Resilience and stability of Posidonia oceanica (L.) Delile to global warming and human pressures

The growing climate crisis requires a greater understanding of the response of ecosystems to rapid environmental change. Extreme climatic events, such as marine heatwaves, affect the structure, functioning, and stability of marine ecosystems. In the Mediterranean Sea, the seagrass Posidonia oceanica (L.) Delile is a foundation species that provides key ecosystem services, including primary production, carbon sequestration, habitat provision, and coastal protection. However, P. oceanica meadows have declined by about 34% over the last 50 years, due to climatic and anthropogenic pressures. Previous studies examined the effect of marine heatwaves (MHWs) and increasing sea surface temperatures (SSTs) on the growth, physiology, and biochemical composition of P. oceanica meadows. However, the long-term effects of climatic stressors on the growth and stability of P. oceanica, together with the role of local anthropogenic pressure, remain poorly understood. At the same time, restoration initiatives are increasingly adopted to counteract seagrass decline. Most interventions rely on “cutting transplantation”, yet little is known about the structural effects of rhizome removal on donor meadows and the recovery process of the transplanted fragments in new environments. My PhD project focuses on restoration strategies to conserve P. oceanica meadows, with particular attention to “cutting transplantation” techniques and the potential effects of mechanical extraction of rhizomes in the donor meadow. In addition, it aims to assess the effects of climatic and anthropogenic stressors on the growth, physiology, and stability of P. oceanica meadows at different organisational levels, from individuals to populations and up to metapopulations in the Mediterranean basin. In the second chapter, I experimentally assessed the effects of different rhizome-harvesting intensities and spatial positions (margin vs. inside) on the resistance and recovery capacity of P. oceanica donor meadows. This chapter provides insights into how rhizome harvesting can induce measurable structural degradation in donor meadows, with implications for the sustainability of transplantation practices and for the management of source sites used in restoration interventions. Chapter 3 combines a demonstrative transplantation intervention on dead matte within an area currently under full protection with a transplantation experiment to identify possible bottlenecks in reforestation interventions, such as the physiological stress imposed on the transplanted fragments during transportation and the characteristics of the origin and destination sites. These results clarify the factors shaping the early stages of establishment and help define the conditions that determine the initial success of transplantation. In chapter 4, I used a hierarchical sampling design to assess the effects of MHWs and anthropogenic pressure on production, stability, and spatial synchrony of P. oceanica and how such metrics change across multiple spatial scales (from a few meters to hundreds of kilometers) in the Mediterranean basin. These findings suggest that MHW exposure, seasonal thermal conditions, and local environmental and anthropogenic pressures may leave distinct signals on production, physiological functioning, and local variability, offering a forward-looking perspective on the functioning of this key Mediterranean primary producer. Overall, this thesis provides new insights into the ecological responses of P. oceanica meadows to climatic stress and human disturbance and offers an empirical basis to improve restoration practices in the Mediterranean Sea. From a practical perspective, when direct harvesting from donor meadows is necessary, rhizome collection should preferentially target the meadow interior rather than the margin, which appears more vulnerable to mechanical disturbance. Restoration planning should also consider that donor-origin effects may be more relevant during early establishment, whereas destination-site conditions become important for subsequent transplant performance, emphasizing the need to select receiving sites with favourable environmental conditions. Handling stress should be minimized by limiting unnecessary exposure of cuttings out of the water.