Moving wrack from beaches to sand dunes: A sustainable beach-dune management practice?

The potential benefits of adding wrack (i.e., litter comprised of algae and seagrasses), removed from beaches during cleaning, on coastal dunes for improving their resilience have been recently explored. Yet, how this approach affects the establishment of native plant communities remains to be elucidated. This is of great relevance as plants contribute to dune formation and stabilization and are particularly vulnerable to environmental stresses during their early establishment phase. In this study, we evaluated the effects of placing a layer of Posidonia oceanica wrack of different thickness (2, 4 or 8 cm) and composition (wrack without or with sand) on a Mediterranean embryo dune on (i) the survival of already established seedlings and (ii) the seed recruitment and performance of new seedlings. As models, we selected three common dune plants, Thinopyrum junceum, Euphorbia paralias, and Cakile maritima. Changes in elemental composition of wrack and its leachate during decomposition were also assessed. For all species, the thickest wrack layer decreased the survival of already established seedlings by up to 70 %. Regardless of thickness and composition, wrack reduced seed recruitment success. The thickest layer reduced the growth of T. junceum and E. paralias seedlings while increased that of C. maritima ones. Wrack released carbon and nutrients thorough leaching, and the release efficiency was the highest for phosphorus. These results indicate that covering dunes with wrack layers even few cm thick can impair their resilience by hindering plant colonization. They also pose doubts about the ecological sustainability of moving beachcast wrack to dunes.