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International audience; For decades, diatoms have been recognized as powerful bio-indicators of modern water quality. They have also been utilized in the design of transfer functions, which can be applied to diatom assemblages in lake sediment cores to infer aspects of past lake hydrochemistry and estimate variables that can be incorporated into paleohydrology models. The Ounianga lakes, in the heart of the Chadian Sahara, possess unique and well-preserved sediment records that extend back beyond the middle Holocene. Today, the lakes display a range of hydrochemical conditions, from fresh to hypersaline. Mainly fed by fossil groundwater that originates in the Nubian Sandstone Aquifer System, measured conductivity across these lakes varies from 217 to 352,000 μS cm −1, values that are influenced by factors such as hydrology, local geomorphology (e.g., depth and area), and aquatic vegetation. Although these lakes have been on the UNESCO World Heritage List since 2012, they have never been studied in detail because they are located on the fringes of the Chadian Sahara. The distribution of diatom taxa in the lakes today is closely linked to water-column physical and chemical conditions, especially conductivity. Whereas each lake has particular features that influence its diatom flora, diatoms across a conductivity gradient enabled identification of three distinct waterbody types, freshwater lakes, meso-saline to hyper-saline lakes, and freshwater springs. Relationships between diatom species distributions and environmental variables were examined using multivariate analysis, which revealed that conductivity is the variable that explains most of the variance in the diatom flora. We used modern diatom assemblages from the lakes to develop a predictive model (transfer function) for conductivity, using the weighted averaging method. Our conductivity prediction model is strong, with a coefficient of determination (R 2) of 0.89 between estimated and measured values, and a value of 0.78 using jackknife estimates of prediction. This study better constrained conductivity optima and tolerance values for diatom species found in the Ounianga lakes, thereby enabling development of a model that will yield better inferences forpast conductivity, using diatoms from lake sediment records in the region.

The Sahel, located at the southern edge of the Sahara is defined as the transitional boundary of the Sahara, whose aridification since the late Holocene has profoundly affected landscapes, human occupation and ecosystems. Current climate change, characterized by an increase in temperature, raised the idea of aridification and the disappearance of surface water in the Sahelian zone. However, in central Sahel, the trend towards increasing open water surfaces seems to be confirmed over the last decade, particularly during extreme rainfall episodes. Here, we show the current changes of Lake Fitri, a remnant of the mega-lake Chad during the Holocene 6,000 years ago, whose seasonal flooding has reached unequaled levels in the last 50 years. This terminal and shallow lake and spreading out over a vast flat sedimentary plain irrigates a variable surface area. During exceptional rainy years such as the current one (2020-2021), the waters from the Batha river basin (main tributary of Lake Fitri), in the Sahelian zone, can reach the Chari catchment area (main tributary of Lake Chad) by passing through a succession of basins.The diachronic analysis of Landsat images combined with ground-based observations recorded during the year 2020-2021 will be compared with the previous available time-series, in particular the monthly series already analysed for the year 2015 (Yalikun and al., 2019). The results confirm the trend towards the extension of the lake and the "outflow" of its waters to the west. These observations raise many questions related to the definition of a lake and its precise shoreline in this context. We will also specify the consequences in terms of the recomposition of the social-ecological system (changes in land use, recomposition of activity systems and access to natural resources, governance issues). Finally, we will discuss the timing of the changes observed in the short (annual, decade), medium (25 years), long and very long term (Holocene), on the comparability of these changes and on the elements to be considered in order to envisage the future evolution of such sahelian social-ecological system.

During the Holocene (0-11.7 ka BP), the subtropical regions of Africa were characterized by very significant climatic changes. These changes were evidenced by variations in lake levels which reflect the balance between precipitation and evaporation (P-E) at the watershed scale. These climatic conditions are mainly associated with the dynamics of the African summer monsoon in relation with the location of the Intertropical Convergence Zone (ITCZ), which is modulated by summer insolation induced by the Earth's orbital parameters. This mechanism explains the wetter conditions observed from 11 ka BP to 5.5 ka BP in the Sahelo-Saharan zone. This period, called "Green Sahara" or "African Humid Period", was characterized by a green landscape, covered by grasslands and trees, dotted with numerous lakes, and incised by large river networks.Despite the numerous studies carried out on the African Humid Period, there is a scarcity of data for quantifying source and origin of precipitation. The Lake Chad Basin (BLT) is a key region for paleoclimatic research because of its position reflecting main tropical atmospheric mechanisms and its endorheic morphology amplifying climatic signals. More particularly, the crater palaeolakes of Trou au Natron (Pic Toussidé) and Era Kohor (Emi Koussi) in the Tibesti mountains offer unique sedimentary archives that may record the climatic history of the Sahara.This work aims to reconstruct the evolution of these crater palaeolakes, thanks to the oxygen isotopic composition of diatoms (18Odiatom), from the termination of the last deglaciation until the African Humid Period. The identification of the fossil diatom assemblages combined with the 18Odiatom values should give insights on the evolution of the limnological parameters, the relative depth, the chemistry, and the water isotopic composition of these palaeolakes.The measurement of 18Odiatom is carried out using the IR-laser fluorination-isotope ratio mass spectrometry technique at the CEREGE stable isotope laboratory, after dehydration under a flow of nitrogen gas. The diatoms are purified at the CEREGE micropaleontology laboratory after decarbonation and organic matter oxidation. The taxonomic determination of diatoms is carried at CEREGE (France) and at the University of N’Djamena (Chad).Preliminary results from the two Tibesti mountains records cover the Holocene wet period. They show significant variations in the d18Odiatom values and a distinct evolution of ecosystems as demonstrated by taxonomic assemblage of fossil diatoms. These results are compared with a reconstruction of the d18Odiatom from Lake Chad for the same period. This comparison evidenced substantial data for the reconstruction of the Holocene wet period, in terms of origin of water inflows in the basin, eg Tropical versus Mediterranean and lowland versus mountainous atmospheric processes, and on the reconstruction of the migration and the position of the ITCZ. These two questions are still speculative and will certainly provide data for global climate circulation models which are struggling to reproduce the climate in Saharan latitudes.

International audience

Reconnues comme hauts lieux de biodiversité (hot spots), les zones humides africaines sont sources de nombreux services écosystémiques. Les sociétés y mènent des activités de subsistance (pêche, agriculture, élevage, chasse, cueillette) intégrées au fonctionnement des écosystèmes. De plus en plus sollicités tout en restant très vulnérables, ces territoires jouent désormais un rôle moteur dans les économies locales et régionales, en exportant une large diversité de produits vers les villes. Or en ce début du XXIe siècle, ils cumulent les défis de l’Anthropocène – changement climatique, croissance démographique, urbanisation, mondialisation des échanges – et ceux d’une crise profonde de gouvernance. Leur étude est donc devenue un enjeu scientifique et sociétal important. En proposant plusieurs angles de lecture, cet ouvrage collectif contribue à une meilleure connaissance des zones humides sahéliennes à partir de l’étude des lacs du Tchad. Il articule les résultats des recherches à différents pas de temps : le temps long (géologie, archéologie), le temps moyen des dernières décennies (hydrologie, histoire, écologie, géographie) et le temps court de l’année et des saisons (géographie, anthropologie). Étudiants, chercheurs, acteurs locaux et décideurs trouveront dans cet ouvrage un regard nouveau, pluridisciplinaire, sur ces socioécosystèmes lacustres. Il enrichit la vision des possibles pour les sociétés à venir et invite à un débat nécessaire sur la conception de politiques publiques intégrant les enjeux imbriqués de conservation, de préservation des services écosystémiques et de développement.