Unprecedented Proliferation of Novel Pelagic Sargassum Form has Implications for Ecosystem Function and Regional Diversity in the Caribbean

Pelagic Sargassum is a planktonic macroalgae comprised of two species, S. fluitans and S. natans, each exhibiting a variety of morphological forms; it is found throughout the North Atlantic, Caribbean Sea, and Gulf of Mexico. Drifting open ocean Sargassum provides essential habitat and food resources to organisms across multiple trophic levels, from resident shrimp to migratory sea turtles. Historic observations, including Sea Education Association’s (SEA) 22-year field sampling dataset, indicate that S. natans-I and S. fluitans-III are most common and that S. natans-VIII is rare. Furthermore, SEA's long-term record shows very low pelagic Sargassum abundance in the Eastern Caribbean in contrast to the Sargasso Sea. During April 2014, Sargassum began washing ashore along Caribbean coastlines in unprecedented quantities. Shipboard observations of the recent inundation event occurred November 2014 to May 2015. In total, 30.5 kg of pelagic Sargassum was collected in 92.6% of surface neuston tows, sorted and weighed by morphological form. Notably, the predominant Sargassum form observed during the 2014/15 event is S. natans-VIII, a documented change in Sargassum diversity. Strong spatial patterns were also observed, with S. natans-VIII dominant in the Western Tropical Atlantic (87.3% wet weight) and Eastern Caribbean (95.3% wet weight) and S. natans-I most common in the South Sargasso Sea (87.5% wet weight). S. fluitans-III was observed in low abundance across all regions. These sudden assemblage and abundance changes, the biophysical mechanisms of which are not yet understood, have significant ecological consequences at multiple scales. Impacts to associated mobile fauna diversity and community structure, dependent fisheries and iconic species, and coastal ecosystem function will echo throughout the Caribbean, and should comprise focal areas of future research efforts across the region.