Gabon Coastal Land Cover Classification 2018

Wetlands of coastal Gabon provide many ecosystems services including flood protection, water purification, and wildlife habitat. Effective sustainable management of this coastal zone is hindered by a lack of accurate wetland maps. Here we describe a novel method used to map the wetland ecosystems of nearly 100,000 km2 of wetland and upland habitat mosaic in the delta of the Ogooué River using an object-based classification of optical and radar satellite imagery based on training data collected from UAV (unmanned aerial vehicle) and a post-classification accuracy assessment using helicopter-based video. We identified 15 land cover classes, of which nine were wetland. A stratified random sample accuracy assessment of the final classification yielded an overall accuracy of 0.80. Despite the important role that wetland habitats play for wildlife and ecosystem functioning across the region, our results indicate these wetlands cover only 22% of the project area. As expected, most of the wetland habitats are found close to major water bodies, including the Ogooué River, estuaries near the cities of Libreville and Port Gentil, and coastal lagoons to the south of these cities. When considering the six Wetlands of International Importance designated under the Ramsar Convention within the project area, only 33% of mapped wetlands fall within the Ramsar site boundaries, and only 10% of mapped wetlands fall within official protected areas. Furthermore, within the Ramsar sites, only 31% of the land cover was classified as wetland. In order to better manage these wetland resources, more effective Ramsar boundaries would include the extensive wetland habitats found along the coast from Port Gentil south to Loango National Park. These data are now available for Ramsar managers to improve wetland management within the currently designated Ramsar sites, for example by protecting wetland connectivity and other ecosystem processes

Supply-limited weathering regime in a tropical shields basin (Ogooué River basin, Gabon)

International audienceAt the global scale and on geological time scales, mechanical erosion and chemical weathering budgets are linked. Together, these processes contribute to the formation and the degradation of the Earth's critical zone and to the biogeochemical cycles of elements. While the weathering of hot and humid shields areas exhibit low weathering rates because of the depth of the mature depleted soil mantle there, shields areas dominate the continents areas over intertropical regions and, therefore, represent a significant proportion of the global delivery of dissolved matter to the oceans. In addition, these environments are under supply-limited conditions (the weathering rate is limited by the low rates of the erosion) and thus particularly sensitive to long-term variability erosion rates. Despite this importance, weathering-erosion budgets and rates estimation in these environments is sparse, and generally performed at a local scale (soil profiles) or, when performed at a larger catchment scale, the intra cratonic characteristics variabilities (e. g. the diversity of mechanical erosional regimes) are usually not singled out.In the present study, we explored the variability of the weathering intensity of the Ogooué sub-basins (Western central Africa, Gabon) as a function of their geomorphologic, tectonic and lithological setting variability. We analyzed major and trace elements concentration and the strontium and neodymium isotopes of water, suspended matter sediments and bedload sampled in 24 Ogooué tributaries (September 2017 campaign). Our results show that shield areas exhibit a high variability of chemical weathering intensity, which follows the erosional regime characteristics of the studied sub-basins, likely related to their tectonic activity. Three regions can be distinguished: The Bateke plateau (East sub-basins - PB), is composed of pure sandstones (quartz) and is inert in term of tectonic activity and therefore in term of erosion and weathering budget; the northern sub-basins (NB) are subjected to low tectonic activity and exhibit slightly higher erosion and weathering intensity than PB region and, by comparison, southern sub-basins (SB) exhibits uplift activity which is traduced by more intensive erosion and weathering processes.The annual dissolved solid budget of the Ogooué basin is ~2.52 t.yr-1 for a rate of 11.7 t.km-2.yr-1. According to the source discrimination method performed based on the geochemical analysis, the atmospheric inputs contributes to around 20% to the TDS, the silicate weathering contribution dominates the dissolved exports throughout 70% of its production while the carbonates weathering lowly contributes to the TDS production.By comparison to the other large shields rivers, this basin exhibit a lower range of chemical silicate weathering rate than most of the world's large rivers, with values similar to those of the Congo River. This new dataset provides a key information to complete the World River chemistry database, which is limited for inter-tropical regions, especially in tectonically quiescent environments. Moreover, this study provides new data for tropical shields contexts allowing for the exploration of the interactions between erosion rates and climate in the control of continental weathering rates, and their relationships with long-term carbon cycle and short-term biogeochemical cycles

Cartographie de la végétation

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Monitoring Water Levels and Discharges Using Radar Altimetry in an Ungauged River Basin: The Case of the Ogooué

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Contrasted Chemical Weathering Rates in Cratonic Basins: The Ogooué and Mbei Rivers, Western Central Africa

International audienceDespite the absence of tectonic activity, cratonic environments are characterized by strongly variable, and in places significant, rock weathering rates. This is shown here through an exploration of the weathering rates in two inter-tropical river basins from the Atlantic Central Africa: the Ogooué and Mbei River basins, Gabon. We analyzed the elemental and strontium isotope composition of 24 water samples collected throughout these basins. Based on the determination of the major element sources we estimate that the Ogooué and Mbei rivers total dissolved solids (TDS) mainly derive from silicate chemical weathering. The chemical composition of the dissolved load and the area-normalized solute fluxes at the outlet of the Ogooué are similar to those of other West African rivers (e.g., Niger, Nyong, or Congo). However, chemical weathering rates (TZ+sil rate expressed as the release rate of the sum of cations by silicate chemical weathering) span the entire range of chemical weathering intensities hitherto recorded in worldwide cratonic environments. In the Ogooué-Mbei systems, three regions can be distinguished: (i) the Eastern sub-basins draining the Plateaux Batéké underlain by quartz-rich sandstones exhibit the lowest TZ+sil rates, (ii) the Northern sub-basins and the Mbei sub-basins, which drain the southern edge of the tectonically quiescent South Cameroon Plateau, show intermediate TZ+sil rates and (iii) the Southern sub-basins characterized by steeper slopes record the highest TZ+sil rates. In region (ii), higher DOC concentrations are associated with enrichment of elements expected to form insoluble hydrolysates in natural waters (e.g., Fe, Al, Th, REEs) suggesting enhanced transport of these elements in the colloidal phase. In region (iii), we suggest that a combination of mantle-induced dynamic uplift and lithospheric destabilization affecting the rim of the Congo Cuvette induces slow base level lowering thereby enhancing soil erosion, exhumation of fresh primary minerals, and thus weathering rates. The study points out that erosion of lateritic covers in cratonic areas can significantly enhance chemical weathering rates by bringing fresh minerals in contact with meteoric water. The heterogeneity of weathering rates amongst cratonic regions thus need to be considered for reconstructing the global, long-term carbon cycle and its control on Earth climate

Hydro-climatology study of the Ogooué River basin using hydrological modeling and satellite altimetry

International audienceHydrological models are important tools for the simulation of water storage and hydrological fluxes in large basins and complex river systems. The hydrological models can compensate the lack of observed data in ungauged basins. In this study, the hydrological model of large basins MGB (for Model of Large Basins in Portuguese) is used to evaluate the hydrological processes of the Ogooué River Basin (ORB), which has been mostly unmonitored for about three decades. Simulations were carried out over an 18-year period from 1998 to 2015 using TRMM 3B42 daily rainfall data from the Tropical Rainfall Measurement Mission (TRMM) as forcing and in situ and altimetry-based river discharges from Envisat, Saral Altika and Jason-2 for calibration and validation. The results of the model were in good agreement with the flows measured at stations upstream and downstream of the Ogooué basin (Nash-Sutcliffe Efficiency (NSE) > 0.56 for all calibration gauges). The MGB model efficiently describes the seasonal and interannual variations of the flow in the Ogooué River and its major tributaries which were found to be highly correlated to the rainfall (r ranging from 0.72 to 0.90 and 0.56 to 0.87 at seasonal and interannual time-scales respectively). Interannual variations of precipitation and river discharge of the ORB are linked to the El Niño Southern Oscillation (ENSO) in the tropical eastern Pacific Ocean and southeastern tropical Atlantic Niño. Also, the Ogooué River discharge was found to be strongly correlated with Sea Surface Temperature (SST) at annual and semi-annual time-scales

Physical erosion rates in Ogooué and Mbei Rivers (Gabon, Western Central Africa): insights for Cratonic Catchments.

International audienceWe measured the long term physical denudation of the Ogooué River catchment using 10Be. These measurements are averaged over 25-200 ka (average 40 ka), depending on the physical denudation rate. The denudation rate of the Ogooué River catchment is slow (38 t/km2/a, 15 m/Ma), slightly higher than the Equatorial West Africa (from Senegal to Angola, 26 t/km2/a, 10 m/Ma). Physical denudation and chemical weathering fall within the same order of magnitude. Thus, although low, chemical weathering, is substantial compared to physical denudation, its contribution is likely over 30% of the total denudation.Denudation rates are spatially variable (from 10 to 60 t/km2/a) within this large Ogooué River catchment. Over the long term, this variability exhibits a fairly close match of physical denudation/chemical weathering, except in the Batéké Plateaux area, because they are made up of already weathered detrital material and their modern flux of solutes is therefore very low (~9.5 t/km2/a). The spatial distribution is similar to the one described in Moquet et al. (2021)'s work, i.e. the southern part of the catchment is denuding twice as fast as the northern part. We show here that the whole picture did not vary much since 100 ka, as shown by both methods giving consistent results. Faster denudation in the south of the catchment may be related to some more uplift than in the north caused by the south African superswell

Spatial planning for a green economy: National-level hydrologic ecosystem services priority areas for Gabon

<div><p>Rapidly developing countries contain both the bulk of intact natural areas and biodiversity, and the greatest untapped natural resource stocks, placing them at the forefront of “green” economic development opportunities. However, most lack scientific tools to create development plans that account for biodiversity and ecosystem services, diminishing the real potential to be sustainable. Existing methods focus on biodiversity and carbon priority areas across large geographies (e.g., countries, states/provinces), leaving out essential services associated with water supplies, among others. These hydrologic ecosystem services (HES) are especially absent from methods applied at large geographies and in data-limited contexts. Here, we present a novel, spatially explicit, and relatively simple methodology to identify countrywide HES priority areas. We applied our methodology to the Gabonese Republic, a country undergoing a major economic transformation under a governmental commitment to balance conservation and development goals. We present the first national-scale maps of HES priority areas across Gabon for erosion control, nutrient retention, and groundwater recharge. Priority sub-watersheds covered 44% of the country’s extent. Only 3% of the country was identified as a priority area for all HES simultaneously, highlighting the need to conserve different areas for each different hydrologic service. While spatial tradeoffs occur amongst HES, we identified synergies with two other conservation values, given that 66% of HES priority areas intersect regions of above average area-weighted (by sub-watersheds) total forest carbon stocks and 38% intersect with terrestrial national parks. Considering implications for development, we identified HES priority areas overlapping current or proposed major roads, forestry concessions, and active mining concessions, highlighting the need for proactive planning for avoidance areas and compensatory offsets to mitigate potential conflicts. Collectively, our results provide insight into strategies to protect HES as part of Gabon’s development strategy, while providing a replicable methodology for application to new scales, geographies, and policy contexts.</p></div

Servicesheds for erosion control, nitrogen retention, and phosphorus retention weighted by downstream beneficiary population size.

<p>(A) “All population” serviceshed. (B) “Rural-weighted” serviceshed. Pixel values represent the number of downstream people within each serviceshed that would benefit from a watershed conservation activity on that pixel. Therefore, pixels with the highest values are those with the largest downstream population.</p