Carbonate system and acidification of the Adriatic Sea

Abstract

Although the marginal seas represent only 7 % of the total area of the ocean, CO₂ fluxes are important for the carbon budget, exposing them to the intense process of anthropogenic ocean acidification. The Adriatic Sea is currently a CO₂ sink (−0.5 to −1 mol C m⁻² y⁻¹) with an annual flux comparable to the net sink rates in the NW Mediterranean. Based on a comparison of two winter cruises carried out in the 25-years interval between 1983 and 2008, an acidification rate of 0.003 pH_T units y⁻¹ was estimated in the northern Adriatic which is similar to the Mediterranean open waters (with recent estimations of −0.0028 ± 0.0003 pH_T units y⁻¹) and the surface coastal waters (−0.003 ± 0.001 and − 0.0044 ± 0.00006 pH_T units y⁻¹). The computed Revelle factor for the Adriatic Sea (approximately 10) indicates that the buffer capacity is rather high and that the waters do not appear to be particularly exposed to acidification. Total alkalinity (TA) in the Adriatic (2.6–2.7 mmol kg⁻¹) is in the upper range of TA measured in the Mediterranean Sea. This is primarily due to the riverine inputs which transport carbonates dissolved from the Alpine dolomites and karstic watersheds. The Adriatic Sea is the second sub-basin (319 Gmol y⁻¹), following the Aegean Sea (which receives the TA contribution from the Black Sea), that contribute to the riverine TA discharges into the Mediterranean Sea. About 60 % of the TA inflow into the Adriatic Sea is attributed to discharge from the Po River with a TA of ∼3 mmol kg⁻¹ and TA decreases with increasing salinity. The north Adriatic dense water spreading and cascading is an efficient mechanism for exporting TA and DIC at depth, from the northern Adriatic towards the bottom of the South Adriatic Pit and possibly to the eastern Mediterranean. Saturation states indicate that the waters of the Adriatic are supersaturated throughout the year with respect to aragonite (Ω_Ar). However, the saturation state is considerably lower in the bottom water layers, due to the prevalence of the bottom layer and benthic remineralisation in the stratification period. Effects on calcifying organisms and phytoplankton are expected in the future.