. However, dealing with
its multiscale nature in models of coupled fluid and biological dynamics is a
formidable challenge. Advection by coherent structures, as those related to
winter convection and Langmuir circulation, is also recognized to play a role
in the survival and localization of phytoplankton. In this work we revisit a
theoretically appealing model for phytoplankton vertical dynamics, and
numerically investigate how large-scale fluid motions affect the survival
conditions and the spatial distribution of the biological population. For this
purpose, and to work with realistic parameter values, we adopt a kinematic flow
field to account for the different spatial and temporal scales of turbulent
motions. The dynamics of the population density are described by an
advection-reaction-diffusion model with a spatially heterogeneous growth term
proportional to sunlight availability. We explore the role of fluid transport
by progressively increasing the complexity of the flow in terms of spatial and
temporal scales. We find that, due to the large-scale circulation,
phytoplankton accumulates in downwelling regions and its growth is reduced,
confirming previous indications in slightly different conditions. We then
explain the observed phenomenology in terms of a plankton filament model.
Moreover, by contrasting the results in our different flow cases, we show that
the large-scale coherent structures have an overwhelming importance. Indeed, we
find that smaller-scale motions only quite weakly affect the dynamics, without
altering the general mechanism identified. Such results are relevant for
parameterizations in numerical models of phytoplankton life cycles in realistic
oceanic flow conditions.
