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

Seawater acidification can be induced both by absorption of atmospheric carbon dioxide (CO₂) and by atmospheric deposition of sulfur and nitrogen oxides and ammonia. 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 CO ₂-induced acidification but are relatively least affected by additional atmospheric deposition-induced acidification. In contrast, the pH of CO₂-supersaturated systems is most sensitive to atmospheric deposition. The projected increment 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%. Key Points Seawater acidification by absorption of CO₂ and atmospheric deposition co-occur CO₂- supersaturated seawater is especially susceptible to atmospheric deposition By 2100 coastal seas become more sensitive to any proton transferring process