Diel vertical migration in stratified tidal flows: Implications for plankton dispersal

Abstract

This paper examines the kinematics of the horizontal motion induced in marine plankton which undergo diel vertical migration (DVM) in oscillatory tidal currents with depth varying amplitude and phase. Particular attention is paid to the transport-producing effects of DVM in shallow, stratified seas where frictional decoupling associated with a reduction in the vertical eddy viscosity in the pycnocline can cause abrupt changes in tidal phase across the pycnocline. M 2 -period tidal currents, horizontal displacements of up to 5 upper layer tidal excursions can be produced over a 15-day cycle and in S 2 -period currents, net unidirectional horizontal transport at a rate of up to 1 tidal excursion per tidal cycle is possible. The transport producing effects of DVM in spatially varying tidal fields are illustrated by the case of the northwest European shelf and with the example of an idealized stratified gulf. The results have potentially far reaching consequences since marine zooplankton and phytoplankton capable of DVM are often concentrated in the pycnocline and, for those organisms with swimming behavior unaffected by the temperature/salinity gradient at the pycnocline, vertical excursions of just a few meters would be sufficient to allow significant horizontal transport.