Increase in marginal sea alkalinity may impact air–sea carbon dioxide exchange and buffer acidification

Total alkalinity (TA) has increased in the Baltic Sea, with implications for atmospheric CO₂-induced acidification and CO₂ uptake. We compiled extensive data of TA in surface waters of the Baltic Sea, aiming to (i) identify new tendencies in the relationship between TA and salinity (TA–S relationship), (ii) update the TA trend analysis, (iii) investigate spatial–temporal patterns, and (iv) discuss potential drivers and implications. We observed a progressive decrease in the slopes and increase in the intercepts of the TA–S overtime due to the persistent process of TA enhancement. A weak seasonal pattern was identified, with warmer months presenting lower salinity and TA. Lower rates of TA increase were observed in high salinities (Skagerrak–Kattegat; +1.00 to +2.20 μmol kg⁻¹ yr⁻¹), intermediate trends in low salinities (Gulf of Bothnia; +3.28 to +3.57 μmol kg⁻¹ yr⁻¹), and maximal trends in the Central Baltic Sea (+3.70 to +4.57 μmol kg⁻¹ yr⁻¹) and Bornholm Basin (+4.82 to +5.32 μmol kg⁻¹ yr⁻¹). The increase in the intercept of the TA–S in the Gulf of Bothnia suggests a progressive increase in the external supply of TA, although lower than previously thought. The maximum trend in the Bornholm Basin suggests an increase in external supply from the Southern catchment and/or the accumulation of internal production. The positive TA–phosphorus correlations underscore a significant internal source. The TA increase amplifies the CO₂ uptake by 1.8–7.8% during spring/summer and reduces the CO₂ outgassing by 3.4–7.7% in autumn/winter. The TA enhancement has the potential to buffer CO₂-induced acidification by 39–60% by 2050.