Composition, abundance, distribution and seasonality of zooplankton in the Sundays Estuary, South Africa

Abstract

The zooplankton assemblage in the permanently open Sundays Estuary, on the south-east coast of South Africa, was studied seasonally. Samples were collected once per season between winter 2007 and autumn 2009 by means of boat-based plankton tows. A total of 19 taxa was recorded. Numerically, the copepod Pseudodiaptomus hessei dominated the zooplankton, with Paracartia longipatella and Halicyclops sp. also contributing significantly to the assemblage. Mysids were dominated by Mesopodopsis wooldridgei and Rhopalopthalmus terranatalis. Among the Brachyura, Hymenosoma orbiculare larvae and Paratylodiplax edwardsii larvae were prevalent. Zooplankton density varied with season, site and salinity zone, as has been reported in previous studies of temperate estuaries worldwide. Density peaked in summer, with means of 3 331 ind. m⁻³ in 2008 and 8 031 ind. m⁻³ in 2009. Highest densities of P. longipatella occurred in the lower estuary (Sites 1–3) and P. hessei in the upper estuary (Sites 8–10). Highest density occurred in the euhaline zone, due largely to the high density of P. longipatella. Site 1, closest to the mouth, typically supported the highest diversity of zooplankton. Patterns in zooplankton density and diversity resulted from the combined influence of salinity, temperature and fresh water inflow.