In this paper, a first attempt at analysing the macronutrients' mass balance between the north Adriatic coastal sea and the lagoon of Venice by means of a nested and coupled model is presented. The hydrodynamic part of the model simulates the evolution of the sea-surface elevation and of the three-dimensional field of velocity, temperature, salinity and density. Tides, winds, river discharges, thermal and evaporative fluxes are included as forcings. Two models are nested in order to handle the correct spatial scales. The first one, with a resolution of about 10km, is able to describe the basin and sub-basin scale hydrodynamical features; the second one, with a resolution of 1·2km, describes the interactions between the open sea and the lagoon. This last circulation model has been coupled with a simple primary production submodel, in order to investigate the short-term dynamic of the ecosystem during spring time. Results show that, in some instances, the primary production can be sustained by macronutrients' fluxes coming from the coastal area.