Effects of alkalinity and carbon in submarine groundwater discharge on coastal acidification in the Baltic Sea

With increasing atmospheric concentration of carbon dioxide, more CO₂ is dissolved in seawater, resulting in ocean acidification (OA). The consequences of OA can be altered by alkalinity (TA), dissolved inorganic carbon (DIC) and freshwater inputs. Here, we study wheter submarine groundwater discharge (SGD) may impact coastal acidification. The DIC and TA relationship in rivers and SGD across six countries and 16 beaches along the Baltic Sea coastline were resolved. TA was greater in groundwater (average = 2521 µmol/kg) than river (1324 µmol/kg) and surface seawater (1307 µmol/kg) samples. The average concentration of DIC in SGD was more than twice the concentration of river and surface Baltic Sea water. SGD contributes with a TA deficiency relative to DIC. 84% of the groundwater samples were acidifying compared to 74% and 38% for river and surface water respectively. Mixing plots revealed that 7 out of 9 basins experienced non-conservative TA and DIC in the subterranean estuary. The surplus of both DIC and TA indicates diagenetic sources such as sulfate reduction and oxic respiration. Overall, SGD can acidify the Baltic Sea and therefore should be included in regional carbon budgets to assess regional ocean acidification.