Carbon accumulation, storage and provenance in the Portuguese continental shelf

The field of Blue Carbon research has traditionally focused on the carbon sequestration capacity of coastal vegetated habitats, despite these habitats comprising only a small fraction of oceanic sediment. However, continental shelf sediments also play a significant role in carbon sequestration and represent a significantly larger surface area. While the majority of organic carbon deposited in the shelf sediment is initially fixated by phytoplankton, and then potentially cycled through other marine organisms, some of it is originated from coastal and terrestrial producers, such as marine macroalgae, then transported, deposited and sequestered into shelf sedimentary basins. In this study, we investigated the sedimentary organic carbon (OC) stocks and sequestration rates at two sites of the continental shelf of Portugal, each adjacent to major wetland systems dominated by saltmash and seagrasses: the northern site is located off the Sado estuary at the Arrábida coast where macroalgae forests are also present, and the southern site off the Ria Formosa coastal lagoon. We also assessed the contributions of marine and terrestrial primary producers to sedimentary OC using various proxies such as C/N ratios, carbon isotopic signature (δ13 C), magnetic susceptibility, lipid contents and sedimentary DNA metabarcoding. Our findings revealed similar OC sequestration rates at both sites (23.3 ± 7.1 g OC m⁻² yr⁻¹ and 20.9 ± 5.7 g OC m⁻² yr⁻¹ for the northern and southern sites, respectively) and comparable OC stocks in the top 25 cm of sediment (29.5 ± 2.33 g OC cm⁻² and 21.1 ± 3.01 g OC cm⁻², respectively). Clear differences were observed on the contributions of terrestrial versus marine sources to the sediment organic matter, with the northern site showing lower terrestrial contribution as opposed to the southern site. This conclusion is supported by the different proxies used. For example, the northern site consistently exhibited higher OC contents at comparable particle sizes, indicative of a greater deposition rate of organic carbon not adhered to sediment particles, typical of oceanic primary productivity. Sedimentary DNA metabarcoding detected seagrass and saltmarsh genetic material in sedimentary organic matter from both sites, indicating that detritus from the two wetlands are being exported to the continental shelf. Further investigation is needed to quantify the relative magnitude of this export. Understanding this process is essential to accurately assess the role of coastal vegetated habitats in the global carbon cycle, as current estimates focus solely on in-situ sequestration and often overlook the potential contribution of exported organic matter. Our study highlights the need to expand our perspective on the interconnectedness of coastal and oceanic carbon dynamics.