Classification of estuaries in the Ciskei and Transkei regions based on physical and botanical characteristics

Abstract

For the first time a comprehensive botanical survey has taken place in the Ciskei and Transkei estuaries, Eastern Cape Province, South Africa. In total 54 plant species were found in the 92 estuaries surveyed. These plants could be divided into the following five habitat complexes; reed and sedge beds (23 species), salt marsh (20 species), mangrove forest (4 species), macroalgal assemblages (3 species) and swamp forest (4 species). Ordination showed that salinity and depth were important in influencing the distribution of species. Salinity separated salt marsh from reed and sedge species and depth separated mangrove and associated species (e.g. Zostera capensis , Halophila ovalis and Acrostichum aureum ) from reed and sedge species. Based on the plant species composition the estuaries could be divided into those that were permanently open versus those that were temporarily open/closed. The characteristic habitat complexes for the permanently open estuaries were intertidal salt marsh and mangrove forest. This region is a transition between the warm temperate and subtropical biogeographic zones and both permanently open and temporarily open/closed estuaries showed divisions at the Great Kei and Mngazana estuaries. In permanently open estuaries mangrove forest occurred north of the Great Kei Estuary and swamp forest north of the Mngazana Estuary. In temporarily open/closed estuaries reed and sedge beds occurred north of the Great Kei River and swamp forest, reed and sedge beds occurred north of the Mngazana Estuary where they replaced the salt marsh. The temporarily open/closed estuaries in the Ciskei were characterised by salt marsh and macroalgae. Submerged macrophytes such as Ruppia cirrhosa and Potamogeton pectinatus were also common. Salt marsh occurred because of high water column salinity and wide intertidal and supratidal zones. These estuaries were characterised by seawater washing into the estuary over the berm at the mouth and thus had high salinity. The variation in physical environment, changes in climate and transition between biogeographic regions has resulted in regions of high biodiversity. Mangrove, salt marsh and swamp forest species are found in the same region.