Nutrient Budget in the Seasonal Wetland of the Okavango Delta, Botswana

Abstract

The nutrient (P and N species) and chloride budgets were investigated in a representative floodplain in the seasonal wetlands of the Okavango Delta, Botswana. A variety of sources of nutrients in the surface water were considered, namely ion species coming with the floodwater, those generated from dry floodplain soils and those from water-soluble dust deposition (both local and long-range sources). Concentrations of total-nitrogen and chloride in surface water were below 1 mg l À1 . Total-phosphorus concentrations were 0.05 mg l À1 , reflecting the oligotrophic character of the system. Dust deposition rates were highest for chloride at 2.44 g m À2 year À1 followed by 0.79 g m À2 year À1 for total-N, 0.40 g m À2 year À1 for ammonia and only 0.02 g m À2 year À1 for total-P, respectively. Chloride was derived primarily from long-range transport, while N and P species were of more local origin. Dissolution rates for these ions combined were calculated to be 3.9 g m À2 for the flooded area in the 1999 season and thus all dry deposits must be re-dissolved. The accumulation of dust deposits on dry surfaces and their subsequent dissolution causes 2-5 times higher concentrations of nitrogen, phosphorus and chloride with the onset of the flood, thus boosting the nutrient stock in the crucial phase of the onset of flooding. Chloride dissolved from dry soil surfaces and dust contributed approximately 40% to the overall floodplain budget. Although contributions from the soil surface and dust to the nitrogen and phosphorus pools of the floodplain are less prominent (with 10% of total), they nonetheless represent a significant source of nutrients in the entire system. Extrapolation to annually flooded swamps (10,000 km 2 ) indicates a maximum contribution of 40% for total-nitrogen and 60% for total-phosphorus from dust deposition on wet or dry surfaces to the nutrient pool of the water body.

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

“…However, TN and TP have been measured in every region during each stage of the flood cycle, and these still serve as useful macronutrient indicators. Inorganic NO 3 concentrations did not vary significantly with seasonal changes in hydrology (Table 1), perhaps confirming observations by Krah et al (2006) who also did not detect any nitrate "boost" as the flood arrived and expanded. Garstang et al (1998) suggests that this may be because most of the nitrogen has been taken up by plant growth at the end of the previous flood period or has undergone denitrification.…”

“…However, TN concentrations are significantly higher during the period of high flood, perhaps because the floodwaters contain slightly higher nitrogen concentrations than wetland soils (Cronberg et al 1996a). Mean TP concentrations are low (<0.04 mg/L), similar to levels determined by Krah et al (2006) for a Boro floodplain, but lower than values obtained by Lindholm et al (2007). Statistically, however, TP shows no significant regional variation (Table 1).…”

“…2). The increase in SiO 2 and cations is due to evapo-transpiration Axes 1 2 3 4 Total variance processes (Dinçer et al 1978) and an increase in area of seasonally inundated floodplains (Sawula and Martins 1991;Krah et al 2006). Yet total concentration of dissolved ions even in the distal regions of the Delta is still very low, despite evaporation being 2-3 times greater than precipitation (Ramberg and Wolski 2008).…”

“…Floodplains contain greater concentrations of chemical constituents because flooding acts to mobilise ions and nutrients from previously dry soil surfaces into solution (Cronberg et al 1996b). Nutrients in the floodplains are derived from a number of sources, including river flow (Cronberg et al 1996b), in situ from accumulated sedimentary stores over many thousands of years (Mendelsohn et al 2010), from wind-blown dust (Garstang et al 1998;Krah et al 2006), and animal faeces (Lindholm et al 2007). Lindholm et al (2007) measured highly elevated nutrient concentrations during a period of low flood extent in a BOR floodplain, which they attributed to low dilution of stored nutrients.…”

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

“…The introduction of solutes into wetland systems occurs with the inflow of water as dissolved chemical species, or as a result of the dissolution of riverine or aeolian particulates (Garstang et al, 1998;Krah et al, 2004;Krah et al, 2006;Baker et al, 2009). Once in the system the solutes may undergo a series of transformations due to interactions between hydrological, biological and geochemical processes (Mitsch and Gosselink, 2007).…”