Sargassum-associated mobile fauna communities in the Caribbean, Gulf of Mexico, and Sargasso Sea

Sargassum natans and S. fluitans are entirely pelagic, offering a pseudo-benthic structural habitat for an associated community of mobile fauna. In turn, the mobile fauna community supports foraging seabirds, fish, and turtles. Recent satellite observations suggest Sargassum in the Sargasso Sea is seeded annually from the Gulf of Mexico. Furthermore, the Caribbean is in the midst of a Sargassum inundation that appears disconnected in origin from the Sargasso Sea and Gulf of Mexico. Sargassum and fauna were collected via dip net during spring and summer 2015 from the Gulf of Mexico, Sargasso Sea and Eastern Caribbean to study the impacts that region, aggregation pattern (isolated clump, windrow, mat), and Sargassum variety morphology have on mobile fauna community composition. Sargassum from all three regions shared five common (frequency >10%) species: flatworm spp., Portunus sayi, Litiopa melanostoma, Leander tenuicornis, and Latreutes fucorum). The Gulf presented the most unique species (9 unique / 16 total) followed by the Sargasso Sea (5 unique / 12 total) and the Caribbean (1 unique / 6 total). The majority of species unique to the Gulf of Mexico were juvenile fish while those in the Caribbean and Sargasso Sea were benthic-like species residing on the Sargassum itself. Differences in the morphological forms of Sargassum had a marked effect on fauna diversity and abundance. In all three regions, fewer individuals and species were found on the broad-leafed, less compact S. natans VIII than on the denser S. natans I and S. fluitans III. This study identifies the differences in macrofauna abundance and diversity between varieties of Sargassum and highlights the potential for dramatic community changes that could result from largescale Sargassum blooms and species shifts. Any shift in these keystone communities could result in negative cascading effects on seabirds, economically important fish populations, and juvenile turtles which use the seaweed as a nursery habitat.