Figure 2-6 - uploaded by Bruce Kelbe
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South Africa has 280 estuaries, of which about 75% are classed as Temporarily Open/Closed Estuaries (TOCEs). The ecology of TOCEs is very much dictated by mouth closure events, the frequency of closures, how long the mouth remains closed each time and how much backing up of water occurs behind the closed mouth.
This study focused on the Mlalazi E...
Citations
... This estuary has been permanently open since 1952 ( Hill, 1966 ;Papadopoulos et al., 2002 ) and is characterized by the presence of sandbanks near the river mouth. Nonetheless, Kelbe and Taylor (2019) note that since the early 1800s, the catchment and estuary have experienced escalating human impacts. The significant alteration has been the recurring breaching of the estuary mouth ( Kelbe and Taylor, 2019 ;Taylor, 2020 ), typically occurring within a few weeks after closure (with the earliest recorded instance dating back to the late 1890s). ...
... Nonetheless, Kelbe and Taylor (2019) note that since the early 1800s, the catchment and estuary have experienced escalating human impacts. The significant alteration has been the recurring breaching of the estuary mouth ( Kelbe and Taylor, 2019 ;Taylor, 2020 ), typically occurring within a few weeks after closure (with the earliest recorded instance dating back to the late 1890s). The microtidal nature of the study site, uMlalazi Estuary, is characterized by a maximum tidal range of 2.13 m at Mtunzini Beach ( Ortega-Cisneros et al., 2011 ). ...
... The Mlalazi River flows through a range of different geological formations, including the Karoo Supergroup, the Table Mountain Group, and the Natal Group ( Kelbe and Taylor, 2019 ). They consist of sedimentary rocks, including sandstone, shale, conglomerate and basalt. ...
... It is set in the southernmost part of the Maputaland Coastal Plain. Draining into the estuary from beyond the coastal plain is the perennial Mlalazi River, a river that is periodically subjected to severe flooding (Kelbe and Taylor 2019). In its regional context, the largest major estuary complex to the north is Richards Bay and the Mhlathuze Estuary (26 km away), which have the largest stands of mangroves in South Africa. ...
... To obtain an estimate of the total area of habitat that can support mangroves in the Mlalazi Estuary, the depth-area curve developed by Kelbe and Taylor (2019) was used to estimate the area that lies in the range from mean water level to extreme high tidal level. This provides a low-resolution quantification of the maximum possible area of mangroves that the Mlalazi Estuary can support. ...
... This provides a low-resolution quantification of the maximum possible area of mangroves that the Mlalazi Estuary can support. This depth-area curve is derived from the digital elevation model (DEM) used by Kelbe and Taylor (2019). ...
The Mlalazi Estuary has 40 ha of mangroves, but before the 1930s there were none. The purpose of this study is to understand why this change occurred, and how the understanding thus gained can provide ecological information that will help understand the dynamics of similar estuaries in the region. The approach was to accumulate evidence gathered from historical records, published literature and scientific investigations. In the past 6 000 years, the Mlalazi Estuary evolved from an open bay to a permanently open estuary and then to its current state of a temporarily closed estuary. There is evidence that the mangroves that occurred in the system were lost when the estuary mouth started closing for extended periods. Since 1896, Mlalazi has been artificially breached whenever closure resulted in the backing up of water. This mouth-breaching regime resulted in physical conditions suitable for mangrove growth. Mangrove propagules would have dispersed from adjacent estuaries; taking more than 30 years for Avicennia marina and Bruguiera gymnorrhiza to colonise, and approximately 100 years for Rhizophora mucronata. Once established the mangrove population expanded and there is evidence that available habitat is becoming limiting. Any alteration to the current mouth management strategy is likely to affect the mangroves and must not be implemented without a full understanding of its consequences. The Mlalazi Estuary is a unique estuary in South Africa in that mouth manipulation has led to the colonization of mangroves not previously current in the system.
