Epibiont Community Compositions on Floating Turbinaria Rafts, Novel Ecosystems in an Increasingly Stormy Anthropocene

The loss of herbivorous fish and coastal eutrophication are driving shifts from coral to algal dominated reefs. The macroalga Turbinaria ornata, in particular, is greatly expanding habitat usage in the French Polynesia and the South Pacific, achieving densities so abnormally high that, when dislodged by storms, they create large floating rafts of aggregating algae. Although algal rafts are a common and important ecosystem in regions like the Sargasso Sea, T. ornata rafts are an entirely new tropical ecosystem observed the last few decades and do not have the ecological and evolutionary history that Sargassum rafts possess. T. ornata rafts form in shallow lagoons before transiting to open ocean environments, and are covered in epibionts, potentially increasing connectivity among reef ecosystems. Epibionts can include an array of epiphytes, invertebrates, and other microscopic organisms. Of particular concern are ciguatoxic dinoflagellates (e.g. Gambierdiscus and Ostreopsis) and coral pathogens. With climate change increasing the intensity and frequency of storms, T. ornata rafts will likely become a common feature of South Pacific reef ecosystems, but their impact on these ecosystems remains unknown. As such, it is essential to understand how these novel algal ecosystems may facilitate the dispersal of harmful epibionts.

This temporal study documents how the epibiont community on T. ornata rafts may change over time and compares these communities to attached T. ornata that plague the reefs of the South Pacific, providing critical insights into the potential of algal rafts of facilitate novel ecological connectivity among tropical reef ecosystems. I simulated T. ornata rafts by collecting T. ornata attached to the benthos to create rafts in mesocosms (1.2 m diameter, 1 m tall) with continuous flow of unfiltered sea water. I employed DNA metabarcoding to examine community composition and monitor epibiont changes across the timepoints that I sampled in three-day intervals over 30 days. For DNA metabarcoding, I used a suite of primers to target multiple groups: D1R/305R for dinoflagellates; 16S for microbes; 18S for protists; and, CO1 for metazoans. Preliminary investigations using microscopy revealed ciguatoxic dinoflagellate densities change over time on T. ornata rafts, but in ways that are not clear. For example, Ostreopsis density sharply declines over time while the highly toxic Gambierdiscus increased in density as well as variation in its density. Additionally, metabarcoding preliminary results regarding the microbial community indicate significant differences in alpha (Kruskal−Wallis: p = 0.0197) and beta diversity (PERMANOVA: p = 0.02) between microbes on attached and rafting T. ornata. Given the proliferation of T. ornata and the floating habitats they create, it is critical to better understand the composition and temporal dynamics of their associated epibiont communities to elucidate how this novel dispersal pathway may facilitate transport of epibionts among coral reef ecosystems, with unknown ecological consequences.