Discovering a beach “cemetery” of a seagrass Posidonia oceanica barrier reef: search for clues to reconstruct its origins

Barrier reefs are peculiar structures of high ecological importance formed by the Mediterranean seagrass Posidonia oceanica in very shallow areas through the accretion of biogenic matte. This study reports for the first time the multiple discovery of several matte blocks of P. oceanica stranded on the southern coast of Sicily (central Mediterranean) in front of a barrier reef. Unmanned Aerial Vehicle (UAV) mapping of the barrier reef and morphological analysis of the blocks made it possible to hypothesise that the original substrate of the stranded blocks was rocky, and that the colonisation of this substrate may have been facilitated by boring bivalve molluscs through a so far unexplored plant-animal interaction. Furthermore, the indirect estimation of the theoretical bathymetric origin of the blocks through a shoot density depth model, combined with the retrospective analysis of the time series of the wave regime prior to stranding, suggests that a severe storm caused this collective uprooting and stranding event from very shallow depths, raising new questions about the vulnerability of these important ecosystems to the increase in frequency and intensity of storms expected in the climate change scenarios.