Elevated River Inputs of the Total Alkalinity and Dissolved Inorganic Carbon in the Northern Adriatic Sea

The response of coastal systems to global acidification depends strongly on river inputs, which can alter the total alkalinity (A(T)) and dissolved inorganic carbon (DIC) in seawater. The northern Adriatic Sea (NAd) is a shallow continental shelf region that currently receives about 15% of the total freshwater input in the Mediterranean Sea, where the role of riverine discharges on the carbonate system has been poorly studied. In particular, river discharges can alter the carbonate system in the sea, affecting both the equilibrium chemistry and biological processes. For the main rivers flowing into the NAd (the Po, Adige, Brenta, Piave, Livenza, Tagliamento, Isonzo, Timavo and Rizana), data were collected for the pH, concentrations of the total alkalinity (A(T)), Ca2+ and Mg2+ and the isotopic ratio of stable carbon in the dissolved inorganic carbon (delta C-13(DIC)). The DIC fluxes were estimated using the THINCARB (THermodynamic modeling of INOrganic CARBon) model for the compilation of the A(T) and pH data. The results show that the total transport of the A(T) in the rivers was 205 Gmol yr(-1) while the transport of the DIC was 213 Gmol yr(-1), of which about 70% was from the Po River. About 97% of the DIC in the river waters was in the form of bicarbonates. The high Mg2+/Ca2+ ratios indicate that dolomite weathering is predominant in the Adige, Piave, and Livenza river basins, while lower ratios in the Timavo and Rizana rivers indicate a greater proportion of calcite. The mean delta C-13-DIC value was estimated to be -10.0 +/- 1.7 parts per thousand, a value nowadays considered typical for the DIC flux inputs in oceanic carbon cycle modeling. The DIC flux depends on the mineral weathering and biological activity in each river basin. However, these natural processes can be modified by anthropogenic disturbances that should be better quantified.