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 (pCO(2)), dissolved inorganic carbon (DIC), carbonate ion concentration (CO3=), aragonite saturation state (Omega(Ar)) and total alkalinity (A(T)). During winter, low seawater temperature (9.0 +/- 0.4 degrees C) and a weak biological activity (-10.7 < AOU < 15.7 mu mol O-2 kg(-1)) in a homogeneous water column led to the lowest average values of pCO(2) (328 +/- 19 mu atm) and Omega(Ar) (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 = -29.3 mu mol O-2 kg(-1)) and respiration processes in the lower layer (average AOU = 26.8 mu mol O-2 kg(-1)). These conditions caused the decrease of DIC (50 mu mol kg(-1)) and the increase of Omega(Ar) (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 = 86.9 mu mol O-2 kg(-1)), the intense remineralisation of organic carbon caused the rise of pCO(2) (1043 mu atm) and the decreases of pH(T) and Omega(Ar) values down to 7.732 and 1.79 respectively. On an annual basis, surface pCO(2) 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 air-sea gas exchange (FCO2 up to -11.9 mmol m(-2) d(-1)). During summer, the temperature-driven increase of pCO(2) was dampened by biological CO2 uptake, as consequence a slight over-saturation (pCO(2) = 409 mu atm) turned out. River plumes were generally associated to higher A(T) and pCO(2) values (up to 2859 mu mol kg(-1) and 606 mu atm 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 (pH(T) 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. (C) 2012 Elsevier Ltd. All rights reserved.