Influence of karstic rivers and southern Adriatic waters on the CO2 system of the Gulf of Trieste (Mediterranean Sea)

The carbon cycle in coastal areas is particularly dynamic due to the complex interactions between different carbon sources and sinks. The present study focuses on the influence of carbonate aquifers on the CO2system in the Gulf of Trieste (GoT, northeast Adriatic Sea). Three marine stations (in the north, centre and south GoT) were investigated monthly between September 2021 and December 2022 at 4 different depths for key water chemistry and carbonate system parameters. In the surface waters under the influence of river water, total alkalinity was highest (2728±19 µmol kgsw-1), pCO2 was 425±35 μatm and δ13C-DIC was 0.66±0.39 ‰. Total alkalinity (TA) was inversely correlated with salinity, indicating a high contribution of bicarbonates and carbonates from river discharges. The highest pCO2 (594±68 μatm) and the lowest pH (7.951±0.040) and δ13C-DIC (0.61±0.35‰) occurred in bottom water isolated below the pycnocline during summer/early fall, reflecting the contribution of respiration and decomposition processes. The Eastern Adriatic Current brings southern waters with increasingly high salinity into the GoT, which when they cool in winter, form very dense water (1029.7±0.2 kg m-3) with a high TCO2content (2401±24 μmol kg-1). These southward-flowing waters contribute to the formation of the Eastern Mediterranean Deep Water.The observed low river discharges amplified the effect of the intrusion of high salinity waters with a lower buffer capacity. A future increase in prolonged droughts could therefore, reduce the buffering capacity of GoT