Due to a long-lasting drought afflicting the Sahel, the Casamance River has been transformed into a hyperhaline estuary, with salinities up to 170 °/°° at a distance of 210 km from the sea. Foraminifera and zooplankton populations both show a marked decrease in the number of species in increasingly confined water, the distribution of species being closely related to the evolution ofabiotic variables. Our three-prong study allowed us to identify six zones, upwards from the sea. The uppermost ones are characterized by drastic conditions which considerably reduce the number of species. Therefore, the populations of Foraminifera, zooplankton and even fish become oligo or mono specific. The Casamance river appears to fit quite well into the general rules concerning hyperhaline environments. However, it shows some peculiar features which are : the large dimensions of the hyperhaline estuary (over 230 km long and over 5 km wide in the lower course); peak salinities among the highest known for a permanently open estuary (up to 170°/°°); and water remaining trapped inside the river for several years. (Résumé d'auteur
