2. The Marine CO₂ System and Its Peculiarities in the Baltic Sea

Carbonic acid formed by the dissolution of CO₂ in seawater dissociates to yield hydrogen carbonate, carbonate, and hydrogen ions, which are linked to each other by dissociation constants and constitute the marine CO₂ system. To determine the composition of the CO₂ system, requires that the values of two of four measurable variables are known: total CO₂ (sum of the CO₂ species), alkalinity (excess of base equivalents over hydrogen ions), pH (log of the hydrogen ion concentration), and the CO₂ partial pressure (pCO₂ in air at equilibrium with the respective water). Alkalinity plays a central role because it controls the status of the CO₂ system in case that the sea is at equilibrium with CO₂ in the atmosphere. In the Baltic Sea, alkalinity inputs via river water are subject to strong regional differences. Together with the alkalinity input by inflowing North Sea water different alkalinity regimes are formed which lead to characteristic regional distributions of the total CO₂ and pH. Alkalinity also affects the relationships between the variables of the CO₂ system. The magnitude of the change in pCO₂ in response to a change in total CO₂ increases with decreasing alkalinity. This effect is important when pCO₂ measurements are used to estimate biological production, but it also influences CO₂ gas exchange with the atmosphere, by increasing the equilibration time at high alkalinities.