Hitchhiking in the East Australian Current: rafting as a dispersal mechanism for harmful epibenthic dinoflagellates

Due to their small size, planktonic marine microorganisms have large dispersal capacity in the global ocean. However, it is not known how epibenthic microalgae dispeise across long distances because they are generally associated with a substrate. In this study, we examined a long-term data series (similar to 50 yr) of microalgal composition from a coastal station in southeast Australia foi the presence of epibenthic dinoflagellates in the plankton. In addition, we collected drifting macrophytes (i.e. macroalgae and seagrass) and plastic debris from the East Australian Current, identified the associated microalgal assemblage, assessed their viability, and used phylogenetic analyses to taxonomically identify cryptic harmful epibenthic dinoflagellate species. We found no occurrences of epibenthic dinoflagellates from the genera Gambierdiscus, Fukuyoa, Ostreopsis, and Coolia at the long-term coastal station, concluding that entrainment of cells in ocean currents is an unlikely mechanism for transport of these taxa. The epibenthic microalgal communities associated with macrophyte rafts and plastic debris were primarily comprised of diatom taxa. However, intact cells of potentially harmful epibenthic dinoflagellates from the genera Coolia, Amphidinium, and Prorocentrum were also observed, and their viability was confirmed by division of isolated cells and establishment into clonal cultures. Phylogenetic analyses confirmed the presence of C. pahnyrensis on a drifting Sargassum sp. raft, the first report of this potentially harmful epibenthic species in temperate Australian waters. This study shows that epibenthic dinoflagellates can attach to, and remain viable, when associated with macrophyte fragments that drift in the open ocean, therefore revealing rafting as a potential vector for dispersal of these organisms.