Assessing sargassum pressure on coastal habitats using a spatial and temporal approach at the territorial scale

• Airborne and land-based camera monitoring programmes were conducted on Martinique. • Indicators were developed to quantify sargassum pressure along the coast. • Three sargassum states were considered: stranded, stagnant, brown water. • Mangroves are heavily exposed to dense and persistent sargassum accumulations. • Seagrass meadows are mainly exposed to brown water from sargassum decomposition. Since 2011, coastal areas of the Caribbean Sea and the Tropical Atlantic Ocean have been exposed to massive influxes of pelagic sargassum algae. Coastal sargassum accumulations and brown water associated with their decomposition represent a new pressure threatening ecosystems, human health and the socio-economic sector. There is still a lack of well-calibrated monitoring methods to accurately characterise sargassum pressure at the scale of a territory. This study aimed to develop a methodology for quantifying, locating, and monitoring the dynamics of sargassum pressure on Martinique’s coastal environment (Lesser Antilles). Monthly aerial monitoring and daily records of in situ cameras were used to develop pressure indicators for sargassum accumulations and brown water. Thanks to these indicators, we identified the areas most exposed to sargassum pressure: the innermost parts of the bays characterised by low hydrodynamic conditions, particularly the mangroves, and very exposed to prevailing winds. In these areas, dense accumulations persist all over the year, even during lulls in sargassum influxes. Coastal marine habitats, such as seagrass meadows, are mainly exposed to brown water, particularly during heavy rainfall, which causes brown water to spread offshore. Our methodology has proved to be an effective tool for territorial diagnosis, enabling the identification of sites at stake and periods at risk. It provided a better understanding of spatial variations and drivers of exposure to three different decomposition states of sargassum (stranded, stagnant and brown water). These data are invaluable to managers, researchers, and entrepreneurs and can be helpful to other regions that are exposed to sargassum influxes as part of health, socio-economic, or environmental impact studies.