Since 2011 pelagic Sargassum strandings in the Caribbean basin have had significant environmental, health, economic and socio-political consequences and a substantial impact on tourism. Research so far tends to confirm the existence of a new ecosystem subjected to movements of water masses across the equatorial Atlantic. The phenomenon occurred in 2011 and the 2012-2018 period, along with increases in Sargassum biomass. Satellite imagery is used to detect Sargassum rafts in the equatorial Atlantic at medium resolution. Sea surface current data for the same area are used to infer annual passive drifting. The combination of these two data sources have made it possible to design coarse-scale approximate prediction tools that remain, however, difficult to operate at the scale of insular coastal zones. This context makes it more difficult to manage Sargassum influxes. The CESAR program will describe and explain the dynamics of coastal Sargassum flows at local spatial scales, their impacts on Caribbean coasts and the various paths explored by political institutions to address this environmental issue. This implies improving knowledge on local forcing parameters and Sargassum dynamics in the Lesser Antilles. The accumulation of Sargassum on the coastline impacts also the morphosedimentary dynamics of beaches and the biogeomorphic functioning of coastal marine ecosystems, especially seagrass beds. The characterization of these impacts at the local and regional level is a major challenge for the development of governance adaptation strategies within the Caribbean context, linking environmental, economic, health, regulatory and decision-making policies. CESAR will deliver operational systems to prepare communities and governments to the apparent increasing future risk of Sargassum washing ashore.