Perched wetlands: An explanation to wetland formation in semi-arid areas

- Wetlands in the NMBM are precipitation dominated.
- Wetlands were perched on calcrete, bedrock and clay across the study area.
- Clay and bedrock perches were associated with various underlying geological formations.
- Perched water tables are key to wetland development in some semi-arid areas.

This research demonstrates the importance of “perches”, a hard, impermeable sub-surface layer, for ephemeral wetland development in a semi-arid area. Over 1700 wetlands were identified in the Nelson Mandela Bay Municipality (NMBM), Eastern Cape, South Africa. The average wetland density in the Municipality is approximately nine wetlands per 10 km2. This density is high considering the climate of the NMBM, where annual evapotranspiration rates are approximately three times higher (1800 mm) than the average annual rainfall (613 mm). The NMBM is diverse in terms of its climate, vegetation types, geomorphology and underlying geology. This environmental diversity has resulted in a range of wetland types. Previous research in the Municipality has indicated that the majority of these systems are precipitation driven, with minimal groundwater input. Therefore, other environmental processes facilitate the formation of wetlands. For this study, data from 46 wetland sites were used that were situated across the different environments in the NMBM (geology, climate etc.). Thirty-four wetlands were perched, and were from all three of the different wetland types studied: depressions, seeps and wetland flats. A dense clay layer was found at 14 sites, across different geological and sediment types. Calcrete lenses were recorded at seven sites that were associated with aeolian deposits. A shallow bedrock layer was also observed at nine sites on quartzitic sandstones of the Peninsula Formation. In conclusion, this study has highlighted that perches are key for wetland development in some semi-arid areas.