Carbon dynamics at the mudflat-marsh edge of a temperate tidal bay

Vegetated coastal ecosystems are still under represented in regional/global carbon cycling and flux studies. Despite their small surface area, they represent a significant part in annual global ocean productivity (mudflats, up to 20%), carbon stored on Earth and in air-sea CO₂ fluxes (wetlands, 17%). In the context of rising greenhouse gas emissions, climate change and mitigation, it is paramount to increase integrative and multidisciplinary approaches to better understand processes, flux dynamics and controlling factors at terrestrial-aquatic-atmospheric exchange interfaces at the various spatiotemporal scales. In this objective and the framework of the La Rochelle Zero Carbon Territory project (LRTZC 2019-27), the carbon dynamic was studied within a temperate intertidal bay (Aiguillon, French Atlantic coast) at its mudflat - salt marsh edge, colonized by microphytobenthic communities and halophytic plants respectively, and likely to influence associated carbon processes and fluxes. An atmospheric Eddy Covariance station was deployed with simultaneous seasonal in situ and satellite measurements within the different compartments (air, soil/sediment, water) and associated interfaces to temporally and spatially characterize atmospheric CO₂ exchanges and controlling factors. Our first results showed mean CO₂ sink values higher in spring than winter (-2.42±4.21 and -0.73±1.83 µmol m⁻² s⁻¹), at low than high tides (-1.26±3.14 and -0.49±2.45 µmol m⁻² s⁻¹) and from mudflats than marsh habitats (-1.47±2.79 and -0.98±3.56 µmol m⁻² s⁻¹). Photosynthetically active radiation, air temperature as well as wind direction significantly controlled CO₂ fluxes through photosynthesis and respiration activities of involved habitats. At the small-time scales (diurnal, tidal), the tide clearly influenced carbon processes and fluxes at the studied site, though 90% of the time emerged. However, at larger time scales (seasonal, annual), observed incoming CO₂ oversaturated coastal waters did not question the overall CO₂ sinks ensured by the mudflat – marsh system. These ongoing LRTZC measurements and results take part in the better carbon process/flux understanding of under studied blue carbon ecosystems in France and elsewhere and will earn to be discussed in the context of the climate change and its potential impact on carbon budgets.