| | | 18.4 Northern basin |
The oceanography of the northern Adriatic Sea is influenced by the physiography of the basin and by the meteorology of the area. The wide periodical modification of the density field and of the water column vertical stability is a result of the above factors, the density structure of the basin varying from complete instability in winter to a highly stratified pattern during the rest of the year.
The most important factors driving the processes, which determine the annual modification of the thermohaline characteristics of the water, the generation of water masses and their circulation is the wide fluctuation of the surface heat fluxes and the large volume of fresh water inflowing into the shallow basin.
During the winter, when the total heat budget is negative, the cold waters diluted by the western riverine inflow remain confined in a coastal belt, separated from the offshore waters by a frontal system, and flowing southward. In the offshore area of the basin the waters are highly saline, being advected from the southern basins: they are actively mixed by wind driven surface cooling and mechanical stirring. Under these conditions of vertical instability, high density waters are generated, which flow towards the central basin during periods of strong NE (Bora) wind. In the early spring the inversion of the total heat budget leads to the generation of a thermocline, and the vertical stabilization of the surface layer to the injection of Po River waters. The dilution increases the buoyancy of the surface layer, which expands to a large part of the northern Adriatic. Successive heating and dilution processes, wind mixing, and advection from the southern basins in the summer generate highly stratified water column, in which as a rule three layers can be identified, separated by strong density gradients. The increase in hydrostatic stability resulting from the pycnoclines reduces the vertical turbulent diffusion and affects vertical mixing, and in different ways the transport processes of dissolved and particulate matter.
During the wintertime a significant part of the nutrient input from river sources is retained inside the coastal water belt, in which their concentration becomes very high (e.g. up to 10 M dm-3 for nitrate-nitrogen). In offshore saline water the nutrients show quite low concentrations, and seem to originate mainly by recycling, by advection from the southern basin, or, to a lesser extent, by cross frontal turbulent diffusion from the coastal waters. Both in the coastal and in the offshore zone the vertical mixing causes a homogeneous distribution along the water column of dissolved and suspended matter.
In spring and summer diffusive transport of the terrigenous and riverine inputs are the prevailing mechanism of nutrient transfer to the basin waters. Part of the input from sources distributed along the coast is retained in the coastal waters, but a large part of the input from the River Po, driven by the river plume dynamics, is distributed with the fresh diluting water over the whole basin. In the surface layer the nutrient concentrations range from one to three orders of magnitude from the input point to the periphery of the diluted area, as an effect both of turbulent diffusion and of biological uptake, and of sedimentation of the particulate matter produced in the deeper layers.
The vertical flux of nutrients in the dissolved and particulate phases is strongly discriminated by the density condition. The decrease of vertical turbulence at the pycnoclines hinders the diffusive transport of dissolved matter between the layers, and the steep density gradients regulate the downwards diapycnic flux of different dimensional ranges of the biogenic particulates, through the control of their settling speed. The level of nutrient concentrations becomes periodically high in the dense water mass, confined by the deep pycnocline. In this layer the stock of dissolved nutrients increases in consequence of the mineralization processes of the organic matter settling from the overlying layers, with a related decrease of dissolved oxygen. A significant fraction of the nutrients advected in the surface layer by the River Po is stored in this way into the near bottom layer, from which is very slowly released upwards through turbulent diffusion, or periodically exported by lateral advection, and quantitatively diffused into the upper part of the water column only during the early winter mixing phase.
| | | 18.4 Northern basin |