High-frequency pCO₂ and ancillary data were recorded for seven years during the first deployment of a CARbon Interface OCean Atmosphere (CARIOCA) sensor in the surface waters of a temperate coastal ecosystem, the Bay of Brest, which is impacted by both coastal (via estuaries) and oceanic (North Atlantic via the Iroise Sea) water inputs. The CARIOCA sensor proved to be an excellent tool to constrain the high pCO₂ variability in such dynamic coastal ecosystem. Biological processes (e.g. pelagic photosynthesis/respiration) were the main drivers of the seasonal and diurnal pCO₂ dynamics throughout seven years of observations. Autotrophic processes were responsible for abrupt pCO₂ drawdown of 100 to 200 μatm in spring. During the spring bloom, diurnal variations were driven by diel biological cycle. The average daily drawdown due to autotrophy (observed during highest daily PAR) was equivalent to 10 to 60% of the total pCO₂ drawdown observed every year during the spring season. From late summer to fall, heterotrophic processes increased pCO₂ in the surface water of the Bay back to the pre-bloom level. The average daily increase due to heterotrophy (observed during lowest daily PAR) corresponded to 10 to 70% of the total pCO₂ increase observed every year during the late summer to fall period. Air-sea CO₂ fluxes estimates based on hourly, daily and monthly calculations showed that careful consideration of the diurnal variability was needed to accurately estimate air-sea CO₂ fluxes in the Bay of Brest. Sampling only during daytime or night-time would induce 8 to 36% error on monthly air-sea CO₂ fluxes. This would in turn reverse the direction of the fluxes at annual level for the Bay. The annual emissions of CO₂ from the surface waters of the Bay to the atmosphere showed relatively low inter-annual variations with an average of + 0.7 ± 0.4 mol C m^-2 yr^-1 computed for the study period. Further, air-sea CO₂ fluxes computed for the adjacent inner-estuaries and Iroise Sea for an annual cycle were + 17 ± 3 mol C m^-2 yr^-1 and − 0.2 ± 0.2 mol C m^-2 yr^-1 , respectively. The spatial gradient showed a clear pattern from strong source to sink of CO₂, from the inner-estuaries to the open oceanic waters of the North Atlantic. We suggest that semi-enclosed Bays act as buffers for sea to air emissions of CO₂ from inner estuaries to adjacent costal seas.