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This study examined the CO 2 system in the estuary of the Oder River, one of the largest rivers entering the Baltic Sea. Three measurable parameters describing the CO 2 system, namely total alkalinity (A T), total CO 2 (C T), and the partial pressure of CO 2 (pCO 2), were investigated together with dissolved oxygen, salinity (S), and temperature during two RV Oceania cruises, in May and November of 2016. Large spatial variabilities of A T (1771–2940 μmol kg−1) and C T (1676–2972 μmol kg−1) were determined along the S gradient between the open Baltic Sea and river mouth. In November, the relationships of A T –S and C T -S indicated conservative mixing whereas in May both were strongly affected by biomass production and calcium carbonate formation. The waters of the Oder were oversaturated with CO 2 compared to the atmosphere , irrespective of the season, with pCO 2 values of 1351 ± 42 μatm in May and 1120 ± 32 μatm in November. In the Szczecin Lagoon, however, pCO 2 levels dropped significantly, to 63 μatm, in May, accompanied by an O 2 saturation of up to 134% during the same period. The inverse correlation of pCO 2 and O 2 saturation indicated that the distribution of CO 2 and O 2 in the estuary at the time of sampling was controlled mostly by biological activity. The very large drop in the pCO 2 of the Szczecin Lagoon induced an extreme oversaturation of CaCO 3 that triggered mineral calcite precipitation. The mineral precipitation of carbonates in the lagoon may have accounted for as much as 40% of the C T depletion determined in May, with the remaining 60% attributed to the joint effect of net ecosystem production and CO 2 air/water gas exchange. • The CO 2 system in the Oder River Estuary was investigated for the first time. • OM production and remineralization affect the CO 2 system in the Oder River Estuary. • Extreme primary production may initiate mineral precipitation of calcite in high A T. • Estuarine processes may modify the riverine loads of A T and C T to the Baltic Sea. [ABSTRACT FROM AUTHOR]