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Abstract :
[en] The transfer of carbon between the continent, the ocean and the atmosphere is investigated in ten European estuaries. A major fraction of the organic carbon (COrga) transported by rivers is mineralised in estuaries. This mineralisation affects in majority the particulate fraction and depends on the origin (fresh water phytoplankton, anthropogenic loads and soils) and the lability of this COrga and the residence time of particles. Mineralisation processes are investigated in two macrotidal estuaries. In the Gironde (France) because of fluid mud settling and resuspension cycles at tidal and neap-spring time scales, particulate material experiences oxic/anoxic oscillations. Particulate organic matter is mineralised alternatively by aerobic respiration, denitrification and metal-oxides reduction. In the fluid mud, a carbonate dissolution significantly reduces the quantity of CO2 produced by the respiration and denitrification produces nitrous oxide. In the hypoxic and highly polluted area of the Scheldt (Belgium, the Netherlands), ammonification, nitrification and denitrification processes modify the equilibrium between the chemical species of dissolved inorganic carbon by producing and consuming protons. The CO2 fluxes from the water to the atmosphere measured in the ten studied sites allow an estimation of the total European estuarine emission of 30-60.106 tC.YEAR-1, corresponding to 5-10% of the anthropogenic emissions from the continent. Because of a physical ventilation of CO2 carried by rivers, the estuarine emission exceeds the mineralisation of COrga. From one estuary to the other, the origin of the total CO2 atmospheric flux (heterotrophic activity, nitrification, carbonate dissolution and physical ventilation) appear to be very different, according to the characteristics of the hydrological basin and the residence time.
