Papers by Mangaliso Gondwe
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2021
Data-poor tropical wetlands constitute an important source of atmospheric CH4in the world. We stu... more Data-poor tropical wetlands constitute an important source of atmospheric CH4in the world. We studied CH4fluxes using closed chambers along a soil moisture gradient in a tropical seasonal swamp in the Okavango Delta, Botswana, the sixth largest tropical wetland in the world. The objective of the study was to assess net CH4fluxes and controlling environmental factors in the Delta's seasonal floodplains. Net CH4emissions from seasonal floodplains in the wetland were estimated at 0.072 ± 0.016 Tg a−1. Microbial CH4oxidation of approximately 2.817 × 10−3 ± 0.307 × 10−3 Tg a−1in adjacent dry soils of the occasional floodplains accounted for the sink of 4% of the total soil CH4emissions from seasonal floodplains. The observed microbial CH4sink in the Delta's dry soils is, therefore, comparable to the global average sink of 4–6%. Soil water content (SWC) and soil organic matter were the main environmental factors controlling CH4fluxes in both the seasonal and occasional floodplains...
The data resource consists of half hourly time series of heat (latent and sensible) and trace gas... more The data resource consists of half hourly time series of heat (latent and sensible) and trace gas (carbon dioxide and methane) fluxes obtained by eddy-covariance, gas concentrations and ancillary meteorological data (e.g. air temperature, relative humidity, pressure, photosynthetically active radiation, total incoming radiation, wind speed and direction). The data were collected at Guma Lagoon (18°57'53.01"S; 22°22'16.20"E), in the perennially flooded area of the Okavango Delta, Botswana, for the purpose of quantifying greenhouse gas fluxes over a Cyperus papyrus stand. The measurement period was 01/01/2018 to 31/12/2020. The instrumentation was installed the UK Centre for Ecology and Hydrology; monthly maintenance and data collection visits were effected by the Okavango Research Institute, University of Botswana. The research was funded through NERC grant reference NE/N015746/2 - The Global Methane Budget.
<p>Wetland methane (CH<sub>4</sub>) emissions are the l... more <p>Wetland methane (CH<sub>4</sub>) emissions are the largest natural source in the global CH<sub>4</sub> budget, contributing to roughly one third of total natural and anthropogenic emissions. As the second most important anthropogenic greenhouse gas in the atmosphere after CO<sub>2</sub>, CH<sub>4</sub> is strongly associated with climate feedbacks. The different pathways of biochemical cycling of CH<sub>4</sub>, which exert a primary control on atmospheric CH<sub>4</sub> concentrations through its production and biological consumption, remain poorly constrained. It is therefore crucial to understand and, if possible, quantify these variable CH<sub>4 </sub>sources to natural climate variability.</p> <p>We studied a soil transect (up to seven sites, 250 m long) across a seasonal floodplain at Nxaraga on the south-west part of the Chief’s Island, Okavango Delta, Botswana, over three years (2018 – 2020, 50 samples in total). Previous studies showed a clear link between CH4 fluxes and soil water content in the area, with CH<sub>4</sub> fluxes in the seasonally flooded soils of up to 492 nmol m<sup>-2</sup> s<sup>-1</sup>.</p> <p>To constrain biomass active in CH<sub>4 </sub>production (specifically, methanogenic archaea) intact and core isoprenoid lipids (and their stable carbon isotope signature) were quantified on a High Performance Liquid Chromatograph (HPLC) and on an high-resolution mass spectrometer ("Orbitrap"). To constrain biomass of CH<sub>4 </sub>oxidizers (i.e. bacterial methanotrophs), core (hopanol) and intact lipids (i.e., bacteriohopanepolyols (BHPs)) were analyzed non-derivatized on an Orbitrap. Confirming their proposed methanotroph source, BHP-aminopentol and methylcarbamate-BHP were detected and their variation correlated positively with those of hopanols and archaeol lipids. Methyl-amino BHPs however were not detected in the soils. In-depth study of their environmental variation points towards two bacterial communities depending on the pH, EC and water content of the soils. This will be confirmed or refuted by bacterial community profiling based on 16S RNA genes, and functional genes for methane oxidation</p>
AGU Fall Meeting Abstracts, Dec 1, 2018
We present one year of continuous monitoring of methane emissions at two contrasting sites in the... more We present one year of continuous monitoring of methane emissions at two contrasting sites in the Okavango Delta, a UNESCO World Heritage site located in North-Western Botswana. 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). We set up two eddy-covariance systems 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. We also conduct monthly measurements of methane and carbon dioxide fluxes by using a clear dynamic chamber at Nxaraga along transects chosen to span the natural soil moisture gradient (very dry to waterlogged soils). We observed contrasting spatial and temporal patterns of methane emissions between the sites as well as significant differences in emission intensities. Methane hotspots were observed in the Guma Lagoon papyrus swamp whereas fluxes at Nxaraga increased gradually with distance into the floodplain, which correlates with the spatial gradient of soil moisture in the seasonal floodplain. The differences observed at the two measurement sites suggest different controls, with soil moisture likely to be the dominant one at the seasonal floodplain. In contrast, we expect the emissions from the permanent wetland to be driven by a complex set of mechanisms, which could include plant-mediated transport, diffusion at the water-air interface, bubbling and convective flow within the water column
We report on 3 years of continuous monitoring of carbon dioxide (CO<sub>2</sub>) and ... more We report on 3 years of continuous monitoring of carbon dioxide (CO<sub>2</sub>) and methane (CH<sub>4</sub>) emissions in two contrasting wetland areas of the Okavango Delta, Botswana: a perennial swamp and a seasonal floodplain. The hydrographic zones of the Okavango Delta, possess distinct attributes (e.g. vegetation zonation, hydrology) which dictate their respective greenhouse gas (GHG) temporal emission patterns and magnitude. The perennial swamp was a net source of carbon (expressed in CO<sub>2</sub>-eq units), while the seasonal swamp was a sink in 2018. Despite differences in vegetation types and lifecycles, the net CO<sub>2</sub> uptake was comparable at the two sites studied in 2018/2020 (−894.2 ± 127.4 g m<sup>−2</sup> yr<sup>−1</sup> at the perennial swamp, average of the 2018 and 2020 budgets, and −1024.5 ± 134.7 g m<sup>−2</sup> yr<sup>−1</sup> at the seasonal floodplain). ...
The atmospheric methane (CH<sub>4</sub>) burden is rising sharply, but the causes are... more The atmospheric methane (CH<sub>4</sub>) burden is rising sharply, but the causes are still not well understood. One factor of uncertainty is the importance of tropical CH<sub>4</sub> emissions into the global mix. Isotopic signatures of major sources remain poorly constrained, despite their usefulness in constraining the global methane budget. Here, a collection of new <i>δ</i><sup>13</sup>C<sub>CH<sub>4</sub></sub> signatures is presented for a range of tropical wetlands and rice fields determined from air samples collected during campaigns from 2016 to 2020. In summary, tropical wetland <i>δ</i><sup>13</sup>C<sub>CH<sub>4</sub></sub> generally range between −62 and −52‰ and rice paddies between −59 and −57‰. Long-term monitoring of <i>δ</i><sup>13</sup>C<sub>CH<sub>4</sub></sub> in ambient air has been conducted at...
Detailed site location information along with extra information on Botswana and Bolivian long-ter... more Detailed site location information along with extra information on Botswana and Bolivian long-term time series data
The dataset consists of a continuous time series of heat (latent and sensible) and trace gas (car... more The dataset consists of a continuous time series of heat (latent and sensible) and trace gas (carbon dioxide and methane) fluxes obtained by eddy-covariance, gas concentrations and some ancillary meteorological data (e.g. air temperature, relative humidity, pressure, photosynthetically active radiation, total incoming radiation, wind speed and direction). The data were collected at Nxaraga, on the south edge of Chief's Island (19°32'53''S; 23°10'45"E), in the seasonally flooded area of the Okavango Delta, Botswana, for the purpose of quantifying greenhouse gas fluxes from seasonal floodplains. The data is reported at half-hourly interval for the measurement period 01/01/2018 to 31/12/2020. Missing data were caused by instrumentation downtime and are reported as -9999.
African Journal of Aquatic Science, 2016
The Okavango Delta depends on water quantity and quality to sustain its ecosystem services. Where... more The Okavango Delta depends on water quantity and quality to sustain its ecosystem services. Whereas many studies have been carried out on its hydrology, few have been done on water quality in the delta. Water pH, electrical conductivity (EC), dissolved oxygen (DO), turbidity, total suspended solids (TSS) and dissolved organic carbon (DOC) were monitored at 10 sites along the Okavango–Boro–Thamalakane–Lake Ngami system almost fortnightly from June 2008 to June 2010. Water quality in the delta was generally good, despite high evapotranspiration rates which would normally produce very saline waters. Electrical conductivity and water temperature increased with distance from Mohembo to Lake Ngami, the former most likely due to evapoconcentration. In contrast, pH, DO, turbidity and TSS decreased with distance from Mohembo to Boro at the lower end of the seasonal floodplain, before increasing again to Lake Ngami. Dissolved oxygen and TSS most likely declined due to biological uptake and particle sedimentation, respectively. Strong and significant relationships were observed between TSS and turbidity and between DOC and EC, indicating that turbidity and EC could be useful proxies for routine estimations of TSS and DOC, respectively, in the delta.
African Journal of Aquatic Science, 2014
The quality of water in aquatic systems is subject to temporal and spatial variations due to vary... more The quality of water in aquatic systems is subject to temporal and spatial variations due to varying effects of natural and anthropogenic factors. This study assessed the dynamics of water quality in the Boro-Thamalakane-Boteti river system along an upstream–downstream gradient above and below Maun during February, May, September and December 2012. Temperature, conductivity, pH, dissolved oxygen, turbidity, Escherichia coli and faecal streptococci were monitored in the settlements of Boro, Maun, Xobe, Samedupi, Chanoga and Motopi along the river system. Comparisons of water quality among settlements using ANOVA and Tukey's honest significant difference test highlighted a significant decline in water quality from upstream to downstream, indicated by increases in microbial numbers and turbidity. The quality of water improved as the water level rose during the peak flooding season in September and declined thereafter. This study highlights the possible influence of human settlements and associated developments on waterbodies, and lends strong support to management efforts to maintain river water quality to ensure the suitability of the water for various ecosystem uses.
Hydrobiologia, 2007
... Planktonic nitrogen fixation in Lake Malawi/Nyasa Mangaliso J. Gondwe Æ Stephanie J. Guildfor... more ... Planktonic nitrogen fixation in Lake Malawi/Nyasa Mangaliso J. Gondwe Æ Stephanie J. Guildford Æ Robert E. Hecky ... MJ Gondwe (&amp;) SADC/GEF Lake Malawi Biodiversity Conservation Project, PO Box 311, Salima, Malawi e-mail: [email protected] ...
Aquaculture, 2012
Cage culture of native tilapiine cichlids was initiated in Lake Malawi in 2004. The lake is well ... more Cage culture of native tilapiine cichlids was initiated in Lake Malawi in 2004. The lake is well known for its highly endemic ichthyodiversity, and it is estimated to have more species of fish than any other lake in the world. Consequently there is concern about the impact of cage farming operations on the wild fish communities. In 2007, high densities of diverse wild fish were observed around the cages and stable isotopes, δ 13 C and δ 15 N analyses, established that cage wastes were incorporated in the food web that supported diverse wild fishes in the vicinity of the cages. Comparison of δ 13 C and δ 15 N signals of caged and wild fish caught in 2007 in the vicinity of the fish farm and signals of fish samples caught between 1995 and 1997 before the fish farm was started in 2004 established a shift in the isotopic signatures of wild fish indicating the incorporation of cage wastes into the wild fish diet. Sedimentation of cage wastes collected in sediment traps below the fish cages was also confirmed using the δ 13 C and δ 15 N analyses. The accumulation of the cage wastes in the sediments below the cages was, however, minimal as indicated by the small differences in the isotopic ratios between the bulk sediments and some sedimentary organisms (bivalves, snails and earthworms) under the cages relative to ratios in similar organisms at control stations. The low impact of cage wastes on underlying sediments and benthic organisms was due to the rapid and efficient dispersion of the cage wastes facilitated by water currents through the fish farm which averaged 9.3 cm s − 1 as well as the consumption and subsequent dispersion of cage wastes by the large numbers of wild fishes which aggregated around the cages. This study has also shown that in Lake Malawi, fish rather than benthic organisms and plankton material may be a more sensitive monitor of the dispersion of cage wastes.
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences
We report on three years of continuous monitoring of carbon dioxide (CO 2 ) and methane (CH 4 ) e... more We report on three years of continuous monitoring of carbon dioxide (CO 2 ) and methane (CH 4 ) emissions in two contrasting wetland areas of the Okavango Delta, Botswana: a perennial swamp and a seasonal floodplain. The hydrographic zones of the Okavango Delta possess distinct attributes (e.g. vegetation zonation, hydrology) which dictate their respective greenhouse gas (GHG) temporal emission patterns and magnitude. The perennial swamp was a net source of carbon (expressed in CO 2 -eq units), while the seasonal swamp was a sink in 2018. Despite differences in vegetation types and lifecycles, the net CO 2 uptake was comparable at the two sites studied in 2018/2020 (−894.2 ± 127.4 g m −2 yr −1 at the perennial swamp, average of the 2018 and 2020 budgets, and −1024.5 ± 134.7 g m −2 yr −1 at the seasonal floodplain). The annual budgets of CH 4 were however a factor of three larger at the permanent swamp in 2018 compared to the seasonal floodplain. Both ecosystems were sensitive to d...
Eddy-covariance data processing details, extended site descriptions, flux partioning.
Nature Communications
Tropical wetlands are a significant source of atmospheric methane (CH4), but their importance to ... more Tropical wetlands are a significant source of atmospheric methane (CH4), but their importance to the global CH4 budget is uncertain due to a paucity of direct observations. Net wetland emissions result from complex interactions and co-variation between microbial production and oxidation in the soil, and transport to the atmosphere. Here we show that phenology is the overarching control of net CH4 emissions to the atmosphere from a permanent, vegetated tropical swamp in the Okavango Delta, Botswana, and we find that vegetative processes modulate net CH4 emissions at sub-daily to inter-annual timescales. Without considering the role played by papyrus on regulating the efflux of CH4 to the atmosphere, the annual budget for the entire Okavango Delta, would be under- or over-estimated by a factor of two. Our measurements demonstrate the importance of including vegetative processes such as phenological cycles into wetlands emission budgets of CH4.
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
Remote Sensing
Lake Malawi is an important water resource in Africa. However, there is no routine monitoring of ... more Lake Malawi is an important water resource in Africa. However, there is no routine monitoring of water quality in the lake due to financial and institutional constraints in the surrounding countries. A combination of satellite data and a semi-analytical algorithm can provide an alternative for routine monitoring of water quality, especially in developing countries. In this study, we first compared the performance of two semi-analytical algorithms, Doron11 and Lee15, which can estimate Secchi disk depth (SD) from satellite data in Lake Malawi. Our results showed that even though the SD estimations from the two algorithms were very highly correlated, the Lee15 outperformed the Doron11 in Lake Malawi with high estimation accuracy (RMSE = 1.17 m, MAPE = 18.7%, R = 0.66, p < 0.05). We then evaluated water transparency in Lake Malawi using the SD values estimated from nine years of Medium Resolution Imaging Spectrometer (MERIS) data (2003–2011) with the Lee15 algorithm. Results showed ...
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences
The atmospheric methane (CH 4 ) burden is rising sharply, but the causes are still not well under... more The atmospheric methane (CH 4 ) burden is rising sharply, but the causes are still not well understood. One factor of uncertainty is the importance of tropical CH 4 emissions into the global mix. Isotopic signatures of major sources remain poorly constrained, despite their usefulness in constraining the global methane budget. Here, a collection of new δ 13 C CH 4 signatures is presented for a range of tropical wetlands and rice fields determined from air samples collected during campaigns from 2016 to 2020. Long-term monitoring of δ 13 C CH 4 in ambient air has been conducted at the Chacaltaya observatory, Bolivia and Southern Botswana. Both long-term records are dominated by biogenic CH 4 sources, with isotopic signatures expected from wetland sources. From the longer-term Bolivian record, a seasonal isotopic shift is observed corresponding to wetland extent suggesting that there is input of relatively isotopically light CH 4 to the atmosphere during periods of reduced wetland exte...
Water SA
Water chemistry is important for the maintenance of wetland structure and function. Interpreting ... more Water chemistry is important for the maintenance of wetland structure and function. Interpreting ecological patterns in a wetland system therefore requires an in-depth understanding of the water chemistry of that system. We investigated the spatial distribution of chemical solutes both in soil pore water and surface water, along island-floodplain-channel hydrological gradients in seasonally and permanently inundated habitats between major regions in the Okavango Delta, Botswana. Our results show that major cations (Ca, Na, Mg, and K), dissolved silica (DSi), dissolved boron (B), dissolved organic matter (DOC) and electrical conductivity increased significantly, at p ≤ 0.05, from the inlet of the Delta (the Panhandle) to the distal downstream reaches, suggesting the influence of evapoconcentration. Concentrations of dissolved Fe, Al, Zn, Cu, and Mn significantly decreased, at p ≤ 0.05, from the inflow of the Delta to the distal reaches. Only Na, Mn, Fe, Al, and DOC showed significant differences, at p ≤ 0.05, along the local floodplain-channel hydrological gradients, with higher solute concentrations in the floodplains than the channels. Solute concentrations in soil water exhibited similar distribution patterns to those in surface water, but concentrations were higher in soil water. Based on the results, we hypothesise that floodplain emergent vegetation and the channel-fringing vegetation in the Panhandle (a faultbounded entry trough to the Delta) and the permanently inundated eco-region together influence the cycling of solutes that enter the Delta through uptake.
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Papers by Mangaliso Gondwe