Global trends in air-water CO2 exchange over seagrass meadows revealed by atmospheric Eddy Covariance

Coastal vegetated habitats like seagrass meadows can mitigate anthropogenic carbon emissions by sequestering CO₂ as “blue carbon” (BC). Already, some coastal ecosystems are actively managed to enhance BC storage, with associated BC stocks included in national greenhouse gas inventories. However, the extent to which BC burial fluxes are enhanced or counteracted by other carbon fluxes, especially air-water CO₂ flux (FCO₂) remains poorly understood. To this end, we synthesized all available direct FCO₂ measurements over seagrass meadows made using a common method (atmospheric Eddy Covariance), across a globally-representative range of ecotypes. Of the four sites with seasonal data coverage, two were net CO₂ sources, with average FCO₂ equivalent to 44 - 115% of the global average BC burial rate. At the remaining sites, net CO₂ uptake was 101 - 888% of average BC burial. A wavelet coherence analysis demonstrates that FCO₂ was most strongly related to physical factors like temperature, wind, and tides. In particular, tidal forcing appears to shape global-scale patterns in FCO₂, likely due to a complex suite of drivers including: lateral carbon exchange, bottom-driven turbulence, and pore-water pumping. Lastly, sea-surface drag coefficients were always greater than prediction for the open ocean, supporting a universal enhancement of gas-transfer in shallow coastal waters. Our study points to the need for a more comprehensive approach to BC assessments, considering not only organic carbon storage, but also air-water CO₂ exchange, and its complex biogeochemical and physical drivers.