Sediment characteristics and vegetation dynamics as indicators for the potential rehabilitation of an estuary salt marsh on the arid west coast of South Africa

The Orange River estuary is a Ramsar site that has been placed on the Montreux Record following the degradation of 300 ha of salt marsh. The groundwater and surface sediments are hypersaline and above the tolerance range of the dominant plant Sarcocornia pillansii. This study measured the sediment characteristics in 2005 (dry conditions) and in 2006 (flood and high rainfall). The sediment EC ranged from 120.9±4.7 mS cm−1 in 2005 to 52.5±4.8 mS cm−1 in 2006 with sediment moisture content sufficient for the germination of seed during both years (9.7±0.7%). The small areas of remaining S. pillansii have the potential to produce 40 billion seeds, which would be sufficient to revegetate the desertified marsh, but due to hypersaline sediment conditions the successful establishment and survival of the seedlings remain low. Many seeds (3616 m−2) and seedlings (1296 m−2) were found in drift lines in 2006. Microhabitats created by the adult vegetation also provided favourable conditions for seed germination. Laboratory studies indicated that seeds of S. pillansii germinate best in freshwater (40% at 0 psu compared to 5% at 35 psu). Exposure to hypersaline conditions (70 psu) decreased the viability of the seeds. The potential of the marsh to rehabilitate naturally therefore depends on sufficient freshwater entering the marsh.