Influence of typology and management practices on water pCO2 and atmospheric CO2 fluxes over two temperate shelf – estuary – marsh water continuums

Within the coastal zone, salt marshes often behave as atmospheric CO2 sinks, allowing for blue carbon (C) sequestration associated with intense autotrophic metabolism. However, C dynamics over salt marshes are complex since various biogeochemical processes and fluxes occur at different terrestrial – aquatic – atmospheric exchange interfaces and spatiotemporal scales. This study focuses on seasonal, tidal and diurnal variations of water pCO 2, estimated water-air CO2 fluxes and controlling factors along two temperate shelf – estuary – marsh continuums. The latter include typical coastal systems with artificial salt marshes that have contrasting water management practices and primary producer types. Our high-frequency biogeochemical measurements (seasonal 24-hour cycles) highlighted a strong control of ecosystem typology on inorganic C dynamics with lower water pCO 2 values in the artificial salt marshes, due to stronger biological activity and longer water residence times, than in the tidal estuary. In the marine-dominated estuary, water pCO 2 variations (267 - 569 ppmv) were strongly controlled by tidal effects and phytoplankton activity particularly in spring/summer. On the contrary, the greatest amplitudes in water pCO 2 were recorded in the artificial salt marshes (6 - 721 ppmv) due to intense macrophyte activity. In the rewilded marsh, nutrient inputs favoured spring/summer fast-growing macroalgae produced, in turn, strong fall atmospheric CO2 outgassing from degraded algae waters and thus a net annual source of CO2 to the atmosphere (17.5 g C m−2 yr −1). Conversely, specific management practices at the working marsh for salt-farming activity favoured rather slow-growing macrophytes (i.e. seagrasses) which greatly contribute to the yearly observed atmospheric CO2 sink (-97.7 g C m−2 yr −1). In this work, we suggest that salt marsh management can be used to control the contribution of primary producers to marsh C budget as atmospheric CO2 (sink and/or source).