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Mangrove dynamics and local perception in the Department of Oussouye (Basse-Casamance, Senegal) between 1972 and 2018. In Lower-Casamance, the mangrove is a particular ecosystem that is home to many resources and remains the seat of the main rural activities. In the Oussouye area, this mangrove has experienced a significant decline during the 1970s and 1980s. The study of its spatio-temporal dynamics, using geomatics tools coupled with field surveys, made it possible to apprehend its decline between 1972 and 1994 of -42 %; and its positive evolution of +22.44 % between 1994-2018. The factors behind the decline during the 1970s and 1980s were mainly the rainfall deficit, with the consequences of salinization and acidification of the land, and the weakness of natural regeneration. The study also showed that anthropogenic actions such as cutting wood for various reasons and stilt roots have long harmed mangrove ecosystems. The recovery observed in recent decades is to be seen in the context of the return of rainfall to a better situation and the awareness of the populations of the strategic and important role of the mangrove.

In West Africa, as in many other estuaries, enormous volumes of marine water are entering the continent. Fresh water discharge is very low, and it is commonly strongly linked to rainfall level. Some of these estuaries are inverse estuaries. During the Great Sahelian Drought (1968−1993), their hyperhaline feature was exacerbated. This paper aims to describe the evolution of the two main West African inverse estuaries, those of the Saloum River and the Casamance River, since the end of the drought. Water salinity measurements were carried out over three to five years according to the sites in order to document this evolution and to compare data with the historical ones collected during the long dry period at the end of 20th century. The results show that in both estuaries, the mean water salinity values have markedly decreased since the end of the drought. However, the Saloum estuary remains a totally inverse estuary, while for the Casamance River, the estuarine turbidity maximum (ETM) is the location of the salinity maximum, and it moves according to the seasons from a location 1−10 km downwards from the upstream estuary entry, during the dry season, to a location 40−70 km downwards from this point, during the rainy season. These observations fit with the functioning of the mangrove, the West African mangrove being among the few in the world that are markedly increasing since the beginning of the 1990s and the end of the dry period, as mangrove growth is favored by the relative salinity reduction. Finally, one of the inverse estuary behavior factors is the low fresh water incoming from the continent. The small area of the Casamance and Saloum basins (20,150 and 26,500 km² respectively) is to be compared with the basins of their two main neighbor basins, the Gambia River and the Senegal River, which provide significant fresh water discharge to their estuary.