Societal Impact Statement: Seagrass ecosystems are of fundamental importance to our planet and wellbeing. Seagrasses are marine flowering plants, which engineer ecosystems that provide a multitude of ecosystem services, for example, blue foods and carbon sequestration. Seagrass ecosystems have largely been degraded across much of their global range. There is now increasing interest in the conservation and restoration of these systems, particularly in the context of the climate emergency and the biodiversity crisis. The collation of 100 questions from experts across Europe could, if answered, improve our ability to conserve and restore these systems by facilitating a fundamental shift in the success of such work.Summary: Seagrass meadows provide numerous ecosystem services including biodiversity, coastal protection, and carbon sequestration. In Europe, seagrasses can be found in shallow sheltered waters along coastlines, in estuaries & lagoons, and around islands, but their distribution has declined. Factors such as poor water quality, coastal modification, mechanical damage, overfishing, land‐sea interactions, climate change and disease have reduced the coverage of Europe’s seagrasses necessitating their recovery. Research, monitoring and conservation efforts on seagrass ecosystems in Europe are mostly uncoordinated and biased towards certain species and regions, resulting in inadequate delivery of critical information for their management. Here, we aim to identify the 100 priority questions, that if addressed would strongly advance seagrass monitoring, research and conservation in Europe. Using a Delphi method, researchers, practitioners, and policymakers with seagrass experience from across Europe and with diverse seagrass expertise participated in the process that involved the formulation of research questions, a voting process and an online workshop to identify the final list of the 100 questions. The final list of questions covers areas across nine themes: Biodiversity & Ecology; Ecosystem services; Blue carbon; Fishery support; Drivers, Threats, Resilience & Response; Monitoring & Assessment; Conservation & Restoration; Governance, Policy & Management; and Communication. Answering these questions will fill current knowledge gaps and place European seagrass onto a positive trajectory of recovery.

Combined use of optical (satellite imagery) and acoustic (sidescan sonar) remote sensing techniques, as well as in situ methodologies (visual census; SCUBA diving, Towed Underwater Cameras, and Remotely Operated Vehicles) was employed to map the spatial distribution of seagrass habitats in the coastal waters of the Greek territory. Seagrass meadows were recorded over approximately 70% of the Greek coastline (Eastern Ionian, Aegean and Levantine Seas), and their surface area exceeded 2749.07 km ² . Posidonia oceanica is clearly the dominant seagrass species in Greek Seas, covering the vast majority of seabed at depths between the shoreline and 25–30 m (or deeper in insular areas), followed by the species Cymodocea nodosa , Zostera noltei , and Halophila stipulacea , that are not widely distributed but are locally abundant in specific areas. Habitat suitability in terms of the of the seabed that is available for the growth of seagrass meadows (i.e., the spatial extent of coastal areas between the shoreline and the isobath of 20 m) and the seawater clarity conditions are highlighted as the critical factors for the formation of well-structured and extensive meadows. The results of this study are of great importance and usefulness for the effective management and conservation of valuable marine ecosystems and are given in the form of an openly accessible shapefile.

Non-indigenous species (NIS) are among the main drivers of native biodiversity loss, habitat alteration, and degradation of ecosystem services, with some NIS posing risks to human health. Efficient monitoring strategies are necessary to assess the distribution and impacts of NIS. In this study, we compared the performance of environmental DNA (eDNA) metabarcoding and visual surveys with SCUBA diving to detect marine fish NIS in Greek waters. We collected water samples from 20 coastal sites across the Aegean, Ionian, and Levantine Seas in both warm and cold periods, targeting the 12S rRNA region. A reference 12S Mediterranean NIS sequence database was created to improve regional monitoring. Underwater visual surveys were performed at the same sites to visually detect fish NIS. Overall, 15 non-indigenous fishes were detected, five with both eDNA and visual surveys, seven exclusively by eDNA, and three by visual surveys alone. The southern stations yielded more NIS detections than the northern stations in both periods. Our findings demonstrate that eDNA can provide a rapid, low-cost, and effective tool, advocating for its integration into systematic NIS monitoring in the eastern Mediterranean Sea. A comprehensive barcode reference database is essential in enhancing the effectiveness of eDNA approaches. Thus, the combination of eDNA metabarcoding and traditional underwater visual surveys is recommended for comprehensive monitoring of NIS in marine environments.