Monitoring and simulating threats to aquatic biodiversity in the Okavango Delta: field and laboratory methods

The Okavango Delta, situated in northwest Botswana between E22.0°-E24.0° and S18.0°-S20.5°, is the world's second largest inland wetland region. The Delta is actually an alluvial fan and is fed mainly by the Okavango River whose catchment lies largely in the highlands of central Angola (Fig 1). The river flows south-east through the Caprivi Strip in eastern Namibia, before entering into Botswana as a large river, some 200 m in width. The size of the Delta changes significantly throughout the year - during the dry season, the Delta is approximately 7,000 km2, and more than doubles in size to over 15,000 km^{2} during the wet season (Ramberg et al. 2006)

Spatial and Seasonal Variability in Surface Water Chemistry in the Okavango Delta, Botswana: A Multivariate Approach

The annual flood pulse in the Okavango Delta (Botswana), has a major influence on water chemistry and habitat. We explore spatial and temporal patterns in a suite of chemical variables, analysed from 98 sample points, across four regions, taken at different stages of the flood cycle. The major pattern in water chemistry is characterised by an increasing gradient in ionic concentration from deep-water sites in the Panhandle to more shallow, distal regions to the south. Concentrations of cations, anions, dissolved organic carbon, and SiO2 are significantly higher in the seasonally inundated floodplains than in permanently flooded regions. Several variables (including Na and total nitrogen) significantly increase from low flood to high flood, while others (including HCO3, SiO2, and Cl) increase in concentration, initially between low flood and flood expansion, before declining at maximum flood extent. Redundancy analysis (RDA) revealed that hydrological variables (water depth, flow velocity, flood frequency, and hydroperiod class) significantly explain 17% variation in surface water chemistry. Predictions of increasing flood volume in the near future may result in a decline in alkalinity and dilution of DOC. Our study provides an important baseline from which to monitor future change in the Delta

Characterization of arsenic occurrence in the water and sediments of the Okavango Delta, NW Botswana

Detailed chemical analyses were performed on surface water, groundwater and sediment samples collected from the Okavango Delta between February and November 2003 in order to examine the distribution and geochemistry of naturally occurring As in the area. Surface water in the Okavango Delta, which is neutral to slightly acidic and has high dissolved organic C (DOC), was found to be slightly enriched in As when compared to a global value for stream water. Of the 20 new borehole analyses from this project, six were found to have values exceeding 10 μg/L, the current World Health Organization provisional guideline value for As. The results from field speciation indicate that As(III) is slightly more predominant than As(V). There is a positive correlation between As and pH and between As and DOC in the groundwater samples. For the sediment samples, there is a positive correlation between As and Co, As and Fe, As and loss on ignition (LOI) and between As and the percent fines in the sample. Reductive dissolution of oxides and hydroxides in the sediments with organic C as an electron acceptor is the likely mechanism for the release of As from the sediments into the groundwater

Deltamethrin in sediment samples of the Okavango Delta, Botswana

Deltamethrin concentrations were determined in 35 sediment samples collected from three different habitats: channel, lagoon and pool sites from Xakanaxa in the Okavango Delta, NW Botswana. The samples were Soxhlet-extracted in acetone to extract deltamethrin residues and subsequently cleaned-up with silica gel 60. The final determination was carried out with a gas chromatograph equipped with an electron capture detector (GC-ECD). The sample work-up and determination gave deltamethrin recoveries of 54 to 97%, and detection limits of 0.004 mg/kg dw. The concentration of deltamethrin residues in the sediment samples collected from the three sprayed areas in the Okavango delta ranged between 0.013 and 0.291 mg/kg dw, with the highest concentrations observed in samples obtained from the pool sites. Analysis of samples for organic matter content showed percentage total organic carbon (% TOC) ranging between 0.19% and 8.21%, with samples collected from the pool having the highest total organic carbon. The concentrations of deltamethrin residues and the % TOC in sediment samples showed a similar trend with the highest levels recorded in the pool samples. These data confirmed that a simple method based on GC-ECD, after Soxhlet extraction, was robust enough to enable quantification of deltamethrin in the sediments, because comparable results were obtained with a more sophisticated system consisting of a GC coupled to a mass spectrometer with a time of flight (TOF) analyser. Water SA Vol.32 (4) 2006: pp.483-48

Cryptic indicators of provenance from the geochemistry of the Okavango Delta sediments, Botswana

The siliciclastic sediments of the Okavango inland Delta of northwest Botswana have a modal composition of quartz arenites and result from a complex history, including transport by river and deposition in a nascent rift basin located in a desert environment with input of aeolian sands. The geochemical composition of sediments from the Okavango Delta was determined in order to constrain the role of weathering at the source and the composition of the source rocks. The chemical analyses and the interelement ratios show a broad compositional range usually encompassing the PAAS composition. The chemical index of alteration (CIA) values and the A–CN–K diagram define an evolution trend which can be interpreted using a mixing model involving a strongly weathered component which corresponds to the sedimentary fraction transported by the Okavango River and a relatively immature component which corresponds to the aeolian sand component of the Okavango sediments. Field geological data supported by geochemical ratios involving elements with affinity for mafic–ultramafic and felsic rocks such as Th/Cr, Th/Sc, La/Sc, La/Co and Eu/Eu* support a source area including mafic–ultramafic and felsic rocks, with or without intermediate rocks. The relationships between certain elements (Cr–Ni, Na2O–Al2O3, K2O–Al2O3) refine the interpretation by pointing to the existence of at least three source rock end-members, including a felsic rock source and pyroxene-rich and olivine-rich mafic–ultramafic source rocks. Proterozoic granitoid–gabbro and related volcanic and ortho-metamorphic rock complexes exposed in NW Botswana and adjacent Angola and Namibia are the source rocks of the sediment component which was mixed with aeolian sand and interacted with a variable proportion of diagenetic carbonates to produce the Okavango sediments

Indicators of desiccation-driven change in the distal Okavango Delta, Botswana

This work seeks to determine whether riparian woody plant variables respond to drying and salinity regimes in the semi-arid distal Okavango Delta, northern Botswana. Structural and compositional variables were obtained from 47 field sites. Mapping using satellite imagery illustrated differences in the character of riparian zones in terms of species composition and provided data on flood frequency. Salinity data plots show increases downstream. Results imply that woody plant variables respond to desiccation-driven change due to water-table lowering (reduced recharge) and increased salinization through distinct changes in tree and shrub height, plant density and species richness. In the wetter, intermediate distributaries, key biotic indicators of ecosystem change comprise structural variables such as decreases in canopy cover per cent and tree height and increases of shrub height, which are indicative of mainly ground-water declines. Biotic indicators in the less frequently flooded receiver channels comprise plant density and species richness increases involving mainly brackish ground-water-tolerant and dryland species which are indicative of both ground-water declines and/or salinization. These indicators could provide useful parameters for use in long- and short-term monitoring aimed at assessing desiccation-driven change in different parts of the Okavango Delta and possibly other semi-arid wetlands. The indicators are important as a less-expensive alternative to drilling as a means of verifying ground-water declines and/or salinization

Rift basins in their early stage of development: Examples from the Makgadikgadi-Okavango-Zambezi Basin and the southern Tanganyika Basin. Geochemical composition of sediments as tracers of climate change and tectonic evolution

1 p.We present the results of a geochemical study of Quaternary sediments from two rift basins in the East African Rift, the Makgadikgadi-Okavango-Zambezi (MOZ) basin and the Mpulungu sub-basin of Lake Tanganyika. The early initiation stage in the development of a continental rift is characterized by the development of shallow half-graben basins where nascent faults exert a primary control in the evolution of drainage catchment formation. This stage is illustrated by the present Okavango fan, located within the larger Makgadikgadi-Okavango-Zambezi basin (MOZ) in NW Botswana. The immediate successive stage concerns the development of basins where shallow lacustrine environments occur, as the result of fault growth represented by the larger MOZ basin. A more mature stage concerns the development of well-defined, actively subsiding half-graben basins, where propagation and interaction between fault segments lead to basin linkage illustrated by the southern Mpulungu sub-basin of Lake Tanganyika. The use of geochemical compositions of sediments in diagnostic diagrams has been shown to be useful in determining tectonic settings although the field defined for passive margins (PM) include a wide range of tectonic environments, from rifted continental margins of the Atlantic-ocean type, sedimentary basins near to collision orogens and inactive or extinct convergent margins. In this work, based on the major element geochemistry, we define two new fields within the PM setting being 1)alluvial fan (AF) and 2) lacustrine basin (LB).The provenance of the sediments of both the Okavango Delta and Mpulungu sub-basin sediments was determined in this study. For the Mpulungu sub-basin, our results indicate that the immediate source material is a quartzoze sedimentary rock represented by the sandstones of the Mbala formation. The ultimate source material was a felsic source represented by the basement granites. These results agree very well with the current hydrological conditions in the basin, where the Lufubu River traverses these two source rocks. For the Okavango Delta sediments, the interpretation is more complex as the sediment source includes a felsic rock source and a pyroxene and olivine rich mafic-ultramafic rock complexes which are mixed with sand and diagenetic carbonates to produce the Okavango sediments. Geochemical data was used as proxies for climate change in both the Lake Ngami (SW Okavango) and the Mpulungu sub-basin of Lake Tanganyika. The Lake Tanganyika data shows a remarkable excursion coinciding with the Younger Dryas (YD) event, reflecting the input of previously chemically weathered material into the basin due to a change of vegetatio