The Role of Vegetation in the Okavango Delta Silica Sink

Abstract

We assessed the role of vegetation and hydrology in the Si cycle in the Okavango Delta. Our results show a large storage of biogenic Si (BSi) in vegetation and the sediments. The biological storage is among the highest observed so far for any ecosystem worldwide. Floodplain vegetation accumulates similar amounts of BSi in both the temporary floodplains and the permanent floodplains, with most values observed between 20 and 100 g Si m −2 . This vegetation Si, after litterfall, contributes to a large biogenic Si storage in the sediments. In temporary floodplains, sediments contain less BSi (375-1950 g Si m −2 in the top 5 cm) than in the permanent floodplains (1950-3600 g Si m −2 in the top 5 cm). BSi concentrations in the floodplain sediments decline exponentially indicating rapid dissolution. In the occasional and seasonal floodplains, unidirectional solute transfer from floodplains to the islands will remove Si from the riverine systems. Our work clearly emphasizes the crucial role of floodplains and wetlands in Si transport through tropical rivers, and the potential interference of hydrology with this role.

Variability in chemistry of surface and soil waters of an evapotranspiration-dominated flood-pulsed wetland: solute processing in the Okavango Delta, Botswana

“…This study suggests that major nutrients (such as P, N, K, Fe, and DSi) brought in solution by surface water may be scavenged by macrophytes in the Panhandle and the permanent swamps, and cycled multiple times before being transported downstream to seasonal floodplains. This is supported by the results of Struyf et al (2015), who showed that DSi trapping by floodplain vegetation results in a large temporary store of biogenic silica. Downstream of Guma (as the hydroperiod becomes more seasonal), the concentrations of all major cations, DSi, and DOC increased proportional to increasing conductivity, suggesting that processes operating on solutes were dominated by evapoconcentration and bulk sequestration in island soils and groundwater.…”

“…Previous studies on Si cycling in aquatic vegetation worldwide have noted the importance of emergent species (amongst which are sedges and grasses) and submerged species as a sink for Si accumulating up to 27 mg·g -1 . Struyf et al, (2015) (especially Fe) and are generally redox sensitive, and hence are usually present only in small quantities in the waters of the Delta. They are thus at a premium and will be scavenged by emergent macrophytes from passing water.…”

“…From mass-balance calculations this results in the accumulation of about 360 000 t of dissolved solutes per year in the Okavango Delta . There are indications that differential partitioning of various solute species occurs in the transition between surface water and island groundwater (Mladenov et al, 2014;Struyf et al, 2015;, possibly as a result of pH or redox potential (Eh) changes. The processes dominating the cycling of solutes in surface-and soil waters of the Delta, before they eventually precipitate or are sequestered in highly saline groundwater beneath island centres, are not fully understood, however.…”

Silicon in aquatic vegetation

“…Exemplary to this is the strong (permanent) Si sink in the Okavango Delta sediments (Botswana) that can be associated with a dominance of tropical giant grasses such as Cyperus papyrus (Struyf et al . ).…”

Orbital forcing of glacial/interglacial variations in chemical weathering and silicon cycling within the upper White Nile basin, East Africa: Stable-isotope and biomarker evidence from Lakes Victoria and Edward

“…Street-Perrott and . Secondly, refilling of the lake basin after 15 ka BP (Berke et al, 2012;Johnson et al, 1996;Stager and Johnson, 2008) may have remobilized large amounts of isotopically depleted BSi from newly flooded wetland soils (Frings et al, 2014b;Struyf et al, 2015). Thirdly, Lake Victoria overflowed into the White Nile from~14.2 to 14 ka BP onwards Williams et al, 2006), shortening the residence time of DSi in the waterbody (Beuning and Russell, 2004).…”