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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.

Seagrass meadows play an important role in “blue carbon” sequestration and storage, but their dynamic metabolism is not fully understood. In a denseZostera marinameadow, we measured benthic O₂fluxes by aquatic eddy covariance, water column concentrations of O₂, and partial pressures of CO₂(pCO₂) over 21 full days during peak growing season in April and June. Seagrass metabolism, derived from the O₂flux, varied markedly between the 2 months as biomass accumulated and water temperature increased from 16°C to 28°C, triggering a twofold increase in respiration and a trophic shift of the seagrass meadow from being a carbon sink to a carbon source. Seagrass metabolism was the major driver of diurnal fluctuations in water column O₂concentration and pCO₂, ranging from 173 to 377 μmol L⁻¹and 193 to 859 ppmv, respectively. This 4.5‐fold variation in pCO₂was observed despite buffering by the carbonate system. Hysteresis in diurnal water column pCO₂vs. O₂concentration was attributed to storage of O₂and CO₂in seagrass tissue, air–water exchange of O₂and CO₂, and CO₂storage in surface sediment. There was a ~ 1:1 mol‐to‐mol stoichiometric relationship between diurnal fluctuations in concentrations of O₂and dissolved inorganic carbon. Our measurements showed no stimulation of photosynthesis at high CO₂and low O₂concentrations, even though CO₂reached levels used in IPCC ocean acidification scenarios. This field study does not support the notion that seagrass meadows may be “winners” in future oceans with elevated CO₂concentrations and more frequent temperature extremes.

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.