Spatial and temporal dynamics of diffusive methane emissions in the Okavango Delta, northern Botswana, Africa

Abstract

Global warming is associated with the continued increase in the atmospheric concentrations of greenhouse gases; carbon dioxide, methane (CH₄) and nitrous oxide. Wetlands constitute the largest single natural source of atmospheric CH₄ in the world contributing between 100 and 231 Tg year⁻¹ to the total budget of 503–610 Tg year⁻¹, approximately 60 % of which is emitted from tropical wetlands. We conducted diffusive CH₄ emission measurements using static chambers in river channels, floodplains and lagoons in permanent and seasonal swamps in the Okavango Delta, Botswana. Diffusive CH₄ emission rates varied between 0.24 and 293 mg CH₄ m⁻² h⁻¹, with a mean (±SE) emission of 23.2 ± 2.2 mg CH₄ m⁻² h⁻¹ or 558 ± 53 mg CH₄ m⁻² day⁻¹. These emission rates lie within the range reported for other tropical wetlands. The emission rates were significantly higher (P < 0.007) in permanent than in seasonal swamps. River channels exhibited the highest average fluxes at 31.3 ± 5.4 mg CH₄ m⁻² h⁻¹ than in floodplains (20.4 ± 2.5 mg CH₄ m⁻² h⁻¹) and lagoons (16.9 ± 2.6 mg CH₄ m⁻² h⁻¹). Diffusive CH₄ emissions in the Delta were probably regulated by temperature since emissions were highest (20–300 mg CH₄ m⁻² h⁻¹) and lowest (0.2–3.0 mg m⁻² h⁻¹) during the warmer-rainy and cooler winter seasons, respectively. Surface water temperatures between December 2010 and January 2012 varied from 15.3 °C in winter to 33 °C in summer. Assuming mean inundation of 9,000 km², the Delta’s annual diffusive emission was estimated at 1.8 ± 0.2 Tg, accounting for 2.8 ± 0.3 % of the total CH₄ emission from global tropical wetlands.