A biophysical model of the interaction between vertical migration of crustacean zooplankton and circulation in the Lower St. Lawrence Estuary

As an essential step in modeling the influence of circulation on the popula tion dynamics of marine planktonic copepods, we define a simple formulation of swimming behavior that can be used in both Eulerian and Lagrangian mode ls. This formulation forces aggregation of the population toward a preferen tial depth and can be stage specific and time varying, thus allowing descri ption of either diurnal or seasonal vertical migration. We use the formulat ion to examine the interaction between the circulation and vertical distrib ution in controlling horizontal distribution of the common planktonic copep od Calanus finmarchicus in the Lower St. Lawrence Estuary, Canada. We first introduce diel migration into a simple one-dimensional model and then into a model of residual two-dimensional circulation patterns representative of conditions encountered in the Lower St. Lawrence Estuary. Results from the latter indicate that interactions between circulation and stage-specific s wimming behaviors are the main mechanisms for aggregation of planktonic cru staceans at the head of the Laurentian Channel and highlight the implicatio ns of flushing of the surface-dwelling young stages for the population dyna mics of C. finmarchicus in the Lower St. Lawrence Estuary.