Zooplankton in a marine bay II. Vertical migration to maintain horizontal distributions

Abstract

Zooplankton populations can maintain their horizontal distributions by the interaction of vertical movement with current shear. In Westernport Bay, Australia, a poorly stratified marine bay, current shear results from bottom friction, and its direction changes with that of the tidal stream. Vertical profiles of zooplankton abundance showed that adults of the most abundant species resident in the bay, Acartia tranteri (medium sized form), migrate vertically in synchrony with the tides, and in a direction to reduce losses to mixing out of the bay. The authors develop a simple model to show that this vertical migration of A. tranteri adults was sufficient on average to overcome mixing losses. Pseudodiaptomus cornutus may have also migrated vertically to avoid mixing out of the bay, but other bay residents did not; these species may have alternative strategies to mitigate against washout. None of the non-resident species migrated in synchrony with the tides, presumably because of a lack of selective pressure to do so.