Factors influencing the recruitment of Gongolaria barbata (Fucales) in the Conero Riviera: implications for conservation and restoration

Gongolaria barbata is a brown macroalga in the Fucales. This species is widely distributed in the Mediterranean Sea, where it can form dense canopies on hard substrata in the infralittoral zone. Gongolaria barbata plays a pivotal role as habitat former and ecosystem engineer, but in recent decades its distribution has declined, mostly due to anthropogenic stressors. Protection under international conventions and several active restoration projects, in the majority of cases, failed to achieve their desired outcomes. We aimed to investigate factors potentially responsible for the failure of these conservation and restoration efforts. We tested two hypotheses: 1) young specimens are sensitive to toxins and/or mucilage produced by blooms of the dinoflagellate Ostreopsis cf. ovata; 2) fast growing, turf forming macroalgae, such as Ulva lacinulata, outcompete G. barbata propagules in settlement. We assessed the condition of G. barbata’s population at an urban beach of the Conero Riviera, Adriatic Sea, Italy. This population appears fertile and reproductive but fails to recruit new individuals. The first hypothesis was tested in the laboratory, by exposing G. barbata germlings to O. cf. ovata. We observed no significant difference in growth among treatments and controls. The second hypothesis was tested in the field, clearing part of the seabed around reproductive G. barbata. Initially, scraped quadrats hosted a significantly higher number of G. barbata recruits, compared to untreated quadrats. However, no recruit survived to adulthood. We attribute this failure to factors besides turf algae, possibly acting synergistically, with high sedimentation rates, human trampling, grazing and heatwaves. This work, adding to others performed in the study area, identifies three main bottlenecks for conservation and restoration of G. establish and global and local stressors, mean that the remnant G. barbata population faces imminent extinction.