Methane and carbon dioxide emissions from two contrasting wetlands in the Okavango Delta, Botswana.

<p>We report on two years of continuous monitoring of methane (CH<sub>4</sub>) and carbon dioxide (CO<sub>2</sub>) emissions at two contrasting sites in the Okavango Delta, North-Western Botswana, an inland delta bordered by the Kalahari Desert. Approximately 60% of the annual water influx into the Okavango Delta results from seasonal river discharges originating in the Angolan Highlands, and the remainder comes from direct rainfall. 96-98% of the 16.1 billion m<sup>3</sup> entering the Delta annually are lost through evapo-transpiration (1500 mm.year<sup>-1</sup>). Flooding is gradual and it takes the pulsed influx ca. 4-5 months to travel the 250 km separating the inlet in Mohembo from the main outlet in Maun. The wetlands of the Okavango Delta are in pristine condition and can be separated into three categories: permanently flooded, seasonally flooded (3-6 months per year) and occasionally flooded (typically once per decade). </p><p>Two eddy-covariance systems were set up in August 2017, one at Guma Lagoon (18°57'53.01" S;  22°22'16.20" E) at the edge of an extensive papyrus bed in the permanently-flooded section of the delta, and the second one at Nxaraga on the SW edge of Chief’s Island (19°32'53'' S; 23°10'45'' E) in the seasonal floodplain. In addition, monthly measurements of methane and carbon dioxide fluxes were taken using a clear dynamic chamber at the Nxaraga site along transects chosen to span the natural soil moisture gradient (very dry to waterlogged soils).</p><p>The emissions of methane exhibited contrasting spatial and temporal patterns between sites. At the seasonal wetland, very low fluxes of CH<sub>4</sub> were typically observed from January to June. Emissions increased abruptly from July-August onwards after flood waters rewetted the flooplain in that area of the Delta. Throughout the year, local emission hotspots of CH<sub>4</sub> were observed along the vegetated river channels within the flux footprint of the eddy-covariance system, whereas CH<sub>4</sub> oxidation was recorded in persistently dry areas where the soil is sandy and salt-crusted. The chamber measurements corroborated the findings of the eddy-covariance measurements and soil moisture is likely the dominant control of methane fluxes at the seasonal wetland.</p><p>The methane emissions from the floating papyrus mat in the permanent wetland exhibited a marked seasonal cycle, characterised by relatively high emissions (of the order of 250 nmol.m<sup>-2</sup>.s<sup>-1</sup>; 2.5 larger than peak emissions recorded at the seasonal wetland) in the summer months (November-March) and minimum emissions in winter (typically 50 nmol.m<sup>-2</sup>.s<sup>-1</sup> in June-August). At the seasonal timescale, methane emissions were strongly correlated to the phenological cycle of papyrus (lowest emissions during the senescence phase), suggesting that plant-mediated transport is the dominant control. The annual budgets of CH<sub>4</sub> and CO<sub>2</sub> in the permanent wetland were estimated at 153.4 ± 27.9 tons.km<sup>-2</sup> (3835.0 ± 697.5 CO<sub>2</sub>-eq) and -874.0 ± 200.4 tons.km<sup>-2</sup> respectively, making the permanent wetland a potent net source of carbon to the atmosphere.</p>

Strategy to determine the existence of small-scale wetlands in tropical Andean zones (Colombia)

The Flandes community, in Tolima, Colombia, has proposed the protection of an ephemeral wetland that has been an object of interest for developers, who, by means of a simple drainage and using the dry season to their advantage, may convert a seasonal wetland into an urban area. The object of this investigation was to demonstrate that said water body may be recovered, as a protection zone, at request of the Flandes community. This matter was approached from multiple biophysical points of view. The results indicate that it is a seasonal wetland, geological examination reveals the presence of clay lenses, historical reconstruction of aerial photos indicate the existence of the wetlands, and the presence of phytoplankton and diatoms is evident. These factors indicate the existence, in past ages, of aquatic and ground-based organisms. The result of this study was the creation of a successful methodology for the determination of the existence of wetlands in small-scale Andean tropical areas, for their possible recovery and use by urban planners.

Rhizosphere to the atmosphere: contrasting methane pathways, fluxes, and geochemical drivers across the terrestrial–aquatic wetland boundary

Although wetlands represent the largest natural source of atmospheric CH4, large uncertainties remain regarding the global wetland CH4 flux. Wetland hydrological oscillations contribute to this uncertainty, dramatically altering wetland area, water table height, soil redox potentials, and CH4 emissions. This study compares both terrestrial and aquatic CH4 fluxes in permanent and seasonal remediated freshwater wetlands in subtropical Australia over two field campaigns, representing differing hydrological and climatic conditions. We account for aquatic CH4 diffusion and ebullition rates and plant-mediated CH4 fluxes from three distinct vegetation communities, thereby examining diel and intra-habitat variability. CH4 emission rates were related to underlying sediment geochemistry. For example, distinct negative relationships between CH4 fluxes and both Fe(III) and SO42- were observed. Where sediment Fe(III) and SO42- were depleted, distinct positive trends occurred between CH4 emissions and Fe(II) ∕ acid volatile sulfur (AVS). Significantly higher CH4 emissions (p < 0.01) in the seasonal wetland were measured during flooded conditions and always during daylight hours, which is consistent with soil redox potential and temperature being important co-drivers of CH4 flux. The highest CH4 fluxes were consistently emitted from the permanent wetland (1.5 to 10.5 mmol m−2 d−1), followed by the Phragmites australis community within the seasonal wetland (0.8 to 2.3 mmol m−2 d−1), whilst the lowest CH4 fluxes came from a region of forested Juncus spp. (−0.01 to 0.1 mmol m−2 d−1), which also corresponded to the highest sedimentary Fe(III) and SO42-. We suggest that wetland remediation strategies should consider geochemical profiles to help to mitigate excessive and unwanted methane emissions, especially during early system remediation periods.

Rhizosphere to the atmosphere: contrasting methane pathways, fluxes and geochemical drivers across the terrestrial-aquatic wetland boundary

Although wetlands represent the largest natural source of atmospheric CH4, large uncertainties remain regarding the global CH4 flux. Wetland hydrological oscillations contribute to this uncertainty, dramatically altering wetland area, water table height, soil redox potentials and CH4 emissions. This study compares both terrestrial and aquatic CH4 fluxes over two distinct seasons in both permanent and seasonal remediated freshwater wetlands in subtropical Australia. We account for aquatic CH4 diffusion and ebullition rates, and plant-mediated CH4 fluxes from three distinct vegetation communities, thereby examining seasonal, diurnal and intra-habitat variability. CH4 emission rates were related to underlying sediment geochemistry. For example, distinct negative relationships between Fe(III) and SO42− and CH4 fluxes were observed, whereas distinct positive trends occurred between CH4 emissions and Fe(II) / AVS, where sediment Fe(III) and SO42− were depleted. The highest CH4 emissions of the seasonal wetland were measured during flooded conditions and always during daylight hours, which is consistent with soil redox potential and temperature being important co-drivers of CH4 flux. The highest CH4 fluxes were consistently emitted from the permanent wetland (1.5 to 10.5 mmol m−2 d−1), followed by the Phragmites australis community within the seasonal wetland (0.8 to 2.3 mmol m−2 d−1), whilst the lowest CH4 fluxes came from a region of forested Juncus sp. (−0.01 to 0.1 mmol m−2 d−1) which also corresponded with the highest sedimentary Fe(III) and SO42−. We suggest that wetland remediation strategies should consider geochemical profiles to help to mitigate excessive and unwanted methane emissions, especially during early system recovery periods.