A mechanistic explanation for the accumulation of dissolved organic carbon (DOC), observed in coastal seas such as the Northern Adriatic Sea, is proposed here on the basis of numerical simulations of the marine ecosystem dynamics carried out with a coupled biogeochemical-circulation model. The biogeochemical model is based on the European Regional Seas Ecosystem Model (ERSEM) upgraded with a more detailed representation of the DOC-bacteria interactions and resolving different level of DOC lability/refractivity. The circulation model is the Adriatic Sea implementation of the Princeton Ocean Model. The analysis of simulations confirms the important role of the Po river nutrient input on the ecosystem dynamics and highlights the presence of a strong across-shelf trophic gradient that, affecting the Bacterial Growth Efficiency (BGE), could be a key factor for the DOC accumulation. The simulations show the importance of circulation features in modulating the exchanges between areas having different trophic structure such as the western coastal strip, strongly influenced by the Po river runoff, and the open sea areas in the centre of the northern Adriatic sub-basin. The DOC produced in the high energy system of the Po runoff coastal strip, characterized by high BGE, is transported toward the open areas, which is a more oligotrophic environment with lower BGE. In this area the DOC turnover time is strongly increased giving rise to the DOC accumulation.