Abstract
The way and extent water-depth, tidal-currents and species-specific migratory behaviour affect plankton distribution were quantified in the bentho-pelagic zooplankton community of the upper Gamtoos Estuary, South Africa. To compare tidal and non-tidal waters, nocturnal plankton densities were also recorded in the temporarily closed Kabeljous estuary. Most species aggregated near the sediment, especially during ebb-tides when plankters avoided faster seaward currents in surface waters. Tidal current direction modified vertical plankton stratification in even very shallow waters (ca. 1 m) by changing the amplitude of vertical migration - probably promoting retention of estuarine populations. Amphipods, and to a lesser degree eumaceans, became planktonic mainly during flood-tides, giving rise to marked differences in community structure between tidal phases and signifi-cantly increasing species diversity. In non-tidal waters, temporal density fluctuations were a function of differences in species-specific migratory behaviours.Under tidal conditions, temporal variability increased significantly with the degree plankters were axially displaced by tidal-currents. Thus, in tidal estuaries, temporal variations in plankton abundance at any given station are the combined outcome of pulses in active vertical migration and passive tidal dispersal. Consequently, multilevel sampling during both ebb- and flood-tides is seen as a minimum prerequisite for density estimation of estuarine zooplankton.