Stranded seaweeds (Gongolaria barbata): an opportunity for macroalgal forest restoration

Macroalgal forests play a crucial ecological role, providing important ecosystem services, but are also among the most vulnerable marine habitats. In the Mediterranean Sea, the forests of Cystoseira sensu lato (s.l.) are undergoing a drastic decline due to the presence of multiple stressors, and among these species, Gongolaria barbata is one of the most threatened. Despite the various attempts to restore these macroalgal forests, the success of the interventions is limited by the availability of fertile apices to promote zygotes release and the embryo development for subsequent replacement in situ. Here, we propose a new approach based on the use of G. barbata stranded on the beach for the restoration of these marine habitats. We developed a protocol based on the collection of stranded macroalgae to collect their fertile apices and produce healthy zygotes and embryos, whose recruits can be then returned at sea (through ad hoc hard substrates). We show that 3 months of incubation in mesocosms allow the recruits to reach an average length of 1-2 mm with an average density of 50-80 recruits per tile. We demonstrate that these recruits can survive and grow vigorously both in mesocosms, and after being outplanted at sea. The protocol presented here has the double advantage: (1) obtaining recruits without impacting the natural populations and (2) providing a second life to macroalgal fragments (through ecological restoration) that would otherwise be lost.