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- Van Dam, Bryce, et al.
(författare)
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Global Trends in Air-Water CO2 Exchange Over Seagrass Meadows Revealed by Atmospheric Eddy Covariance
- 2021
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Ingår i: Global Biogeochemical Cycles. - : American Geophysical Union (AGU). - 0886-6236 .- 1944-9224. ; 35:4
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Tidskriftsartikel (refereegranskat)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. 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. In this study, we synthesized all available direct FCO2 measurements 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 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 demonstrated that FCO2 was 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 CO2 exchange, and its complex biogeochemical and physical drivers.
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2. |
- Van Dam, Bryce R., et al.
(författare)
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Water temperature control on CO2 flux and evaporation over a subtropical seagrass meadow revealed by atmospheric eddy covariance
- 2021
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Ingår i: Limnology and Oceanography. - : John Wiley & Sons. - 0024-3590 .- 1939-5590. ; 66:2, s. 510-527
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Tidskriftsartikel (refereegranskat)abstract
- Subtropical seagrass meadows play a major role in the coastal carbon cycle, but the nature of air-water CO2 exchanges 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 CO2 and water (evaporation), emphasizing the need for direct measurements. We describe the first direct measurements of evaporation and CO2 flux over a calcifying seagrass meadow near Bob Allen Keys, Florida. Over the 78-d study, CO2 emissions were 36% greater during the day than at night, and the site was a net CO2 source to the atmosphere of 0.27 +/- 0.17 mu mol m(-2)s(-1)(x (c) over bar +/- standard deviation). A quarter (23%) of the diurnal variability in CO2 flux was caused by the effect of changing water temperature on gas solubility. Furthermore, evaporation rates were similar to 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 CO2 exchange 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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