Global trends in air-water CO2 exchange over seagrass meadows revealed
by atmospheric Eddy Covariance
Abstract
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 or traded on international markets.
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.