Hydrochemical behavior and modern sedimentation in Azigza and Tiguelmamine lakes (Middle Atlas, Morocco)
Comportement hydrochimique et sédimentation actuelle des lacs Azigza et Tiguelmamine (Moyen Atlas, Maroc)
Résumé
Natural 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.