INVESTIGATION OF DIEL VARIATIONS OF WATER QUALITY AND STABLE CARBON ISOTOPES ACROSS THE OKAVANGO DELTA, NORTHWEST BOTSWANA

The Okavango Delta, located in the semi-arid environment of northwestern Botswana, Africa is a pristine wetland that covers 22,000 km²and consists of ecotones that are permanently, seasonally, and occasionally flooded. Previous studies have shown that evapotranspiration has a profound effect on the river water chemistry, where solute concentrations, organic and inorganic carbon increase downriver with decrease in river discharge. Although vegetative evapo-concentration is an important process controlling river chemistry, the short term effects are unknown.

We conducted four diel observations spatially from the proximal to distal end; Mohembo and Seronga (permanently flooded), Okavango Research Island (ORI) (seasonally flooded) and Maun (occasionally flooded). We measured photosynthetically active radiation (PAR), water level, temperature, electrical conductivity, dissolved oxygen (DO), pH, alkalinity, silica, dissolved inorganic carbon (DIC) and the stable carbon isotope ratio of DIC (d¹³C_DIC). Our objective was to assess changes in water chemistry caused by evapotranspiration over a diel cycle for the river reaches with a different flooding hydroperiod. All locations had a pronounced diel response, with lower water levels and higher temperatures during the day. The major ions and TDS showed a diel response with higher concentrations during the day and lower concentrations at night. The increased solute concentration during the daytime is due to vegetative drawdown by transpiration coupled to evaporation. Alkalinity and DIC had generally lower concentrations during the daytime in response to the photosynthetic drawdown of aquatic CO₂. However, the d¹³C_DICdid not reflect the photosynthetic activity, as isotopic enrichments were not clearly evident during the daytime which suggests a near balance in respiration and photosynthesis in the water column.

The results of this study suggest that diel variations in the Okavango Delta river water properties are affected in part by vegetation. The physical effects of vegetation on water should be considered in the investigation of processes that control water quality and cycling of carbon in river systems with extensive vegetation in the water column and floodplains.