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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 (δ 13 C 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., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

The response of Posidonia oceanica meadows to global warming of the Eastern Mediterranean Sea, where the increase in sea surface temperature (SST) is particularly severe, is poorly investigated. Here, we reconstructed the long-term P. oceanica production in 60 meadows along the Greek Seas over two decades (1997-2018), using lepidochronology. We determined the effect of warming on production by reconstructing the annual and maximum (i.e. August) SST, considering the role of other production drivers related to water quality (i.e. Chla, suspended particulate matter, Secchi depth). Grand mean (±SE) production across all sites and the study period was 48 ± 1.1 mg DW per shoot yr -1 . Production over the last two decades followed a trajectory of decrease, which was related to the concurrent increase in annual SST and SST aug . Annual SST > 20°C and SST aug  > 26.5°C was related to production decline (GAMM, P < 0.05), while the rest of the tested factors did not help explain the production pattern. Our results indicate a persistent and increasing threat for Eastern Mediterranean meadows, drawing attention to management authorities, highlighting the necessity of reducing local impacts to enhance the resilience of seagrass meadows to global change threats., (© 2023 The Authors New Phytologist © 2023 New Phytologist Foundation.)