Developing the First National Blue Carbon Inventory for the Isle of Man

Temperate coastal ecosystems including seagrass, saltmarsh, and shelf-sea sediments are natural, long-term ‘blue carbon’ (BC) sinks, with the potential to be managed for carbon storage and sequestration. These BC hotspots could help offset unavoidable greenhouse gas emissions and contribute to nations' Net Zero ambitions. The Isle of Man, a self-governing British Crown Dependency situated in the Irish Sea, has territorial waters equivalent to approximately 85% of its total jurisdiction. The island's Government is actively developing a comprehensive BC management plan aimed at maximising carbon sequestration and restoring seabed biodiversity and ecosystem services. To inform the management plan, sediment cores were collected from three major BC habitats around the Isle of Man: seagrass, saltmarsh, and shelf-sea sediments. The cores were analysed using elemental analysis and isotope ratio mass spectrometry to quantify organic and inorganic carbon stores. Radioanalytical methods were employed to measure radionuclides (137Cs, 210Pb, 241Am, and 210Po), which were used to determine sedimentation rates and subsequently carbon accumulation rates. Complementary analyses, including grain size analysis, X-ray fluorescence, X-ray scans, and high-resolution imagery, provide a holistic understanding of the chemical, physical, and biological attributes of the sedimentary cores, illuminating the processes influencing BC storage. Furthermore, side scan sonar, drop-down video, and drone imagery have been used to assess the extent of existing seagrass meadows, which is central to informing spatial-management strategies, particularly in the establishment of seagrass conservation zones. Our findings will help to develop the first national BC inventory for the Isle of Man and set a precedent for co-designed, collaborative, evidence-informed approaches for the sustainable management of coastal ecosystems.