Coastal “blue carbon” (BC) habitats like seagrass meadows can mitigate anthropogenic carbon emissions by sequestering CO₂ in their sediments. However, air-water greenhouse gas exchange, especially CO₂ fluxes can alter net carbon sequestration, complicating BC mitigation. A complex suite of biogeochemical and physical drivers challenge the prediction of CO₂ fluxes, presenting a significant challenge especially for BC monitoring, reporting, and verification. I first present a case study in a subtropical seagrass meadow, examining the "bottom-up" biogeochemical forcing and "top-down" physical forcing, which together cause air-water CO₂ exchanges to diverge from what might be expected. Eddy covariance measurements reveal net CO₂ emissions of 700 ± 660 µmol m⁻² hour⁻¹ (6.1 mol m⁻² year⁻¹) over an annual cycle, with CO₂ flux greater during the day than at night. Net alkalinity consumption by ecosystem calcification explains >95% of the observed CO₂ emissions, far exceeding organic carbon burial and anaerobic alkalinity generation. Water temperature was an important driver, with solubilty explaining ~23% of the diurnal CO₂ flux excursion, including the observed night-time convective enhancement of gas transfer rates. These results are placed into the context of a global synthesis of direct CO₂ flux measurements over seagrass meadow, in which we found that CO₂ fluxes are large, relative to carbon burial. Factors like light availability, tidal mixing and temperature were key drivers of global variability. Taken together, we argue that efforts to monitor, report and verify BC sequestration projects will be incomplete if they do not include assessments of air-water CO₂ (and other GHG) exchange. 

Primary Presenter: Bryce Van Dam, Helmholtz-zentrum Hereon (Bryce.Dam@hereon.de)

Authors:

Bryce Van Dam, Helmholtz-zentrum Hereon  (Bryce.Dam@hereon.de)

Mary Zeller, MARUM  (maryazeller@gmail.com)

Christian Lopes, Florida International University  (clope130@fiu.edu)

Ashley Smyth, University of Florida  (ashley.smyth@ufl.edu)

Michael Böttcher, Institute for Baltic Sea Research Warnemuënde  (michael.boettcher@io-warnemuende.de)

Christopher Osburn, North Carolina State University  (closburn@ncsu.edu)

Tristan Zimmermann, Helmholtz-zentrum Hereon  (tristan.zimmermann@hereon.de)

Daniel Proefrock, Helmholtz-zentrum Hereon  (daniel.proefrock@hereon.de)

James Fourqurean, Florida International University  (fourqure@fiu.edu)

Helmuth Thomas, Helmholtz-zentrum Hereon  (Helmuth.Thomas@hereon.de)

Pierre Polsenaere, Ifremer, La Tremblade, France  ()

Aylin Barreras-Apodaca, Instituto Tecnológico de Sonora, Ciudad Obregón, México  ()

Zulia Sanchez-Mejia, Instituto Tecnológico de Sonora, Ciudad Obregón, México  ()

Tatsuki Tokoro, Port and Airport Research Institute, Yokosuka, Japan  ()

Tomohiro Kuwae, National Institute for Environmental Studies, Tsukuba, Japan  ()

Lucia Gutiérrez Loza7 Lucia Gutiérrez Loza, Department of Earth Sciences, Uppsala University, Uppsala, Sweden  ()

Anna Rutgersson, Department of Earth Sciences, Uppsala University, Uppsala, Sweden  ()