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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 dense Zostera 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. Carbon storage is a valuable ecosystem service of seagrass meadows, serving as a possible pathway to draw down atmospheric carbon dioxide (CO2) levels. However, this approach may be unsuccessful if carbon storage in sediments is exceeded by the release of CO2from the water. To better understand the scope of this problem, we compiled all available measurements of air‐water CO2exchange over seagrass meadows. We found that rates of CO2release or uptake were indeed large, even when compared with potential rates of carbon storage in seagrass soils. However, these large air‐water exchanges of CO2did not occur for the same reason everywhere. While light availability was sometimes a strong predictor of air‐water CO2exchange, tidal mixing and temperature were also very important, revealing a much more complex network of drivers than previously thought. Despite these diverse conditions, we found one key similarity across all sites, in that rates of air‐water gas transfer appear to always be greater than would be expected for the open ocean. Taken together, the results of our study show that assessments of carbon storage in coastal seagrass ecosystems will be incomplete if they do not consider exchanges of CO2between the water and air. Direct measurements show that air‐water CO2exchange over seagrass meadows is of similar magnitude to carbon burial ratesKey drivers are tides, temperature, light, and wind, which off in importance over hourly seasonal time scalesSurface drag coefficients were greater than open water prediction, suggesting a near‐universal gas transfer enhancement across all sites Direct measurements show that air‐water CO2exchange over seagrass meadows is of similar magnitude to carbon burial rates Key drivers are tides, temperature, light, and wind, which off in importance over hourly seasonal time scales Surface drag coefficients were greater than open water prediction, suggesting a near‐universal gas transfer enhancement across all sites