Physicochemical controls of diffusive methane fluxes in the Okavango Delta, Botswana

Atmospheric methane (CH4) is one of the three key greenhouse gases (GHGs) driving global climate change. The atmospheric concentration of CH4 has increased by about 150 % above pre-industrial levels of 400-700 ppb due to anthropogenic activities. Although tropical wetlands account for 50-60 % of the global wetland CH4 emissions, the biogeochemistry of these wetlands, including controls of CH4 emissions from the systems, is poorly understood compared to temperate wetlands. This has resulted in large inter-model variations of the magnitude and distribution of CH4 emission estimates from these tropical wetlands. A recent study in the Okavango Delta, Botswana, estimated diffusive CH4 flux at 1.8 +/- A 0.2 Tg year(-1), accounting for 2.8 +/- A 0.3 % of the total CH4 emission from tropical wetlands. In this paper we present an assessment of relationships between diffusive CH4 flux rates and physicochemical variables in the overlying water column to identify and understand regulatory variables for the diffusive CH4 fluxes in the Delta. The results show that diffusive CH4 flux rates from the Delta seem to be controlled by a combination of physicochemical variables. Although site specific fluxes seem to be controlled by different combinations of factors, temperature was the primary predictor of CH4 flux rates at almost all the sampled habitats and sites in the Delta. Most physicochemical variables, especially in the permanent swamps, were correlated with temperature implying that their regulatory effect on diffusive CH4 fluxes could be modified by climate change feedback as well.