Socio-ecological vulnerability assessment to Sargassum arrivals

Abstract

The proliferation of holopelagic Sargassum spp. (Sargassum) in the tropical North Atlantic Ocean is of concern for populations and coastal ecosystems in the Caribbean Sea, Gulf of Mexico, and West Africa. Satellite detections have enabled rough assessments of the quantity of algae that drifts seasonally in the open ocean, with seasonal peaks of Sargassum biomass reaching 10 to 20 million tons since 2018. Although the impacts on the coast have been widely publicized, there are no estimates of the quantities of Sargassum that accumulate on the coasts. This study proposes novel vulnerability indicators that combine information on Sargassum stranded biomass, ecosystem and socioeconomic factors to assess risks posed by Sargassum on coastal regions. Quantities of Sargassum that accumulate in the coastal strip at the regional scale were derived by combining the satellite detections in the open ocean with an algal growth-transport-stranding model. It shows that the amount of Sargassum accumulating on the Atlantic coast is of the order of 10% of the biomass estimated offshore and has accumulated between 2 and 10 million tons per year over the last five years. Vulnerability indices identify the Mexican Caribbean, northern Lesser Antilles, and eastern Great Antilles as the most vulnerable regions, facing significant ecosystemic and socioeconomic pressures from Sargassum influxes.

Fig. 1

Annual Sargassum cover from MODIS [per mill] and estimation of Sargassum stranding for the period 2018–2023 [in tons per year]. Sargassum cover is shown on a 1/4° and stranding is shown on a grid of 1° resolution. Stranding pixels with less than 1000 tons per year are not considered. Map was created using Geopandas 0.14.2 (https://geopandas.org).

Fig. 2

(a) Monthly time series of Sargassum biomass estimated from MODIS (blue) and obtained from the NEMO-Sarg 1/12° nudged simulation (red). The biomass is integrated over an area ranging from 100°W to 0°E and from 0°S to 30°N. (b) Monthly stranding rate [Million tons per month] and (c) annual stranding rate [Million tons per year] computed from NEMO-Sarg. Time series were plotted using Matplotlib 3.8.2 (https://matplotlib.org).

Fig. 3

Map of coastal vulnerability index computed from estimated stranding from 2018 to 2023: (a) total coastal vulnerability (SVI), (b) ecosystemic coastal vulnerability (SVI_ECOSYS) and (c) socioeconomic coastal vulnerability (SVI_SOCIO). Maps were created using Geopandas 0.14.2 (https://geopandas.org).

Fig. 4

Annual stranding estimated on 1/12° (~ 9 km x 9 km) grid cells for the period 2018–2023, in tons per year for (a–d) the Lesser Antilles, (e–h) the western Caribbean Sea and (i–l) on the coast of Ghana. Maps were created using Geopandas 0.14.2 (https://geopandas.org).

Fig. 5

Estimations of Sargassum annual stranding by country (average over the period 2018–2023) as total biomass per country (left) and as coastal pressure in tons/km of coast (right). The figure was created using Matplotlib 3.8.2 (https://matplotlib.org).