Environmental variability of Moroccan Middle Atlas lakes : hydro-geochemical and hydrological functioning and response to climate change

La région sud de la Méditerranée, située à l’interface des climats aride et tempéré, est extrêmement vulnérable au changement climatique avec un fort impact sur les ressources en eaux. Le Moyen Atlas marocain draine les plus grands fleuves du pays et possède plusieurs lacs d’origine tectono-karstique. Ces lacs peuvent être considérés comme des sentinelles des changements hydroclimatiques, à condition de comprendre les mécanismes qui régissent leur fonctionnement hydrologique en lien avec le climat actuel. Cette thèse s’est concentrée sur la caractérisation des fonctionnements hydrogéochimique et hydro-isotopique lac Azigza à l’échelle mensuelle (octobre 2012 – octobre 2016). Les paramètres physico-chimiques et chimiques ont permis de proposer les mécanismes associés à la minéralisation des eaux lacustres et à la caractérisation des apports en eaux du bassin versant. Les données isotopiques ont permis de préciser l’origine des eaux, l’altitude de recharge, le renouvellement des eaux et d’établir une estimation des flux d’eau. L’acquisition des données journalières du niveau du lac a mis en évidence la réponse rapide du lac aux évènements de précipitations. De plus, le suivi du niveau de lac sur la période d’observation montre une diminution d’environ 4 m. Ce qui nous a permis de simuler les variations du niveau du lac sur la période instrumentée et d’aboutir à la quantification des apports en eaux du bassin versant et des pertes par voie souterraine. De façon exploratoire, le modèle hydrologique a ensuite été testé pour reconstituer les variations historiques du niveau du lac. En prospective, la réponse du lac Azigza face au changement climatique futur est discutéeThe southern Mediterranean region, at the interface between arid and temperate climates, is highly vulnerable to climate change and water availability. The moroccan Middle-Atlas mountains contains several natural lakes regarded as sentinels of hydro-climatic changes, provided that their hydrological functioning is clearly understood. This thesis work was focused on the study of the hydrogeochemical and hydro-isotopic functioning of Azigza lake based on a monthly monitoring (October 2012–October 2016). The physicochemical and chemical parameters (major elements), allowed to propose the mechanisms associated with the mineralization of lake waters and the characterization of groundwater inflows from watershed into the lake. The waters of the Azigza lacustrine system fall under the calcium-magnesium-bicarbonate category. The lake is hot monomictic type. The waters show a seasonal response to climatic variations.The isotopic data allow to specify the origin of waters, the recharge altitude, the residence time of waters. The daily data of water level highlights the fast response of the lake to precipitation. During the whole observation period, the lake level decreased by about 4 meters. This approach was used to simulate the variations of the lake level over the observation period and to quantify the contributions of the groundwater flows. This approach was used to simulate the variations of the lake level over the observation period and to quantify the contributions of the groundwater flows. The hydrological model was then tested to reconstruct historical lake level variations. Finally, the possible use of the model to predict future lake level is discusse

Modelling lake water and isotope mass balance variations under Mediterranean climate: Lake Azigza in the Moroccan Middle Atlas

International audienceAs many Mediterranean headwater catchments, the Moroccan Middle Atlas plays an important role in the highly vulnerable regional water resources. Mountain lakes are numerous in this region, and could be regarded as possible sentinels of hydro-climatic changes, using appropriate modelling tools able to simulate the lake-climate relation. We present a detailed study of Lake Azigza, based on a 4-year (2012-2016) observation period, including lake level measurements, isotope analyses of precipitation, lake and spring waters, and local meteorological data. The approach is based on a calibration of a daily time-step lake water and isotope mass balance model, fed by precipitation and evaporation rates, to estimate the ungauged components of the water balance. Results show the dominance of groundwater exchanges in the lake water balance, with significant interannual variations related to annual precipitation. At the annual time-step, groundwater inflow varies between twice and up to six times the amount of direct precipitation, while the groundwater loss reached up to five times evaporation. However, a significant decrease of groundwater loss is observed in 2016, suggesting that a threshold effect probably limits the seepage when the lake level decreases. This study underlines the importance of groundwater fluxes in the lake level variations for Lake Azigza, probably representative of many similar lakes in the Middle Atlas. The model was able to simulate the continuous lake level decrease (4 m) observed over 2012-2016 and can be further used to explore lake-climate relations at different timescales

Bilans hydrique et hydro-isotopique du lac Aguelmam Azigza (Moyen Atlas, Maroc)

International audienceStable isotopes of water (18O, 2H) and their associated isotope fractionation during water-cycle phase changes produce a natural labelling of the water cycle and can be efficiently use to study hydrological and climatic processes at the local, regional and global scales. Moreover, understanding the seasonal and inter-annualstable isotope behaviour in a lake system is a key for interpretingthe 18O data obtained from inorganic fractions of lake sediments in order to reconstruct past changes in local hydro-climatology. The natural mountain lake system, Aguelmam Azigza (32°58’N, 5°26’W, 1470 m asl), is located in the karstic Tabular Middle Atlas (Morocco), one of the most humid regions of Morocco, which can be seen as the “water tower” of the country. The regional climate receives alternative influences of Mediterranean, Atlantic and Saharan air masses. A lake monitoring has been implemented since October 2012 with a monthly water sampling (precipitation, lake, well and springs) for water isotope measurements,in order to characterize seasonal variations of its water and isotope balance. Precipitation amounts at Azigza lake are very variable and high (1421 mm recorded between Oct. 2012 and Oct. 2013 for ex.). The lake waters isotopic composition plot along an evaporation line, and show a clear seasonality mainly linked to evaporation/rainfall, while groundwaters plot above the GMWL and close to regional rainfall composition suggesting a rapid transfer in the fractured karstic system. A first estimate of the lake water balance using Craig-Gordon equations shows that less than 20% of the water of the lake is lost by evaporation, and that the lake a very sensitiveto climatic parameters

Comportement hydrochimique et sédimentation actuelle des lacs Azigza et Tiguelmamine (Moyen Atlas, Maroc)

International audienceNatural environmental changes may be reconstructed by studying lake sediments that record several climatic, hydrologic and environmental signatures. Such reconstructions must be based on a thorough knowledge of the current lake system conditions. According to this perspective, two karstic lakes in the Moroccan Middle Atlas, Aguelmam Azigza (32°58'N, 5°26'W, 1470 m absl) and Tiguelmamine (32°54'N, 5°21'W, 1650 m absl), have been the subject of hydrochemical and sedimentary investigations. Spatiotemporal monthly monitoring (since October 2012) of the physicochemical characteristics of waters (precipitation, lake, wells, springs and streams) shows that the waters of Azigza lake are alkaline (pH>8) with elevated contents of HCO3-, Ca 2+ and Mg 2+. The Aguelmam Azigza is a monomictic lake (with an overturn water during December to January) and indicates high sensitivity to seasonal climatic changes (temperature and precipitation) that control biogeochemical conditions in the water column. During the wet season, this lake is fed by both meteoric and ground waters. In contrast, during the dry season, the lake supports the water table. Ground waters are highly mineralized and saturated toward carbonates, and lose a large part of their saline load as authigenic, biochemical and biogenic calcite. The waters of Tiguelmamine lake, sampled in July 2012 and April 2013, suggest generally the same physicochemical characteristics such as those of Azigza lake with high contents in Cl-, Na + and SO4 2-due to soil erosion of the lake's watershed. Limnological and sedimentological characterization of subsurface sediments collected across several transects in both lakes, reveal that organic matter contents increase from the edge to the center of the lakes. On the contrary, the carbonate contents of sediments record an opposite distribution

Hydrogeochemical Processes of the Azigza Lake System (Middle Atlas, Morocco) Inferred from Monthly Monitoring

International audienceThe High Oum-Er-Rbia basin, located in the Moroccan Middle Atlas, is a karstic region with significant water sources that have essential functions regarding agriculture, hydropower production, industrial and drinking water. The region contains abundant wetlands, especially springs, rivers and natural lakes. These systems are highly sensitive to the effects of climate change, experiencing considerable lake level, water chemistry, and biological fluctuations in response to regional hydrological balances. This study focuses on the hydrogeochemical processes and mechanisms that control the chemical composition and variability of Azigza Lake, a typical tectono-karstic lake system of the region. Water monitoring was implemented from July 2013 to October 2014 with a monthly water sampling for physicochemical measurements and major ion concentration analyses of lake water and the surrounding groundwater. Both waters show a relatively low salinity due to the fresh input from the Lower Jurassic karst formation. Lake waters are slightly alkaline and of the calcium-magnesium-bicarbonate type. The geochemistry of the lake waters is mainly controlled by carbonate weathering through water–rock interaction and, to a lesser extent, by cation exchange and precipitation of carbonate minerals. The hydrochemistry of the lake showed clear responses to seasonal changes in precipitation and evaporation, with higher conductivity during the wet period. During the beginning of the wet season, groundwater evolution could be explained by a simple first flush stormwater. The rapid response of lake water to subsurface and underground waters confirms the dominance of an underground conduct flow regime. These changes and behaviors highlight the sensitivity of Azigza system to regional hydrological and climatic changes

New lacustrine records from the Moroccan Middle Atlas

International audienc

Recent hydrological variability and extreme precipitation events in Moroccan Middle-Atlas mountains: micro-scale analyses of lacustrine sediments

International audienceSince the 1990s, the Mediterranean basin undergoes an increase in precipitation events and extreme droughts likely to intensify in the XXI century, and whose origin is attributable to human activities since 1850 (IPCC, 2013). Regional climate models indicate a strengthening of flood episodes at the end of the XXI century in Morocco (Tramblay et al, 2012). To understand recent hydrological and paleohydrological variability in North Africa, our study focuses on the macro- and micro-scale analysis of sedimentary sequences from Lake Azigza (Moroccan Middle Atlas Mountains) covering the last few centuries. This lake is relevant since local site monitoring revealed that lake water table levels were correlated with precipitation regime (Adallal R., PhD Thesis in progress). The aim of our study is to distinguish sedimentary facies characteristic of low and high lake levels, in order to reconstruct past dry and wet periods during the last two hundred years. Here, we present results from sedimentological (lithology, grain size, microstructures under thin sections), geochemical (XRF) and physical (radiography) analyses on short sedimentary cores (64 cm long) taken into the deep basin of Lake Azigza (30 meters water depth). Cores have been dated (radionuclides 210Pb, 137Cs, and 14C dating). Two main facies were distinguished: one organic-rich facies composed of wood fragments, several reworked layers and characterized by Mn peaks; and a second facies composed of terrigenous clastic sediments, without wood nor reworked layers, and characterized by Fe, Ti, Si and K peaks. The first facies is interpreted as a high lake level stand. Indeed, the highest paleoshoreline is close to the vegetation, and steeper banks can increase the current velocity, allowing the transport of wood fragments in case of extreme precipitation events. Mn peaks are interpreted as Mn oxides precipitations under well-oxygenated deep waters after runoff events. The second facies is linked to periods of increased detrital input by incising sediments during low lake levels. This interpretation is supported by chronological jumps in this facies (incoherent old 14C ages). Finally, the presence of numerous anhydrous calcium sulfates in the recent low lake level facies supports the observation of a decreasing lake level for the last decades (Flower et al., 1989; Adallal R., PhD Thesis in progress). Our study demonstrates that several lake level changes occurred during the past two hundred years, and highlights the unprecedented lake level drop since the 1980s. Bibliography Flower, R.J., Stevenson, A.C., Dearing, J.A., Foster, I.D., Airey, A., Rippey, B.,Wilson, J.P.F. & Appleby, P.G. (1989). Catchment disturbance inferred from paleolimnological studies of three contrasted sub-humid environments in Morocco. J Paleolimnol 1: 293-322. IPCC, AR 5. Climate Change (2013). The physical Science Report. Tramblay, Y., Badi, W., Driouech, F., El Adlouni, S., Neppel, L. and Servat, E. 2012. Climate change impacts on extreme precipitation in Morocco. Global and Planetary Change 82-83: 104-114

Recent hydrological variability and extreme precipitation events in Moroccan Middle-Atlas mountains: micro-scale analyses of lacustrine sediments

International audienceSince the 1990s, the Mediterranean basin undergoes an increase in precipitation events and extreme droughts likely to intensify in the XXI century, and whose origin is attributable to human activities since 1850 (IPCC, 2013). Regional climate models indicate a strengthening of flood episodes at the end of the XXI century in Morocco (Tramblay et al, 2012). To understand recent hydrological and paleohydrological variability in North Africa, our study focuses on the macro- and micro-scale analysis of sedimentary sequences from Lake Azigza (Moroccan Middle Atlas Mountains) covering the last few centuries. This lake is relevant since local site monitoring revealed that lake water table levels were correlated with precipitation regime (Adallal R., PhD Thesis in progress). The aim of our study is to distinguish sedimentary facies characteristic of low and high lake levels, in order to reconstruct past dry and wet periods during the last two hundred years. Here, we present results from sedimentological (lithology, grain size, microstructures under thin sections), geochemical (XRF) and physical (radiography) analyses on short sedimentary cores (64 cm long) taken into the deep basin of Lake Azigza (30 meters water depth). Cores have been dated (radionuclides 210Pb, 137Cs, and 14C dating). Two main facies were distinguished: one organic-rich facies composed of wood fragments, several reworked layers and characterized by Mn peaks; and a second facies composed of terrigenous clastic sediments, without wood nor reworked layers, and characterized by Fe, Ti, Si and K peaks. The first facies is interpreted as a high lake level stand. Indeed, the highest paleoshoreline is close to the vegetation, and steeper banks can increase the current velocity, allowing the transport of wood fragments in case of extreme precipitation events. Mn peaks are interpreted as Mn oxides precipitations under well-oxygenated deep waters after runoff events. The second facies is linked to periods of increased detrital input by incising sediments during low lake levels. This interpretation is supported by chronological jumps in this facies (incoherent old 14C ages). Finally, the presence of numerous anhydrous calcium sulfates in the recent low lake level facies supports the observation of a decreasing lake level for the last decades (Flower et al., 1989; Adallal R., PhD Thesis in progress). Our study demonstrates that several lake level changes occurred during the past two hundred years, and highlights the unprecedented lake level drop since the 1980s. Bibliography Flower, R.J., Stevenson, A.C., Dearing, J.A., Foster, I.D., Airey, A., Rippey, B.,Wilson, J.P.F. & Appleby, P.G. (1989). Catchment disturbance inferred from paleolimnological studies of three contrasted sub-humid environments in Morocco. J Paleolimnol 1: 293-322. IPCC, AR 5. Climate Change (2013). The physical Science Report. Tramblay, Y., Badi, W., Driouech, F., El Adlouni, S., Neppel, L. and Servat, E. 2012. Climate change impacts on extreme precipitation in Morocco. Global and Planetary Change 82-83: 104-114

Recent hydrological variability and extreme precipitation events in Moroccan Middle-Atlas mountains: micro-scale analyses of lacustrine sediments

International audienceSince the 1990s, the Mediterranean basin undergoes an increase in precipitation events and extreme droughts likely to intensify in the XXI century, and whose origin is attributable to human activities since 1850 (IPCC, 2013). Regional climate models indicate a strengthening of flood episodes at the end of the XXI century in Morocco (Tramblay et al, 2012). To understand recent hydrological and paleohydrological variability in North Africa, our study focuses on the macro- and micro-scale analysis of sedimentary sequences from Lake Azigza (Moroccan Middle Atlas Mountains) covering the last few centuries. This lake is relevant since local site monitoring revealed that lake water table levels were correlated with precipitation regime (Adallal R., PhD Thesis in progress). The aim of our study is to distinguish sedimentary facies characteristic of low and high lake levels, in order to reconstruct past dry and wet periods during the last two hundred years. Here, we present results from sedimentological (lithology, grain size, microstructures under thin sections), geochemical (XRF) and physical (radiography) analyses on short sedimentary cores (64 cm long) taken into the deep basin of Lake Azigza (30 meters water depth). Cores have been dated (radionuclides 210Pb, 137Cs, and 14C dating). Two main facies were distinguished: one organic-rich facies composed of wood fragments, several reworked layers and characterized by Mn peaks; and a second facies composed of terrigenous clastic sediments, without wood nor reworked layers, and characterized by Fe, Ti, Si and K peaks. The first facies is interpreted as a high lake level stand. Indeed, the highest paleoshoreline is close to the vegetation, and steeper banks can increase the current velocity, allowing the transport of wood fragments in case of extreme precipitation events. Mn peaks are interpreted as Mn oxides precipitations under well-oxygenated deep waters after runoff events. The second facies is linked to periods of increased detrital input by incising sediments during low lake levels. This interpretation is supported by chronological jumps in this facies (incoherent old 14C ages). Finally, the presence of numerous anhydrous calcium sulfates in the recent low lake level facies supports the observation of a decreasing lake level for the last decades (Flower et al., 1989; Adallal R., PhD Thesis in progress). Our study demonstrates that several lake level changes occurred during the past two hundred years, and highlights the unprecedented lake level drop since the 1980s. Bibliography Flower, R.J., Stevenson, A.C., Dearing, J.A., Foster, I.D., Airey, A., Rippey, B.,Wilson, J.P.F. & Appleby, P.G. (1989). Catchment disturbance inferred from paleolimnological studies of three contrasted sub-humid environments in Morocco. J Paleolimnol 1: 293-322. IPCC, AR 5. Climate Change (2013). The physical Science Report. Tramblay, Y., Badi, W., Driouech, F., El Adlouni, S., Neppel, L. and Servat, E. 2012. Climate change impacts on extreme precipitation in Morocco. Global and Planetary Change 82-83: 104-114

Recent hydrological variability and extreme precipitation events in Moroccan Middle-Atlas mountains: micro-scale analyses of lacustrine sediments

International audienceSince the 1990s, the Mediterranean basin undergoes an increase in precipitation events and extreme droughts likely to intensify in the XXI century, and whose origin is attributable to human activities since 1850 (IPCC, 2013). Regional climate models indicate a strengthening of flood episodes at the end of the XXI century in Morocco (Tramblay et al, 2012). To understand recent hydrological and paleohydrological variability in North Africa, our study focuses on the macro- and micro-scale analysis of sedimentary sequences from Lake Azigza (Moroccan Middle Atlas Mountains) covering the last few centuries. This lake is relevant since local site monitoring revealed that lake water table levels were correlated with precipitation regime (Adallal R., PhD Thesis in progress). The aim of our study is to distinguish sedimentary facies characteristic of low and high lake levels, in order to reconstruct past dry and wet periods during the last two hundred years. Here, we present results from sedimentological (lithology, grain size, microstructures under thin sections), geochemical (XRF) and physical (radiography) analyses on short sedimentary cores (64 cm long) taken into the deep basin of Lake Azigza (30 meters water depth). Cores have been dated (radionuclides 210Pb, 137Cs, and 14C dating). Two main facies were distinguished: one organic-rich facies composed of wood fragments, several reworked layers and characterized by Mn peaks; and a second facies composed of terrigenous clastic sediments, without wood nor reworked layers, and characterized by Fe, Ti, Si and K peaks. The first facies is interpreted as a high lake level stand. Indeed, the highest paleoshoreline is close to the vegetation, and steeper banks can increase the current velocity, allowing the transport of wood fragments in case of extreme precipitation events. Mn peaks are interpreted as Mn oxides precipitations under well-oxygenated deep waters after runoff events. The second facies is linked to periods of increased detrital input by incising sediments during low lake levels. This interpretation is supported by chronological jumps in this facies (incoherent old 14C ages). Finally, the presence of numerous anhydrous calcium sulfates in the recent low lake level facies supports the observation of a decreasing lake level for the last decades (Flower et al., 1989; Adallal R., PhD Thesis in progress). Our study demonstrates that several lake level changes occurred during the past two hundred years, and highlights the unprecedented lake level drop since the 1980s. Bibliography Flower, R.J., Stevenson, A.C., Dearing, J.A., Foster, I.D., Airey, A., Rippey, B.,Wilson, J.P.F. & Appleby, P.G. (1989). Catchment disturbance inferred from paleolimnological studies of three contrasted sub-humid environments in Morocco. J Paleolimnol 1: 293-322. IPCC, AR 5. Climate Change (2013). The physical Science Report. Tramblay, Y., Badi, W., Driouech, F., El Adlouni, S., Neppel, L. and Servat, E. 2012. Climate change impacts on extreme precipitation in Morocco. Global and Planetary Change 82-83: 104-114