Carbonate system variability in the Gulf of Trieste (North Adriatic Sea)

The seasonal variability of the carbonate system in the waters of the Gulf of Trieste (GoT) was studied at PALOMA station from 2008 to 2009, in order to highlight the effects of biological processes, meteorological forcings and river loads on the dynamics of pHT, CO2 partial pressure (pCO2), dissolved inorganic carbon (DIC), carbonate ion concentration ðCO¼ 3 Þ, aragonite saturation state (UAr) and total alkalinity (AT). During winter, low seawater temperature (9.0 0.4 C) and a weak biological activity (10.7 < AOU < 15.7 mmol O2 kg1) in a homogeneous water column led to the lowest average values of pCO2 (328 19 matm) and UAr (2.91 0.14). In summer, the water column in the area acted as a two-layer system, with production processes prevailing in the upper layer (average AOU ¼ 29.3 mmol O2 kg1) and respiration processes in the lower layer (average AOU ¼ 26.8 mmol O2 kg1). These conditions caused the decrease of DIC (50 mmol kg1) and the increase of UAr (1.0) values in the upper layer, whereas opposite trends were observed in the bottom waters. In August 2008, during a hypoxic event (dissolved oxygen DO ¼ 86.9 mmol O2 kg1), the intense remineralisation of organic carbon caused the rise of pCO2 (1043 matm) and the decreases of pHT and UAr values down to 7.732 and 1.79 respectively. On an annual basis, surface pCO2 was mainly regulated by the pronounced seasonal cycle of seawater temperature. In winter, surface waters in the GoT were under-saturated with respect to atmospheric CO2, thus acting as a sink of CO2, in particular when strong-wind events enhanced airesea gas exchange (FCO2 up to 11.9 mmol m2 d1). During summer, the temperature-driven increase of pCO2 was dampened by biological CO2 uptake, as consequence a slight over-saturation (pCO2 ¼ 409 matm) turned out. River plumes were generally associated to higher AT and pCO2 values (up to 2859 mmol kg1 and 606 matm respectively), but their effect was highly variable in space and time. During winter, the ambient conditions that favour the formation of dense waters on this continental shelf, also favour a high absorption of CO2 in seawater and its consequent acidification (pHT decrease of 0.006 units during a 7-day Bora wind event). This finding indicates a high vulnerability of North Adriatic Dense Water to atmospheric CO2 increase and ocean acidification process.