A baseline assessment of the epiphytic community associated with pelagic sargassum in the Tropical Atlantic

Pelagic sargassum, historically endemic to the North Atlantic where it accumulated in the Sargasso Sea, has now expanded to other accumulation regions namely in the North Equatorial Recirculation Region (NERR) of the Tropical Atlantic. To date, efforts to understand the biological community associated with pelagic sargassum have largely focussed on the motile organisms with very little attention given to the epiphytic flora and fauna, particularly for sargassum from the NERR. This study: (1) assesses the species composition and abundance (percentage cover) of epiphytes associated with the three prevalent forms (Sargassum fluitans III, S. natans I, S. natans VIII) of pelagic sargassum arriving in Barbados over summer and winter months and; (2) uses a backtracking algorithm to determine if differences in the composition of epiphyte species are linked to putative sub-origins within the NERR. Overall, epiphyte diversity was relatively low, comprising just six species of fauna, and an unidentified filamentous alga. However, there were significant differences in the epiphytic community associated with the three prevalent morphotypes. Based on percent cover, the bryozoan, Membranipora tuberculata, was the dominant species observed on S. fluitans III and S. natans I; while S. natans VIII was predominantly occupied by the hydroid, Aglaophenia latecarinata. This research provides the first quantification of the epiphytic community associated with pelagic sargassum in the Eastern Caribbean as it transitions from open Tropical Atlantic Ocean into the Caribbean Sea and will provide a useful baseline for monitoring potential change over time. The results corroborate with previous studies from elsewhere that suggest distinct differences in the epiphytic communities hosted by different sargassum morphotypes. We found no evidence that differences in epiphytic community composition were linked to sub-origin within the NERR, indicating that sargassum morphology is likely a stronger determinant of epiphyte composition than any environmental differences encountered between sub-origins and their associated transport pathways.