Marine Megafauna (cetaceans, turtles, seabirds & elasmobranchs) associate with pelagic Sargassum off Suriname

Recent atypical blooming events of the macroalgae Sargassumhave affected humans and animals within the wider Caribbean region. Little is known how Sargassumaffects marine megafauna offshore in Suriname. Data from dedicated marine megafauna surveys were pooled (2012 and 2015; May -September). A significant higher presence of Sargassumwas found in May and July and particularly over the Demerara plateau. Sargassumconstituted localised ‘hot-mats’ for biodiversity: 77 sightings comprising 25 species associated with Sargassum mats. Young green turtles (Chelonia mydas) basked in Sargassum at a farthest reported (global) distance from the coast. Cetaceans were significantly more abundant in waters with Sargassum. Dolphins (Stenella longirostrisand S. attenuata) showed higher abundance indices in presence of Sargassum, while deep diving cetaceans showed higher indices without Sargassum. All three species of booby (Sulasp.) foraged on flying fish in Sargassum. Terns (Onychoprion fuscatus) and shearwaters (Calonectris diomedea, Puffinus gravis, P. lherminieri) foraged amongst Sargassum. Manx shearwater (P. puffinus) and red-billed tropicbird (Phaethon aethereus) were flying low over Sargassum. Using bird transect-data we found that in 2012, Sargassumattracted significantly larger groups of foraging seabirds compared to 2015. These differences may have been affected by mat-morphology (2012: ‘loose’vs2015: ‘dense’). It is unknown how mat-morphology may affect marine megafauna during future blooming-events. It has been reported that Sargassum natans VIII provides less suitable feeding-mats than other forms, but more research is needed. Nevertheless, Sargassumoffers opportunities to a diverse marine megafauna community in what are otherwise relative nutrient-poor tropical offshore waters.