Salt marshes are blue carbon (C) ecosystems characterized by intense atmospheric CO2 uptake and C sequestration but also organic and inorganic C exports through the tide. However, uncertainties on main biotic factors controlling vertical and horizontal C fluxes imply studying simultaneously terrestrial and aquatic metabolisms at small timescales (diurnal and tidal) and distinguish their contributions to net ecosystem CO2 exchanges (NEE). Within a temperature salt marsh, four sampling 24-h cycles were performed to measure water biogeochemical parameters (carbon and nutrients) and planktonic metabolism simultaneously to NEE successively at high tide (imported coastal waters influenced by the continental shelf) and low tide (exported channel waters influenced by the marsh). At high tide, water CO2 oversaturation due to aquatic heterotrophy was able to significantly reduce marsh atmospheric CO2 uptake at the ecosystem scale (NEE) during the highest immersion levels. At low tide, water pCO2 were also mainly controlled by marsh biological activity inducing large water CO2 oversaturation in winter due to heterotrophy and large water CO2 undersaturation in spring and summer due to autotrophy. In winter, the highest increases of dissolved inorganic carbon (DIC; from 2354 to 3963 µmol kg-1), total alkanity (TA; from 2508 to 4016 µmol kg-1) and dissolved inorganic nitrogen (DIN; from 27.7 to 68.4 µM) were measured at low tide night probably due to intense anaerobic respiration processes in channel waters and/or sediments resulting in the highest water pCO2 (up to 1461 ppmv). On the contrary, in spring and summer, large water pCO2 decreases and dissolved organic carbon (DOC) increases from high to low tide could be related to intense autochthonous and allochthonous aquatic primary production. Over the 24-h cycles, planktonic metabolism strongly influenced water pCO2 variations, especially at low tide, though planktonic communities did not play a major role in the atmospheric C balances at the ecosystem scale (NEE), accounting for only 10 % in spring.