Title : Retreat of the mangrove and risk of marine submersion: the case of Libreville
Abstract:
The urbanization of coastal cities is a process that is often linked to significant urban growth, resulting in intensive coastal development. This often results in continued strong anthropogenic pressure on the coastal zone. The coastal city of Libreville (situated at the north west of Gabon, a central African country) is characterized by a significant increase in its population over the past 50 years (from 7% to 52% of the total Gabonese population, Poitier et al., 2017). In addition to this demographic explosion, there followed an anarchic land occupation described as the "galloping" urbanization of Libreville (Poitier et al., 2017) which leads to the destruction of coastal ecosystems such as mangroves constituting a natural barrier against the risks of erosion and marine submersion. The destruction of mangrove areas exposes the environment to floods and other disasters, often endangering the populations that continue to settle in these areas.
In order to characterize the impact of climate change and human activity on the mangrove, four sites in the Libreville region were the subject of our study. Concerning the first two sites (Malibé and Diba-Diba areas), the processing of aerial photographs associated with field observations and field surveys made it possible to highlight the following results: 1/ in Malibé, the space of the mangrove has receded by about 90m since 1995, that is a retreat rate of 3.5m/year; 2/ in the Diba-Diba area, for an area of 100ha, the mangrove still occupies a relatively large space (70%) with an anthropization of around 15%. This observed decline of the mangrove in these two areas suggests a strong anthropogenic influence on which is added the effects of the sea level rise. The use of micro-organisms (benthic foraminifera and ostracods) as bioindicators of a marine influence on the continent in the two other sites (Akournam and Kouango) has made it possible to show that their presence in the samples analyzed, would translate a marine signal indicating a phase of progressive marine transgression. The two sites could therefore, in the long term, be covered by the waters of the South Atlantic.
In conclusion, the studied areas (Malibé, Diba-Diba, Akournam and Kouango) would constitute areas at risk for populations that are constantly settling there, destroying the ecosystem.
