Effects of prolonged mouth closure in a temporarily open/closed estuary: a summary of the responses of invertebrate communities in the uMdloti Estuary, South Africa

Abstract

Prolonged mouth closure in estuaries is often associated with hypersalinity, mainly as a result of reduced freshwater inflow. However, on the KwaZulu-Natal coast, South Africa, temporarily open/closed estuaries have a tendency towards hyposalinity during closed mouth conditions, primarily because of their perched nature reducing saltwater intrusion, but also due to persistent freshwater inputs, together with seepage of saline water through the sand berm into the sea. We investigated a system, the uMdloti Estuary, experiencing extended periods of mouth closure over two years with the aim of documenting the response of invertebrates to such conditions. Specifically, estuarine invertebrate biodiversity was severely diminished, which was attributed to persistently low salinity, rather than poor water quality due to the absence of flushing. Secondly, a distinct brackish/freshwater community established itself within the mesozooplankton and benthic invertebrate assemblages, including invasive species. Marine overwash of the berm and a short mouth breaching aided the return of estuarine species for only very brief periods (weeks). We conclude that although estuarine species are quick to re-colonise the estuary after a breach, prolonged mouth closure along extended stretches of coast might severely reduce estuarine invertebrate biodiversity. Management options are preferably the provision of an adequate freshwater inflow to allow natural breaching, or else artificial breaching, to facilitate several openings per year as well as for a long enough tidal phase to permit the establishment of an estuarine salinity gradient.