A mechanism for horizontal zooplankton transport by vertical migration in tidal currents

A mechanism is proposed by which net horizontal transport can be induced in an organism which migrates vertically in a purely oscillatory, vertically sheared tidal current. The mechanism contains two elements. First, for all reasonable tidal current profiles, net horizontal transport is induced in any organism which migrates vertically with a period which is an exact multiple of the tidal period. (This is the basis for the widelyknown process of selective tidal stream transport where the migration period and the tidal period are exactly equal.) The second element in the new mechanism is the observation that diel migration, the most common form of vertical migration, has a period of 24 h and is therefore an exact multiple of the principal solar semi-diurnal tidal constituent (S2) which has a period of 12 h. This relation between the S2 and diel migration periods stems from the fact that both phenomena are independently locked to the solar cycle. Diel vertical migration can therefore interact with the S2 tidal current constituent to induce longterm horizontal transport in migrating organisms. For reasonable tidal current and vertical migration parameters it is found that horizontal transport rates of 4 km d^–1 are possible. The direction of net transport depends upon the phase of the S2 tidal current relative to local noon. The hypothesis is therefore proposed that geographical variation in S2 phase is a factor responsible for creating regions which are either retention-favorable or otherwise for diel-migrating marine organisms.