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Abstract Subtropical seagrass meadows play a major role in the coastal carbon cycle, but the nature of air–water CO2exchanges over these ecosystems is still poorly understood. The complex physical forcing of air–water exchange in coastal waters challenges our ability to quantify bulk exchanges of CO2and water (evaporation), emphasizing the need for direct measurements. We describe the first direct measurements of evaporation and CO2flux over a calcifying seagrass meadow near Bob Allen Keys, Florida. Over the 78‐d study, CO2emissions were 36% greater during the day than at night, and the site was a net CO2source to the atmosphere of 0.27 ± 0.17 μmol m−2s−1(x̅ ± standard deviation). A quarter (23%) of the diurnal variability in CO2flux was caused by the effect of changing water temperature on gas solubility. Furthermore, evaporation rates were ~ 10 times greater than precipitation, causing a 14% increase in salinity, a potential precursor of seagrass die‐offs. Evaporation rates were not correlated with solar radiation, but instead with air–water temperature gradient and wind shear. We also confirm the role of convective forcing on night‐time enhancement and day‐time suppression of gas transfer. At this site, temperature trends are regulated by solar heating, combined with shallow water depth and relatively consistent air temperature. Our findings indicate that evaporation and air–water CO2exchange over shallow, tropical, and subtropical seagrass ecosystems may be fundamentally different than in submerged vegetated environments elsewhere, in part due to the complex physical forcing of coastal air–sea gas transfer.
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Global Trends in Air‐Water CO 2 Exchange Over Seagrass Meadows Revealed by Atmospheric Eddy Covariance
Abstract Coastal vegetated habitats like seagrass meadows can mitigate anthropogenic carbon emissions by sequestering CO2as “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 CO2flux (FCO2) remains poorly understood. In this study, we synthesized all available direct FCO2measurements over seagrass meadows made using atmospheric Eddy Covariance, across a globally representative range of ecotypes. Of the four sites with seasonal data coverage, two were net CO2sources, with average FCO2equivalent to 44%–115% of the global average BC burial rate. At the remaining sites, net CO2uptake was 101%–888% of average BC burial. A wavelet coherence analysis demonstrated that FCO2was most strongly related to physical factors like temperature, wind, and tides. In particular, tidal forcing was a key driver of global‐scale patterns in FCO2, likely due to a combination of lateral carbon exchange, bottom‐driven turbulence, and pore‐water pumping. Lastly, sea‐surface drag coefficients were always greater than the 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 CO2exchange, and its complex biogeochemical and physical drivers.
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- PAR ID:
- 10449950
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Global Biogeochemical Cycles
- Volume:
- 35
- Issue:
- 4
- ISSN:
- 0886-6236
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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