Environmental factors controlling the vegetation zonation patterns and distribution of vegetation types in the Olifants Estuary, South Africa

The Olifants Estuary is one of only three permanently open estuaries on the West Coast and is ranked third in terms of conservation importance of all estuaries in South Africa. It has the largest supratidal (143 ha) and floodplain (797.1 ha) salt marshes in the country. Intertidal salt marsh covers 91.94 ha, reeds and sedges 60.05 ha and the submerged macrophyte Zostera capensis 47.72 ha. Correspondence analysis revealed intertidal, supratidal, floodplain and terrestrial plant community types within the studied estuary. The species composition of these communities was the same for the lower, middle and upper reaches of the Olifants Estuary, with the exception of the supratidal community type in the upper reaches that consisted of halophytes with a lower salinity tolerance range rather than the typical dense monospecific stands of Sarcocornia pillansii. The environmental variables that had the greatest influence on the distribution of the dominant salt marsh species, S. pillansii, included soil moisture, distance from the estuary, elevation above mean sea level, and depth to the water table. The most important ecological driver for salt marsh vegetation, especially along the arid West Coast of southern Africa, is moisture. The low rainfall and irregular occurrence of advection sea fog increases the importance of a shallow (< 1.5 m) saline (< 35 psu) water table in the floodplain as a source of moisture during dry periods.