Floating Sargassum has been known since Christopher Columbus to occur in the Atlantic Ocean, off the east coast of Florida, in a region named Sargasso Sea (^{Djakoure et al., 2017}). It consists oftwo pelagic species, Sargassum fluitans and Sargassum natans, which conform a floating ecosystem with a high diversity of species associated to and dependent onto it (H^{offmayer et al., 2005}); the Sargasso Sea is unique because it is the only self-sustaining community of holopelagic algae (^{Trott et al., 2010}). Most species associated to Sargassum are highly adapted, with appendages and coloration mimicking the alga (^{Sterrer, 1992}). Ten species of invertebrates and one species of fish are endemic to the Sargasso Sea (Trott et al., 2010). Many other species, including a diverse array of juvenile and migratory fishes, and at least four species of sea turtles, all endangered, use the ecosystem as nursery habitat or as feeding ground (^{Manzella and Williams, 1991}; ^{Mansfield et al., 2014}).

It has been estimated that the Sargasso Sea harbors about ten million tons of wet biomass (^{Johnson et al., 2013}). Despite the fact that drift Sargassum has historically been reported on the shores along the Gulf of Mexico and the Caribbean Sea (^{Taylor, 1960}), since 2011 the biomass of seaweeds washed ashore has reached unprecedented amounts and has been observed at localities where it was uncommon or unreported before this date (^{Smetacek and Zingone, 2013}; ^{Gavio et al., 2015}, ^{Rodriguez-Martinez et al., 2016}, ^{Louime etal., 2017}). Several hypotheses have been proposed to explain such events, including an excess of nutrient loads, a change in trade currents, and unusually high sea surface temperatures (^{Lopez et al., 2008}; ^{Djakouré et al., 2017}). Wrack Sargassum biomass may be beneficial to the environment at moderate densities, because it provides food and shelter to several species, it may help fight beach erosion and provide nutrients to beach habitats (Lopez et al., 2008). However, when biomass is very high, it may have negative effects: the accumulation of algae on the water surface precludes light penetration, and affects corals and benthic macroalgae (Lopez et al., 2008). The drift algae on the beach may become a barrier to nesting turtles and/or to baby turtles finding their way to the ocean (Maureer et al., 2015; ^{Azanza-Ricardo and Pérez-Martín, 2016}). Cleaning up the excessive biomass along the beaches may enhance beach erosion (Louime et al., 2017), and decomposition of many tons of seaweeds on the shore may change water chemistry, induce anoxia and produce hydrogen sulphide, which is harmful to most organisms, with consequent fish die-off (^{Cruz-Rivera et al., 2015}).

Furthermore, deposits of large quantities of algae are not well-seen by beach users, and tourists have been reported to avoid resorts affected by golden tides, with negative impacts on the tourism industry (^{Milledge and Harvey, 2016}).

Serranilla Bank is an ancient atoll in the Caribbean Sea, at 15° 50' N and 79 ° 50' W (Fig. 1). It has several small cays emerging from the water to form some permanent islands. These oceanic cays, isolated from other emerged territories, have been recognized only recently as important nesting areas for sea turtles (Barrientos-Muñoz and Ramirez-Gallego, pers. comm.). In September 2017, the Colombian Commission for the Ocean (CCO), with the financial support of Colciencias and Dimar, and the logistic support of the Colombian Navy (Armada de Colombia), organized a scientific expedition (Seaflower Expedition 2017) to Serranilla bank, to study the biodiversity of this remote area of the country. During this expedition, we observed great amounts of floating Sargassum reaching the beaches of Beacon Cay, the largest island in Serranilla Bank (Fig. 2). The algae accumulated on the beaches, and formed a thick mat to 40 cm high. Along the beach, a great number of sea turtle nests were reported (Barrientos-Muñoz and Ramirez-Gallego, pers. comm.) which, by the time of the Sargassum wave, were ready to disclose. We were able to observe some baby turtles having troubles passing the barrier posed by the Sargassum mat (Fig. 3), and were vulnerable to predation by ghost crabs, rats and other predators.

Figure 1 Location of Serranilla Bank in the Caribbean Sea. 

Figure 2 Aerial photo of floating Sargassum reaching Beacon Cay. Photo credits: Santiago Estrada-Robledo. 

Figure 3 A baby turtle struggling in the Sargassum mat. 

At other Caribbean localities, it has been reported that baby turtles may have problems to pass through Sargassum mat and reach the sea (Maureer et al., 2015); also adult turtles may be negatively affected by seaweed wrack: in Cuba, there was a decrease of nesting success during Sargassum influx: the most affected species was loggerhead turtle, which is smaller and weaker than green turtle (^{Azanza-Ricardo and Pérez-Martín, 2016}). Considering that all the species of sea turtles in the Caribbean are at extinction risk, large amount of Sargassum in Serranilla Bank may pose an additional threat to the survivorship of these organisms.