Carbonate Chemistry in Groundwater and Rivers Draining to the Baltic Sea: Implications for Coastal Ocean Acidification

Abstract Terrestrial inputs can alter total alkalinity (TA) and dissolved inorganic carbon (DIC) of the coastal ocean and modify seawater pH. Here, we first characterize the carbonate system in river and groundwater draining to the Baltic Sea using observations of TA, DIC, δ 13 C-DIC, and major ions across 6 countries and 17 beaches. We then assess whether submarine groundwater discharge (SGD) may impact coastal acidification. TA and DIC concentrations were about 2 times greater in groundwater than river water. 84% of the groundwater and 72% of river samples showed potential to acidify receiving Baltic Sea waters and degas CO 2 due to low TA/DIC ratios. Mixing plots revealed non-conservative production of TA and DIC in subterranean estuaries. δ 13 C-DIC values imply that organic matter respiration was a main source of DIC to northern catchments, while calcium carbonate (CaCO 3 ) dissolution was more important along the southeastern coast. Fresh SGD contributed only < 2% of TA and DIC, and 5–7% of Ca, Mg, and SO 4 fluxes compared to river discharge when extrapolated to the entire Baltic Sea. However, unquantified total SGD (fresh groundwater plus recirculated seawater) water and chemical fluxes are likely higher. Overall, SGD can locally acidify the Baltic Sea and should be considered in regional carbon budgets.