Seasonal greenhouse gas flux limits the blue carbon potential of intertidal seagrass

<title>Abstract</title> <p> Understanding of seasonal greenhouse gas (GHG) dynamics and their contribution to coastal blue carbon budgets is limited, particularly for northern temperate regions and seagrass ecosystems. Blue carbon (BC) frameworks often focus on carbon stocks and accumulation rates, omitting GHG fluxes which lead to incomplete carbon budgets. We provide the first GHG flux (CO ₂ /CH ₄ ) measurements for intertidal <italic>Zostera noltii</italic> seagrass across an annual cycle, coupled with methanogen community composition, and compared against a synthesis of global GHG values. Emersed <italic>Z. noltii</italic> daytime CO ₂ uptake was 200-fold greater than adjacent unvegetated bare sediments, though lower than previously estimated for this species and seasonal differences were not observed. CH ₄ emissions were small but significantly enhanced in seagrass meadows compared to bare sediments, and elevated during warmer months, likely driven by the dominant methanogenic archaea <italic>Methanomassilicoccus spp</italic> . Our data show including seasonal and species-specific GHG fluxes is essential to avoid overestimations of seagrass BC services. </p>