3-D structure of the marine coastal front and quantitative estimation of nutrient budgets in a seawater volume of the NW Adriatic Sea

A detailed description of a portion of the coastal front (February, 1997) is presented in an area of the north-western Adriatic Sea, south of the Po River Delta. Structure and distribution of the lower salinity waters, generated by freshwater advection, were investigated using the CTD data provided by the undulating tow-fish "SARAGO" and modelling the 3-D salinity field of this area. Of the total seawater volume of 27.42 Km3 the in-shore portion of the front, defined by a salinity value ranging from 28.87 up to 37.00, had a volume of 12.00 Km3. In these lower salinity waters we estimated the budgets of the major nutrients and inorganic nitrogen uptake due to primary production by correlating the 3-D salinity field with analytical data of bottle samples that were collected just a few days later. Budgets of dissolved inorganic phosphorus, silicon and nitrogen (8, 250 and 1417 tons respectively) point very closely to the same proportions of the riverine supply of nutrients in this area, whereas the high amount of dissolved organic nitrogen (1226 tons) and phosphorus (31 tons) stresses the important role of organic fractions in the global balances of these elements in the marine environment. Furthermore, the inorganic nitrogen uptake (34 tons per day) indicates that the main "allochthonous" form of nitrogen, basically constituted by the riverine supply of nitrate, is only partially consumed, whereas the "regenerated" form (i.e. ammonium) seems to sustain an important fraction (53%) of primary production because of its fast recycling in the coastal marine environment.