Biogeochemical context impacts seawater pH changes resulting from atmospheric sulfur and nitrogen deposition

Ocean acidification, the change in seawater chemistry driven by the absorption of increased atmospheric carbon dioxide (CO2), has lowered surface ocean pH by 0.1 units since the Industrial Revolution and is expected to cause a further decrease of 0.4 units by 2100. Additionally, atmospheric deposition of sulfur and nitrogen oxides and ammonia can lead to a decline in seawater pH, especially in coastal regions. Both acidifying processes co-occur and their relative significance, interplay and dependency on water-column biogeochemistry are not well understood. Using a simple biogeochemical model we show that the initial conditions of coastal systems are not only relevant for CO2-induced acidification, but also for additional acidification due to atmospheric acid deposition. Coastal areas undersaturated with respect to CO2 are most vulnerable to CO2-driven acidification, but are relatively least affected by additional atmospheric deposition-induced acidification. However, the pH of CO2-supersaturated systems is most sensitive to atmospheric deposition. The projected increase in atmospheric CO2 by 2100 will increase the sensitivity of coastal systems to atmospheric deposition-induced acidification by up to a factor 4, but the additional annual change in proton concentration is at most 28%.