SMALL-SCALE DISTRIBUTION AND VARIABILITY OF DEMERSAL ZOOPLANKTON IN ASHALLOW, TEMPERATE ESTUARY - TIDAL AND DEPTH EFFECTS ON SPECIES-SPECIFIC HETEROGENEITY

The way and extent water-depth, tidal-currents and species-specific mi gratory behaviour affect plankton distribution were quantified in the bentho-pelagic zooplankton community of the upper Gamtoos estuary, Sou th Africa. To compare tidal and non-tidal waters, nocturnal plankton d ensities were also recorded in the temporarily closed Kabeljous estuar y. Most species aggregated near the sediment, especially during ebb-ti des when plankters avoided faster seaward currents in surface waters. Tidal current direction modified vertical plankton stratification in e ven very shallow waters (ca. 1 m) by changing the amplitude of vertica l migration - probably promoting retention of estuarine populations. A mphipods, and to a lesser degree cumaceans, became planktonic mainly d uring flood-tides, giving rise to marked differences in community stru cture between tidal phases and significantly increasing species divers ity. In non-tidal waters, temporal density fluctuations were a functio n of differences in species specific migratory behaviours. Under tidal conditions, temporal variability increased significantly with the deg ree plankters were axially displaced by tidal-currents. Thus, in tidal estuaries, temporal variations in plankton abundance at any given sta tion are the combined outcome of pulses in active vertical migration a nd passive tidal dispersal. Consequently, multilevel sampling during b oth ebb- and flood-tides is seen as a minimum prerequisite for density estimation of estuarine zooplankton.