River-wetland interaction and carbon cycling in a semi-arid riverine system: the Okavango Delta, Botswana

Abstract

The Okavango River, in semi-arid northwestern Botswana, flows for over 400 km in a pristine wetland developed on a large (>22,000 km2) alluvial fan (Okavango Delta). An annual flood pulse inundates the floodplains of the wetlands and travels across the Delta in 4–6 months. In this study, we assess the effects of long hydraulic residence time, variable hydrologic interaction between river–floodplain–wetland and evapotranspiration on carbon cycling. We measured dissolved inorganic carbon (DIC) concentrations and stable carbon isotopes of DIC (δ13CDIC) from river water when the Delta was not flooded (low water) and during flooding (high water). During low water, the average DIC concentration was 31 % higher and the δ13CDIC 2.1 ‰ more enriched compared to high water. In the lower Delta with seasonally flooded wetlands, the average DIC concentration increased by 70 % during low water and by 331 % during high water compared to the Panhandle with permanently flooded wetlands. The increasing DIC concentration downriver is mostly due to evapoconcentration from transpiration and evaporation with increased transit time. The average δ13CDIC between low and high water decreased by 3.7 ‰ in the permanently flooded reaches compared to an increase of 1.6 ‰ in the seasonally flooded reaches. The lower δ13CDIC during high water in the permanently flooded reaches suggest that DIC influx from the floodplain-wetland affects river’s DIC cycling. In contrast, higher river channel elevations relative to the wetlands along seasonal flooded reaches limit hydrologic interaction and DIC cycling occurs mostly by water column processes and river-atmospheric exchange. We conclude that river-wetlands interaction and evapoconcentration are important factors controlling carbon cycling in the Okavango Delta.