South African coastal outlets and estuaries: what defines an estuary versus an outlet?

This review examines the transition of coastal micro-outlets to micro-estuaries, to fully functional and species rich estuaries of various types. The definitions of the different types of microsystems are reviewed and it is apparent that an estuary is not simply a coastal water body where river and seawater meet. A range of other conditions, especially regular marine-estuarine connectivity and the persistence of a relatively large waterbody, are required for a flourishing estuarine biota to become established. In contrast to many global countries, the South African coastline has a continuum of coastal microsystems that cover the full spectrum of micro-outlets of various sizes, through micro-estuaries in the process of becoming estuarine systems, to a range of fully functional estuaries such as temporarily open/closed and predominantly open estuaries, to large estuarine lakes and bays. The impact of sea-level rise and fall on the exposure or inundation of coastal waterfalls is also examined, thus providing additional insights into the transformation of a freshwater outlet into a functional estuary. Furthermore, the creation of artificial concrete canal outlets in some of the major coastal cities is documented, with some estuaries being lost in the process.