Résumé
Coastal lagoons are among the highest biological productive ecosystems; they offer a variety of habitats and feeding grounds for several species. These ecosystems are besmirched by human activities which entail the degradation to their environmental aspects. The purpose of this study is the identification of the grain-size characteristics and the unprecedented recognition of the vegetation cover, so as to assess the ecological state and the water body quality of Kalâat Andalous lagoon: a fast changing ecosystem under its dynamic sandy spit and human pressures. The methodology is based on the sedimentological analysis and the microfauna identification. The study of the environmental quality of the lagoon was built on an unprecedented identification of the macrophyte communities and the use of diversity indexes and the EXCLAME indicator; the latter combines the taxonomic composition and the species abundance. The sedimentological investigations showed that the northern and southern areas of the lagoon, as well as its channel area are all dominated by a sandy facies provided by the continental shelf i.e. the nearshore zone. The central area of the lagoon is characterized by the finest grain size facies (<0.063 mm) mostly supplied by the aeolian terrestrial fluxes and the delta flood yields. The harvested pelagic and benthic microfauna indicated that the lagoon is supplied by sediments of mixed continental-fluvial and marine origins. The inventory of macrophytes demonstrates the presence of 19 species, five of which are reference species. The dominant taxa are those of Green algae (53%) with several opportunistic and invasive species with a high outbreak altering the ecosystem's natural balance, then Brown algae (21%), Red algae (16%) and angiosperms (10%). Those results, along with the biodiversity indexes outcomes, revealed the quality loss of the ecosystem and the water eutrophication. The lagoon floristic communities are slightly unbalanced, which might be due to the natural disturbance and the anthropogenic activities. Those results conform to the EXCLAME indicator (EQRMAC = 0.24), illustrating the poor ecological state of the Kalâat Andalous lagoon water body.