Planktonic and benthic invertebrate composition of the uMdlotane Estuary (South Africa) during contrasting seasonal flow states

Temporarily Closed Estuaries (TCEs) are known for their ecological importance, yet quantitative biological data are scarce/absent for many South African systems. Given the escalating anthropogenic threats facing these vulnerable systems, baseline data on their ecology and functioning is needed in order to monitor changes over time. The uMdlotane Estuary is a data poor large TCE on the northeast coast of South Africa. This study investigated the zooplankton and macrobenthic invertebrate composition in the uMdlotane Estuary during a summer high flow and winter low flow period, and identified possible environmental drivers. Zooplankton abundance was relatively constant throughout the study period, but composition differed significantly. During the winter low flow period, the system was dominated by estuarine-resident zooplankton, while in the summer high flow period, estuarine taxa were limited to the lower reaches, with the mid to upper reaches characterized by freshwater taxa. A similar pattern was evident in the macrobenthic assemblages, with taxa present dictated by the salinity regime, but the generally low abundance and presence of opportunistic species may be indicative of the episodic poor water quality. Further monitoring is needed to identify pollution pressures and prevent any escalation thereof that could result in degradation of this near natural system.