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

Abstract

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 deepwater sites in the Panhandle to more shallow, distal regions to the south. Concentrations of cations, anions, dissolved organic carbon, and SiO 2 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 HCO 3 , SiO 2 , 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.

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

“…The order of solutes in concentration was Ca > Na > K > Mg, consistent with the findings of Sawula and Martins (1991) and Masamba and Muzila (2005). This was, however, contradictory to results of Mackay et al, (2011) who found a greater concentration of potassium than sodium. There has been little or no systematic change in water chemistry (major ions) over the years, with calcium the dominant cation in the Delta (Sawula and Martins, 1991;Mackay et al, 2011).…”

“…This implies that there must be some differentiation in the processes affecting individual ions. The concentrations of major cations (Ca, K, Mg and Na) increased generally throughout the Delta for surface waters, similar to the trends reported by Sawula and Martins (1991) and Mackay et al (2011). This correlates well with a general increase in conductivity along the Delta distributaries.…”

“…These previous studies, with the exception of those of Mackay et al (2011) andMladenov et al (2005), were either strictly localised or focused mainly along the Jao-Boro channel (Fig. 1).…”

“…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. While there have been studies on water chemistry in the Okavango Delta (Sawula and Martins, 1991;McCarthy and Ellery, 1995;Cronberg et al, 1996a;Cronberg et al, 1996b;Mladenov et al, 2005;Bauer et al, 2007;Mackay et al, 2011;Cawley et al, 2012), the scope was primarily focused on concentrations of solutes in surface water and groundwater;…”

Linking Organic Matter Dynamics to Management, Restoration, and Climate in the Florida Everglades

“…However, as hydro-period has been linked to changes in DOC concentration (Mackay et al 2011) and implicated as a potential driver of molecular diversity in wetlands (Hertkorn et al 2016), the predicted persistence of dry-down conditions may alter both DOC concentration and DOM composition. While % microbial is predicted to increase at TS under this scenario, DOC concentrations and the other three PARAFAC groups are predicted to decrease.…”