Carbonate chemistry dynamics and biological processes along a river–sea gradient (Gulf of Trieste, northern Adriatic Sea)

• We investigated the carbonate system dynamics along a river–sea gradient.
• The conservative mixing was the main driver of DIC seasonal variation.
• The influence of biological activity was significant during spring and summer.
• On annual scale the system was a sink of atmospheric CO2.
• The area close to the river's mouth acted as a source of carbon dioxide.

In this paper we investigated, for two years and with a bi-monthly frequency, how physical, chemical, and biological processes affect the marine carbonate system in a coastal area characterized by high alkalinity riverine discharge (Gulf of Trieste, northern Adriatic Sea, Mediterranean Sea).By combining synoptic measurements of the carbonate system with in situ determinations of the primary production (14C incorporation technique) and secondary prokaryotic carbon production (3H-leucine incorporation) along a river–sea gradient, we showed that the conservative mixing between river endmember and off-shore waters was the main driver of the dissolved inorganic carbon (DIC) distribution and seasonal variation. However, during spring and summer seasons also the influence of biological uptake and release of DIC was significant. In the surface water of June 2012, the spreading and persistence of nutrient-rich freshwater stimulated the primary production (3.21 μg C L− 1 h− 1) and net biological DIC decrease (− 100 μmol kg− 1), reducing the dissolved CO2 concentration and increasing the pHT. Below the pycnocline of August 2012, instead, an elevated bacterial carbon production rate (0.92 μg C L− 1 h− 1) was related with net DIC increase (92 μmol kg− 1), low dissolved oxygen concentration, and strong pHT reduction, suggesting the predominance of bacterial heterotrophic respiration over primary production.The flux of carbon dioxide estimated at the air–sea interface exerted a low influence on the seasonal variation of the carbonate system. A complex temporal and spatial dynamic of the air–sea CO2 exchange was also detected, due to the combined effects of seawater temperature, river discharge, and water circulation. On annual scale the system was a sink of atmospheric CO2. However, in summer and during elevated riverine discharges, the area close to the river's mouth acted as a source of carbon dioxide. Also the wind speed was crucial in controlling the air–sea CO2 exchange, with strong Bora events (a typical ENE wind of the Gulf of Trieste) that drastically increased the absorption (− 32.2 mmol m− 2 day− 1) or the release (5.34 mmol m− 2 day− 1) of carbon dioxide.