The carbonate dissolution plays a major role in the atmospheric CO2 sink and in the riverine transfer of dissolved inorganic carbon from the atmosphere to the critical zone and to the oceans. In this context, the Baget watershed (13.25 km2, altitude 950 m), essentially forested and weakly exposed to local anthropogenic pollution, drains a karst area in the Pyrenees mountains. It has been monitored for more than 40 years to better understand the impact of global changes on carbonate dissolution and hydro-chemistry of streamwaters. The mean annual precipitation exceeds 1500 mm and the air temperature average is 12°C. Calcareous dominates the lithology (around 2/3 of the area), with some flysh and schists. This experimental catchment belongs to the French Karst Network, to the French (OZCAR) and European (LTER) Research Infrastructures. Based on the hydrochemistry survey since 1978, the results focus on carbonate dissolution, stream water chemistry (mainly Ca, Mg and alkalinity) and calcite saturation index (SI) in relation with pCO2, temperature (T) and river discharge (Q). We analysed the long term trends of the instantaneous values but also of the inter-annual fluctuations of the mean monthly values. The long-term hydrochemical survey allows to evidence a net increasing trend in [Ca2++Mg2+] and [HCO3-] that could be related to an increase in air temperature and a decrease in pCO2 and discharge. Indeed, changes in vegetation cover over the period might have been another controlling factor that is currently investigated. Furthermore, mean monthly values based on the long-term trends allow to understand the dynamic of carbonate dissolution and to identify the main key controlling factors such as the water amount (discharge) and the air temperature, which influences pCO2 production. Lastly, the influence of the drainage relative to minor lithology could be evidenced particularly during low water period by an increased proportion of [SO42-] to [HCO3- ] in stream water, due to the relative substitution of [H2CO3] by [H2SO4] from pyrite oxydation.