Benthic community responses to Ostreopsis spp. blooms along the French Mediterranean coast

Blooms of the toxic microalgae Ostreopsis cf. ovata occur annually in the Mediterranean French Riviera, forming mucilaginous aggregates on shallow rocky shores. This benthic dinoflagellate produces toxins of the palytoxin group (PLTX), and its high biomass proliferation can cause skin irritation and respiratory distress in humans, contaminate seafood, and lead to animal mortality events. This study investigates the relationship between the abundance of Ostreopsis spp. and invertebrate community composition among coastal habitats dominated by different marine macrophytes, including turf-forming and erect macroalgal beds and Posidonia oceanica seagrass meadows. During summer 2024, sampling of invertebrates and Ostreopsis spp. cells was conducted at three intervals in each of four sites along the French Riviera. The abundance of Ostreopsis spp. was quantified by deploying artificial substrates overnight in the water column overlying the erect macroalgal beds, and by counting the colonized cells. Mobile invertebrates were collected with both benthic cores and light traps in macroalgal habitats, and with light traps only in Posidonia oceanica. The invertebrate assemblages showed temporal changes in relation to the increase of Ostreopsis spp. cells across all habitats studied. Notably, the amphipods Ampithoe sp. and Elasmopus rapax, as well as the mysid Siriella sp. increased in abundance, while 3 other taxa decreased (the amphipod Caprella sp., the polychaete Polyophtalmus pictus and the tanaid Chondrochelia savignii). Overall, these results indicate that some benthic invertebrate species appear sensitive to fluctuations in Ostreopsis spp. abundance. Changes in benthic community composition could further have an impact on PLTX transfer through the food web and affect human health.