Island forming processes in the Okavango Delta, Botswana

The Role of Vegetation in the Okavango Delta Silica Sink

“…The importance of amorphous Si precipitates in the build-up of islands has been pointed to in earlier research (e.g. Ramberg and Wolski 2008;McCarthy et al 2012).…”

“…It consists of a mosaic of permanent and temporary floodplains, intersected with tree-covered islands (McCarthy et al 2012). The Okavango Delta's water balance is dominated by evapotranspiration: 98 % of all water is lost to the atmosphere before reaching the outlets.…”

“…Islands in the Delta initiate around a termite mound, gradually building from this basis (McCarthy et al 1998). Large trees-which are especially dense on the island fringes (McCarthy et al 2012) -play an essential role in maintaining the hydraulic pressure gradient through transpiration. However, while the role of islands in sequestering Si is known, we have little understanding of the role of vegetation in depositing and sequestering Si in the floodplains, though they actually cover a much greater surface area.…”

Importance of bird seed dispersal in the development of characteristic vegetation on termite mounds in north-eastern Namibia

“…In this study, the term "cone" was used to refer to both active mounds without pediments and the conical parts of active mounds with pediments. Dangerfield et al (1998) and McCarthy et al (1998McCarthy et al ( , 2012 suggested that islands on the floodplain of the Okavango Delta originated as mounds built by Macrotermes michaelseni (Sjöstedt), with subsequent expansion of the original structures through accumulation of minerals in groundwater (as a result of transpiration in plants growing on the mounds). They also suggested that islands in the Okavango Delta developed through repeated colonisation of Macrotermes mounds.…”

Water-Rock Interactions and Seasonal Hydrologic Processes in Constructed Everglades Tree Islands

“…In the Mkuze freshwater-wetland system in South Africa, plants exclude many solutes (Ca, Mg, and Si) in excess to their needs from the water they take up through their roots, driving the chemical sedimentation of CaCO3, amorphous silica, and Fe-smectite (Humphries et al, 2010(Humphries et al, , 2011. In the Okavango Delta, Botswana, the elevated transpiration rate of trees growing on islands establishes a concentric groundwater-flow pattern from the surrounding swamps towards the center of the islands, where groundwater salinity steadily increases inducing the chemical precipitation of calcite and silica (McCarthy et al, 2012). Additionally, the salinity increase in the groundwater beneath the clayey soils of those islands, triggers a process of density-driven groundwater flow transporting solutes downward into the deeper aquifer layers, where saline groundwater accumulates within sand layers contributing to the salt balance of the freshwater-wetland ecosystem (Bauer-Gottwein et al, 2007;Ramberg & Wolski, 2008 …”