The Atlantic Sargassum invasion impedes beach access for nesting sea turtles

Clinging fauna associated with nearshore pelagic sargassum rafts in the Eastern Caribbean: Implications for coastal in-water harvesting

Tropical Atlantic blooms of pelagic Sargassum species are associated with severe inundation events along the coasts of Caribbean and West African nations that cause extensive ecological and socioeconomic harm. The use of in-water harvesting as a management strategy avoids the plethora of challenges associated with shoreline inundations. Moreover, with a growing interest in the valorisation of this raw material, in-water harvesting provides the best opportunity to collect substantial amounts of 'fresh' sargassum that can be used in a variety of applications. However, in-water harvesting of sargassum will remove organisms associated with the floating habitat, resulting in loss of biodiversity, thus creating a potential management dilemma. To address this management concern, we assessed the clinging fauna associated with sargassum rafts at various distances from shore. From a total of 119 dipnet samples of sargassum, we recorded 18 taxa, across 6 phyla (Arthropoda, Mollusca, Chordata, Platyhelminthes, Nemathelminthes, Annelida) with the phylum Arthropoda being the most speciose (n = 10). Our multivariate and model selection analyses support that distance from shore and season are the most important drivers of variability in community composition and that season is the most important driver of individual abundance and number of taxa across samples. Overall, rafts within 0-3000 m of the shoreline of Barbados harbored low biodiversity and were dominated by small invertebrates (mean size: 5.5 mm) of no commercial value. Results suggest that biodiversity trade-offs associated with in-water sargassum harvesting in coastal areas are likely to be negligible.

First Record of Kemp's Ridley Sea Turtle, Lepidochelys kempii (Garman, 1880), in the Waters of Martinique Island (Lesser Antilles)

An adult female Lepidochelys kempii was found trapped in drifting sargassum south of Martinique; this is the southernmost report of this taxon in the Lesser Antilles arc. Determining the limits of distribution and the existence of possible sympatry between L. kempii and L. olivacea in certain subregions of the Caribbean has been hindered by numerous misidentifications. We review the available data and propose a new distribution map in the Caribbean, which can serve as a basis for future studies.

Predominant heterotrophic diazotrophic bacteria are involved in Sargassum proliferation in the Great Atlantic Sargassum Belt

Since 2011, the Caribbean coasts have been subject to episodic influxes of floating Sargassum seaweed of unprecedented magnitude originating from a new area "the Great Atlantic Sargassum Belt" (GASB), leading in episodic influxes and mass strandings of floating Sargassum. For the biofilm of both holopelagic and benthic Sargassum as well as in the surrounding waters, we characterized the main functional groups involved in the microbial nitrogen cycle. The abundance of genes representing nitrogen fixation (nifH), nitrification (amoA), and denitrification (nosZ) showed the predominance of diazotrophs, particularly within the GASB and the Sargasso Sea. In both location, the biofilm associated with holopelagic Sargassum harboured a more abundant proportion of diazotrophs than the surrounding water. The mean δ15N value of the GASB seaweed was very negative (-2.04‰), and lower than previously reported, reinforcing the hypothesis that the source of nitrogen comes from the nitrogen-fixing activity of diazotrophs within this new area of proliferation. Analysis of the diversity of diazotrophic communities revealed for the first time the predominance of heterotrophic diazotrophic bacteria belonging to the phylum Proteobacteria in holopelagic Sargassum biofilms. The nifH sequences belonging to Vibrio genus (Gammaproteobacteria) and Filomicrobium sp. (Alphaproteobacteria) were the most abundant and reached, respectively, up to 46.0% and 33.2% of the community. We highlighted the atmospheric origin of the nitrogen used during the growth of holopelagic Sargassum within the GASB and a contribution of heterotrophic nitrogen-fixing bacteria to a part of the Sargassum proliferation.

Environmental impact of Sargassum spp. landings: an evaluation of leachate released from natural decomposition at Mexican Caribbean coast

Large volumes of pelagic Sargassum spp. have stranded periodically on the Mexican Caribbean shoreline. The aim of this research was to study the mobility of metals through the leachates released into the environment during the natural decomposition process of Sargassum spp. Fresh Sargassum samples were placed in cone-bed reactors: under laboratory and local environmental conditions. The leachate generated naturally by decomposition in both conditions was recovered periodically and analyses of pH, volume, and metal content were carried out. Sargassum biomass was monitored by electron microscopy, FT-IR, and CHNS analysis. The Sargassum biomass studied presented a C: N ratio of 24.39, making it a potential raw feedstock for biofuels and other value-added products. Calculations performed on leachate production allowed inferring that each ton of fresh Sargassum that decomposes at a controlled temperature of 27 °C can produce 316 L of leachate. This leachate can contain 5.67 g of As and other potentially toxic metals (e.g., B, Al, Cu). At the end of both experiments, the biomass that was incubated for 30 days presented a C: N ratio of 28.86, so it can still be used as raw material for biofuels; however, the Sargassum biomass that remained 180 days in incubation decreased its C:N ratio at 8.45 at this point, it can be considered a waste. The leachate generated during the natural decomposition process of Sargassum on beaches or disposal sites represents a high risk of contamination of the Yucatan Peninsula water system due to the high content of arsenic and the presence of potentially toxic metals.

In situ observation of holopelagic Sargassum distribution and aggregation state across the entire North Atlantic from 2011 to 2020

Between 2011 and 2020, 6,790 visual observations of holopelagic Sargassum were recorded across the North Atlantic Ocean to describe regional distribution, presence, and aggregation state at hourly and 10 km scales. Influences of oceanographic region and wind/sea conditions as well as temporal trends were considered; marine megafauna associates documented the ecological value of aggregations. Holopelagic Sargassum was present in 64% of observations from the western North Atlantic. Dispersed holopelagic Sargassum fragments and clumps were found in 97% of positive observations whereas aggregated windrows (37%) and mats (1%) were less common. Most field observations noted holopelagic Sargassum in quantities below the AFAI algorithm detection limit for the MODIS sensor. Aggregation state patterns were similar across regions; windrow proportion increased with higher wind speeds. In 8 of 10 years in the Sargasso Sea holopelagic Sargassum was found in over 65% of observations. In contrast, the Tropical Atlantic and Caribbean Sea exhibited greater inter-annual variability (1-88% and 11-78% presence, respectively) that did not align with extremes in central Atlantic holopelagic Sargassum areal coverage determined from satellite observations. Megafauna association patterns varied by taxonomic group. While some study regions were impacted by holopelagic Sargassum dynamics in the equatorial Atlantic, the Sargasso Sea had consistently high presence and operated independently. Field observations capture important dynamics occurring at fine spatiotemporal scales, including transient aggregation processes and ecological value for megafauna associates, and therefore remain essential to future studies of holopelagic Sargassum.