Role of macroalgal forests within Mediterranean shallow bays in blue carbon storage

Although seaweeds rank among the most productive vegetated habitats globally, their inclusion within Blue Carbon frameworks is at its onset, partially because they usually grow in rocky substrates and their organic carbon (C_org) is mostly exported and stored beyond their habitat and thus, demonstrating its long-term storage is challenging. Here, we studied the sedimentary C_org storage in macroalgal forests dominated by Gongolaria barbata and in adjacent seagrass Cymodocea nodosa mixed with Caulerpa prolifera algae meadows, and bare sand habitats in Mediterranean shallow coastal embayments. We characterized the biogeochemistry of top 30 cm sedimentary deposits, including sediment grain-size, organic matter and C_org contents, C_org burial rates and the provenance of sedimentary C_org throughout stable carbon isotopes (δ¹³C_org) and pyrolysis analyses. Sediment C_org stocks and burial rates (since 1950) in G. barbata forests (mean ± SE, 3.5 ± 0.2 kg C_org m^-2 accumulated at 15.5 ± 1.6 g C_org m^-2 y^-1) fall within the range of those reported for traditional Blue Carbon Ecosystems. Although the main species contributing to sedimentary C_org stocks in all vegetated habitats examined was C. nodosa (36 ± 2 %), macroalgae contributed 49 % (19 ± 2 % by G. barbata and 30 ± 3 % by C. prolifera) based on isotope mixing model results. Analytical pyrolysis confirmed the presence of macroalgae-derived compounds in the sediments, including N-compounds and α-tocopherol linked to G. barbata and C. prolifera, respectively. The sedimentary C_org burial rate linked to macroalgae within the macroalgal forests examined ranged from 5.4 to 9.5 g C_org m^-2 y^-1 (7.4 ± 2 g C_org m^-2 y^-1). This study provides empirical evidence for the long-term (∼70 years) sequestration of macroalgae-derived C_org within and beyond seaweed forests in Mediterranean shallow coastal embayments and thereby, supports the inclusion of macroalgae in Blue Carbon frameworks.