HORIZONTAL ZOOPLANKTON DISPERSAL BY DIEL VERTICAL MIGRATION IN S-2 TIDAL CURRENTS ON THE NORTHWEST EUROPEAN CONTINENTAL-SHELF

A mechanism is discussed whereby diel (24 h period) vertical migration of marine organisms interacts with semi-diurnal tidal currents to induce net horizontal transport. This happens because diel vertical migration, which is locked to the day-night cycle, is synchronized with the S-2 (principal solar semi-diurnal) tidal constituent which has a period of 12 h and is also sun-synchronized. Diel vertical migration has not previously been considered to be implicated in the generation of net zooplankton transport in tidal currents. The key parameter which characterizes migration-induced transport is S-2 current phase. The phase modulates the magnitude and strictly controls the direction of migration-induced transport. S-2 current phase maps determined from a hydrodynamic tidal model of the northwest European continental shelf show regions of expected convergence, divergence and retention of diel-migrating marine organisms.